idnits 2.17.00 (12 Aug 2021) /tmp/idnits62186/draft-ietf-emailcore-rfc5321bis-09.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- == There are 2 instances of lines with non-RFC2606-compliant FQDNs in the document. -- The draft header indicates that this document obsoletes RFC7504, but the abstract doesn't seem to mention this, which it should. -- The draft header indicates that this document obsoletes RFC7505, but the abstract doesn't seem to mention this, which it should. -- The draft header indicates that this document obsoletes RFC1846, but the abstract doesn't seem to mention this, which it should. Miscellaneous warnings: ---------------------------------------------------------------------------- == Line 2067 has weird spacing: '...ivalent and...' -- The document date (1 February 2022) is 108 days in the past. Is this intentional? -- Found something which looks like a code comment -- if you have code sections in the document, please surround them with '' and '' lines. Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) -- Looks like a reference, but probably isn't: '5321bis' on line 2970 -- Looks like a reference, but probably isn't: 'JcK 20210904' on line 2623 -- Possible downref: Non-RFC (?) normative reference: ref. '2' ** Obsolete normative reference: RFC 821 (ref. '3') (Obsoleted by RFC 2821) -- Obsolete informational reference (is this intentional?): RFC 822 (ref. '13') (Obsoleted by RFC 2822) -- Obsolete informational reference (is this intentional?): RFC 974 (ref. '16') (Obsoleted by RFC 2821) -- Obsolete informational reference (is this intentional?): RFC 1869 (ref. '22') (Obsoleted by RFC 2821) -- Obsolete informational reference (is this intentional?): RFC 2821 (ref. '30') (Obsoleted by RFC 5321) -- Obsolete informational reference (is this intentional?): RFC 3501 (ref. '36') (Obsoleted by RFC 9051) -- Duplicate reference: RFC5321, mentioned in '51', was also mentioned in '47'. Summary: 1 error (**), 0 flaws (~~), 2 warnings (==), 14 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 EMAILCORE J. Klensin 3 Internet-Draft 1 February 2022 4 Obsoletes: 5321, 1846, 7504, 7505 (if approved) 5 Intended status: Standards Track 6 Expires: 5 August 2022 8 Simple Mail Transfer Protocol 9 draft-ietf-emailcore-rfc5321bis-09 11 Abstract 13 This document is a specification of the basic protocol for Internet 14 electronic mail transport. It (including text carried forward from 15 RFC 5321) consolidates, updates, and clarifies several previous 16 documents, making all or parts of most of them obsolete. It covers 17 the SMTP extension mechanisms and best practices for the contemporary 18 Internet, but does not provide details about particular extensions. 19 The document also provides information about use of SMTP for other 20 than strict mail transport and delivery. This document replaces RFC 21 5321, the earlier version with the same title. 23 Notes on Reading This Working Draft 25 Early versions of this working draft were extensively annotated with 26 information, primarily in about changes made over the decade since 27 RFC 5321 appeared, especially when those changes might be 28 controversial or should get careful review. Most of those 29 annotations and associated questions are marked in CREF comments 30 ("//" in the text form). Starting with version -09 of the draft, 31 annotations and notes that were no longer relevant are being pruned 32 to improve readability In general, any annotations or comments not 33 marked with "[[Note in Draft", in the contents of an "Editor's note", 34 or are in the "Errata Summary" appendix (Appendix H.1, they are just 35 notes on changes that have already been made and where those changes 36 originated. As one can tell from the dates (when they are given), 37 this document has been periodically updated over a very long period 38 of time. 40 As people review or try to use this document, it may be worth paying 41 special attention to the historical discussion in Section 1.2. 43 This evolving draft should be discussed on the emailcore@ietf.org 44 list. 46 Status of This Memo 48 This Internet-Draft is submitted in full conformance with the 49 provisions of BCP 78 and BCP 79. 51 Internet-Drafts are working documents of the Internet Engineering 52 Task Force (IETF). Note that other groups may also distribute 53 working documents as Internet-Drafts. The list of current Internet- 54 Drafts is at https://datatracker.ietf.org/drafts/current/. 56 Internet-Drafts are draft documents valid for a maximum of six months 57 and may be updated, replaced, or obsoleted by other documents at any 58 time. It is inappropriate to use Internet-Drafts as reference 59 material or to cite them other than as "work in progress." 61 This Internet-Draft will expire on 5 August 2022. 63 Copyright Notice 65 Copyright (c) 2022 IETF Trust and the persons identified as the 66 document authors. All rights reserved. 68 This document is subject to BCP 78 and the IETF Trust's Legal 69 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 70 license-info) in effect on the date of publication of this document. 71 Please review these documents carefully, as they describe your rights 72 and restrictions with respect to this document. Code Components 73 extracted from this document must include Revised BSD License text as 74 described in Section 4.e of the Trust Legal Provisions and are 75 provided without warranty as described in the Revised BSD License. 77 Table of Contents 79 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 7 80 1.1. Transport of Electronic Mail . . . . . . . . . . . . . . 7 81 1.2. History and Context for This Document . . . . . . . . . . 7 82 1.3. Document Conventions . . . . . . . . . . . . . . . . . . 9 83 2. The SMTP Model . . . . . . . . . . . . . . . . . . . . . . . 9 84 2.1. Basic Structure . . . . . . . . . . . . . . . . . . . . . 9 85 2.2. The Extension Model . . . . . . . . . . . . . . . . . . . 11 86 2.2.1. Background . . . . . . . . . . . . . . . . . . . . . 11 87 2.2.2. Definition and Registration of Extensions . . . . . . 12 88 2.2.3. Special Issues with Extensions . . . . . . . . . . . 13 89 2.3. SMTP Terminology . . . . . . . . . . . . . . . . . . . . 13 90 2.3.1. Mail Objects . . . . . . . . . . . . . . . . . . . . 13 91 2.3.2. Senders and Receivers . . . . . . . . . . . . . . . . 14 92 2.3.3. Mail Agents and Message Stores . . . . . . . . . . . 14 93 2.3.4. Host . . . . . . . . . . . . . . . . . . . . . . . . 15 94 2.3.5. Domain Names . . . . . . . . . . . . . . . . . . . . 15 95 2.3.6. Buffer and State Table . . . . . . . . . . . . . . . 16 96 2.3.7. Commands and Replies . . . . . . . . . . . . . . . . 16 97 2.3.8. Lines . . . . . . . . . . . . . . . . . . . . . . . . 17 98 2.3.9. Message Content and Mail Data . . . . . . . . . . . . 17 99 2.3.10. Originator, Delivery, Relay, and Gateway Systems . . 17 100 2.3.11. Mailbox and Address . . . . . . . . . . . . . . . . . 18 101 2.4. General Syntax Principles and Transaction Model . . . . . 18 102 3. The SMTP Procedures: An Overview . . . . . . . . . . . . . . 20 103 3.1. Session Initiation . . . . . . . . . . . . . . . . . . . 20 104 3.2. Client Initiation . . . . . . . . . . . . . . . . . . . . 21 105 3.3. Mail Transactions . . . . . . . . . . . . . . . . . . . . 21 106 3.4. Address Modification and Expansion . . . . . . . . . . . 24 107 3.4.1. Forwarding for Address Correction or Updating . . . . 24 108 3.4.2. Aliases and Mailing Lists . . . . . . . . . . . . . . 25 109 3.4.2.1. Simple Aliases . . . . . . . . . . . . . . . . . 26 110 3.4.2.2. Mailing Lists . . . . . . . . . . . . . . . . . . 26 111 3.5. Commands for Debugging Addresses . . . . . . . . . . . . 26 112 3.5.1. Overview . . . . . . . . . . . . . . . . . . . . . . 26 113 3.5.2. VRFY Normal Response . . . . . . . . . . . . . . . . 29 114 3.5.3. Meaning of VRFY or EXPN Success Response . . . . . . 29 115 3.5.4. Semantics and Applications of EXPN . . . . . . . . . 30 116 3.6. Relaying and Mail Routing . . . . . . . . . . . . . . . . 30 117 3.6.1. Mail eXchange Records and Relaying . . . . . . . . . 30 118 3.6.2. Message Submission Servers as Relays . . . . . . . . 30 119 3.7. Mail Gatewaying . . . . . . . . . . . . . . . . . . . . . 31 120 3.7.1. Header Fields in Gatewaying . . . . . . . . . . . . . 32 121 3.7.2. Received Lines in Gatewaying . . . . . . . . . . . . 32 122 3.7.3. Addresses in Gatewaying . . . . . . . . . . . . . . . 33 123 3.7.4. Other Header Fields in Gatewaying . . . . . . . . . . 33 124 3.7.5. Envelopes in Gatewaying . . . . . . . . . . . . . . . 33 125 3.8. Terminating Sessions and Connections . . . . . . . . . . 33 126 4. The SMTP Specifications . . . . . . . . . . . . . . . . . . . 34 127 4.1. SMTP Commands . . . . . . . . . . . . . . . . . . . . . . 34 128 4.1.1. Command Semantics and Syntax . . . . . . . . . . . . 34 129 4.1.1.1. Extended HELLO (EHLO) or HELLO (HELO) . . . . . . 35 130 4.1.1.2. MAIL (MAIL) . . . . . . . . . . . . . . . . . . . 37 131 4.1.1.3. RECIPIENT (RCPT) . . . . . . . . . . . . . . . . 37 132 4.1.1.4. DATA (DATA) . . . . . . . . . . . . . . . . . . . 39 133 4.1.1.5. RESET (RSET) . . . . . . . . . . . . . . . . . . 40 134 4.1.1.6. VERIFY (VRFY) . . . . . . . . . . . . . . . . . . 41 135 4.1.1.7. EXPAND (EXPN) . . . . . . . . . . . . . . . . . . 41 136 4.1.1.8. HELP (HELP) . . . . . . . . . . . . . . . . . . . 41 137 4.1.1.9. NOOP (NOOP) . . . . . . . . . . . . . . . . . . . 42 138 4.1.1.10. QUIT (QUIT) . . . . . . . . . . . . . . . . . . . 42 139 4.1.2. Command Argument Syntax . . . . . . . . . . . . . . . 43 140 4.1.3. Address Literals . . . . . . . . . . . . . . . . . . 45 141 4.1.4. Order of Commands . . . . . . . . . . . . . . . . . . 47 143 4.2. SMTP Replies . . . . . . . . . . . . . . . . . . . . . . 49 144 4.2.1. Reply Code Severities and Theory . . . . . . . . . . 50 145 4.2.2. Reply Codes by Function Groups . . . . . . . . . . . 53 146 4.2.3. Reply Codes in Numeric Order . . . . . . . . . . . . 54 147 4.2.4. Some specific code situations and relationships . . . 56 148 4.3. Sequencing of Commands and Replies . . . . . . . . . . . 57 149 4.3.1. Sequencing Overview . . . . . . . . . . . . . . . . . 58 150 4.3.2. Command-Reply Sequences . . . . . . . . . . . . . . . 58 151 4.4. Trace Information . . . . . . . . . . . . . . . . . . . . 61 152 4.4.1. Received Header Field . . . . . . . . . . . . . . . . 61 153 4.5. Additional Implementation Issues . . . . . . . . . . . . 65 154 4.5.1. Minimum Implementation . . . . . . . . . . . . . . . 65 155 4.5.2. Transparency . . . . . . . . . . . . . . . . . . . . 65 156 4.5.3. Sizes and Timeouts . . . . . . . . . . . . . . . . . 66 157 4.5.3.1. Size Limits and Minimums . . . . . . . . . . . . 66 158 4.5.3.1.1. Local-part . . . . . . . . . . . . . . . . . 66 159 4.5.3.1.2. Domain . . . . . . . . . . . . . . . . . . . 67 160 4.5.3.1.3. Path . . . . . . . . . . . . . . . . . . . . 67 161 4.5.3.1.4. Command Line . . . . . . . . . . . . . . . . 67 162 4.5.3.1.5. Reply Line . . . . . . . . . . . . . . . . . 67 163 4.5.3.1.6. Text Line . . . . . . . . . . . . . . . . . . 67 164 4.5.3.1.7. Message Content . . . . . . . . . . . . . . . 67 165 4.5.3.1.8. Recipient Buffer . . . . . . . . . . . . . . 67 166 4.5.3.1.9. Treatment When Limits Exceeded . . . . . . . 68 167 4.5.3.1.10. Too Many Recipients Code . . . . . . . . . . 68 168 4.5.3.2. Timeouts . . . . . . . . . . . . . . . . . . . . 69 169 4.5.3.2.1. Initial 220 Message: 5 Minutes . . . . . . . 69 170 4.5.3.2.2. MAIL Command: 5 Minutes . . . . . . . . . . . 69 171 4.5.3.2.3. RCPT Command: 5 Minutes . . . . . . . . . . . 69 172 4.5.3.2.4. DATA Initiation: 2 Minutes . . . . . . . . . 69 173 4.5.3.2.5. Data Block: 3 Minutes . . . . . . . . . . . . 69 174 4.5.3.2.6. DATA Termination: 10 Minutes. . . . . . . . . 70 175 4.5.3.2.7. Server Timeout: 5 Minutes. . . . . . . . . . 70 176 4.5.4. Retry Strategies . . . . . . . . . . . . . . . . . . 70 177 4.5.5. Messages with a Null Reverse-Path . . . . . . . . . . 72 178 5. Address Resolution and Mail Handling . . . . . . . . . . . . 73 179 5.1. Locating the Target Host . . . . . . . . . . . . . . . . 73 180 5.2. IPv6 and MX Records . . . . . . . . . . . . . . . . . . . 75 181 6. Problem Detection and Handling . . . . . . . . . . . . . . . 75 182 6.1. Reliable Delivery and Replies by Email . . . . . . . . . 76 183 6.2. Unwanted, Unsolicited, and "Attack" Messages . . . . . . 76 184 6.3. Loop Detection . . . . . . . . . . . . . . . . . . . . . 77 185 6.4. Compensating for Irregularities . . . . . . . . . . . . . 77 186 7. Security Considerations . . . . . . . . . . . . . . . . . . . 79 187 7.1. Mail Security and Spoofing . . . . . . . . . . . . . . . 79 188 7.2. "Blind" Copies . . . . . . . . . . . . . . . . . . . . . 80 189 7.3. VRFY, EXPN, and Security . . . . . . . . . . . . . . . . 81 190 7.4. Mail Rerouting Based on the 251 and 551 Response Codes . 81 191 7.5. Information Disclosure in Announcements . . . . . . . . . 82 192 7.6. Information Disclosure in Trace Fields . . . . . . . . . 82 193 7.7. Information Disclosure in Message Forwarding . . . . . . 82 194 7.8. Local Operational Requirements and Resistance to 195 Attacks . . . . . . . . . . . . . . . . . . . . . . . . . 82 196 7.9. Scope of Operation of SMTP Servers . . . . . . . . . . . 83 197 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 83 198 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 84 199 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 85 200 10.1. Normative References . . . . . . . . . . . . . . . . . . 85 201 10.2. Informative References . . . . . . . . . . . . . . . . . 86 202 Appendix A. TCP Transport Service . . . . . . . . . . . . . . . 90 203 Appendix B. Generating SMTP Commands from RFC 822 Header 204 Fields . . . . . . . . . . . . . . . . . . . . . . . . . 90 205 Appendix C. Placeholder (formerly Source Routes) . . . . . . . . 92 206 Appendix D. Scenarios . . . . . . . . . . . . . . . . . . . . . 92 207 D.1. A Typical SMTP Transaction Scenario . . . . . . . . . . . 92 208 D.2. Aborted SMTP Transaction Scenario . . . . . . . . . . . . 92 209 D.3. Relayed Mail Scenario . . . . . . . . . . . . . . . . . . 93 210 D.4. Verifying and Sending Scenario . . . . . . . . . . . . . 95 211 Appendix E. Other Gateway Issues . . . . . . . . . . . . . . . . 96 212 Appendix F. Deprecated Features of RFC 821 . . . . . . . . . . . 96 213 F.1. TURN . . . . . . . . . . . . . . . . . . . . . . . . . . 97 214 F.2. Source Routing . . . . . . . . . . . . . . . . . . . . . 97 215 F.3. HELO . . . . . . . . . . . . . . . . . . . . . . . . . . 98 216 F.4. #-literals . . . . . . . . . . . . . . . . . . . . . . . 98 217 F.5. Dates and Years . . . . . . . . . . . . . . . . . . . . . 99 218 F.6. Sending versus Mailing . . . . . . . . . . . . . . . . . 99 219 Appendix G. Other Outstanding Issues . . . . . . . . . . . . . . 99 220 G.1. IP Address literals . . . . . . . . . . . . . . . . . . . 100 221 G.2. Repeated Use of EHLO (closed) . . . . . . . . . . . . . . 100 222 G.3. Meaning of "MTA" and Related Terminology . . . . . . . . 101 223 G.4. Originator, or Originating System, Authentication . . . . 101 224 G.5. Remove or deprecate the work-around from code 552 to 452 225 (closed) . . . . . . . . . . . . . . . . . . . . . . . . 101 226 G.6. Clarify where the protocol stands with respect to 227 submission and TLS issues . . . . . . . . . . . . . . . 101 228 G.7. Probably-substantive Discussion Topics Identified in Other 229 Ways . . . . . . . . . . . . . . . . . . . . . . . . . . 102 230 G.7.1. Issues with 521, 554, and 556 codes (closed) . . . . 102 231 G.7.2. SMTP Model, terminology, and relationship to RFC 232 5598 . . . . . . . . . . . . . . . . . . . . . . . . 102 233 G.7.3. Resolvable FQDNs and private domain names . . . . . . 102 234 G.7.4. Possible clarification about mail transactions and 235 transaction state . . . . . . . . . . . . . . . . . . 102 236 G.7.5. Issues with mailing lists, aliases, and forwarding . 103 237 G.7.6. Requirements for domain name and/or IP address in 238 EHLO . . . . . . . . . . . . . . . . . . . . . . . . 103 240 G.7.7. Does the 'first digit only' and/or non-listed reply 241 code text need clarification? (closed) . . . . . . . 103 242 G.7.8. Size limits (closed) . . . . . . . . . . . . . . . . 103 243 G.7.9. Discussion of 'blind' copies and RCPT . . . . . . . . 103 244 G.7.10. Further clarifications needed to source routes? . . . 104 245 G.7.11. Should 1yz Be Revisited? (closed) . . . . . . . . . . 104 246 G.7.12. Review Timeout Specifications . . . . . . . . . . . . 104 247 G.7.13. Possible SEND, SAML, SOML Loose End (closed) . . . . 104 248 G.7.14. Abstract Update (closed) . . . . . . . . . . . . . . 104 249 G.7.15. Informative References to MIME and/or Message 250 Submission (closed) . . . . . . . . . . . . . . . . . 104 251 G.7.16. Mail Transaction Discussion . . . . . . . . . . . . . 105 252 G.7.17. Hop by hop Authentication and/or Encryption 253 (closed) . . . . . . . . . . . . . . . . . . . . . . 105 254 G.7.18. More Text About 554 Given 521, etc. . . . . . . . . . 105 255 G.7.19. Minimum Lengths and Quantities . . . . . . . . . . . 105 256 G.8. Enhanced Reply Codes and DSNs . . . . . . . . . . . . . . 105 257 G.9. Revisiting Quoted Strings . . . . . . . . . . . . . . . . 106 258 G.10. Internationalization . . . . . . . . . . . . . . . . . . 106 259 G.11. SMTP Clients, Servers, Senders, and Receivers . . . . . . 107 260 G.12. Extension Keywords Starting in 'X-' (closed) . . . . . . 107 261 G.13. Deprecating HELO (closed) . . . . . . . . . . . . . . . . 107 262 G.14. The FOR Clause in Trace Fields: Semantics, Security 263 Considerations, and Other Issues . . . . . . . . . . . . 108 264 G.15. Resistance to Attacks and Operational Necessity 265 (closed) . . . . . . . . . . . . . . . . . . . . . . . . 108 266 G.16. Mandatory 8BITMIME . . . . . . . . . . . . . . . . . . . 109 267 Appendix H. RFC 5321 Errata Summary and Tentative Change Log . . 109 268 H.1. RFC 5321 Errata Summary . . . . . . . . . . . . . . . . . 109 269 H.2. Changes from RFC 5321 (published October 2008) to the 270 initial (-00) version of this draft . . . . . . . . . . . 111 271 H.3. Changes Among Versions of Rfc5321bis . . . . . . . . . . 112 272 H.3.1. Changes from draft-klensin-rfc5321bis-00 (posted 273 2012-12-02) to -01 . . . . . . . . . . . . . . . . . 113 274 H.3.2. Changes from draft-klensin-rfc5321bis-01 (20191203) to 275 -02 . . . . . . . . . . . . . . . . . . . . . . . . . 113 276 H.3.3. Changes from draft-klensin-rfc5321bis-02 (2019-12-27) 277 to -03 . . . . . . . . . . . . . . . . . . . . . . . 113 278 H.3.4. Changes from draft-klensin-rfc5321bis-03 (2020-07-02) 279 to draft-ietf-emailcore-rfc5321bis-00 . . . . . . . . 113 280 H.3.5. Changes from draft-ietf-emailcore-rfc5321bis-00 281 (2020-10-06) to -01 . . . . . . . . . . . . . . . . . 114 282 H.3.6. Changes from draft-ietf-emailcore-rfc5321bis-01 283 (2020-12-25) to -02 . . . . . . . . . . . . . . . . . 114 284 H.3.7. Changes from draft-ietf-emailcore-rfc5321bis-02 285 (2021-02-21) to -03 . . . . . . . . . . . . . . . . . 115 286 H.3.8. Changes from draft-ietf-emailcore-rfc5321bis-03 287 (2021-07-10) to -04 . . . . . . . . . . . . . . . . . 116 289 H.3.9. Changes from draft-ietf-emailcore-rfc5321bis-04 290 (2021-10-03) to -05 . . . . . . . . . . . . . . . . . 117 291 H.3.10. Changes from draft-ietf-emailcore-rfc5321bis-05 292 (2021-10-24) to -06 . . . . . . . . . . . . . . . . . 117 293 H.3.11. Changes from draft-ietf-emailcore-rfc5321bis-06 294 (2021-11-07) to -07 . . . . . . . . . . . . . . . . . 118 295 H.3.12. Changes from draft-ietf-emailcore-rfc5321bis-07 296 (2021-12-04) to -08 . . . . . . . . . . . . . . . . . 119 297 H.3.13. Changes from draft-ietf-emailcore-rfc5321bis-08 298 (2021-12-31) to -09 . . . . . . . . . . . . . . . . . 120 299 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 300 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 123 302 1. Introduction 304 1.1. Transport of Electronic Mail 306 The objective of the Simple Mail Transfer Protocol (SMTP) is to 307 transfer mail reliably and efficiently. 309 SMTP is independent of the particular transmission subsystem and 310 requires only a reliable ordered data stream channel. While this 311 document specifically discusses transport over TCP, other transports 312 are possible. Appendices to RFC 821 [3] describe some of them. 314 An important feature of SMTP is its capability to transport mail 315 across multiple networks, usually referred to as "SMTP mail relaying" 316 (see Section 3.6). A network consists of the mutually-TCP-accessible 317 hosts on the public Internet, the mutually-TCP-accessible hosts on a 318 firewall-isolated TCP/IP Intranet, or hosts in some other LAN or WAN 319 environment utilizing a non-TCP transport-level protocol. Using 320 SMTP, a process can transfer mail to another process on the same 321 network or to some other network via a relay or gateway process 322 accessible to both networks. 324 In this way, a mail message may pass through a number of intermediate 325 relay or gateway hosts on its path from sender to ultimate recipient. 326 The Mail eXchanger mechanisms of the domain name system (RFC 1035 327 [4], RFC 974 [16], and Section 5 of this document) are used to 328 identify the appropriate next-hop destination for a message being 329 transported. 331 1.2. History and Context for This Document 333 This document is a specification of the basic protocol for the 334 Internet electronic mail transport. It consolidates, updates and 335 clarifies, but does not add new or change existing functionality of 336 the following: 338 * the original SMTP (Simple Mail Transfer Protocol) specification of 339 RFC 821 [3], 341 * domain name system requirements and implications for mail 342 transport from RFC 1035 [4] and RFC 974 [16], 344 * the clarifications and applicability statements in RFC 1123 [5], 346 * the new error codes added by RFC 1846 [20] and later by RFC 7504 347 [45], obsoleting both of those documents, and 349 * material drawn from the SMTP Extension mechanisms in RFC 1869 350 [22]. 352 It also includes editorial and clarification changes that were made 353 to RFC 2821 [30] to bring that specification to Draft Standard and 354 similar changes to RFC 5321 [47] to bring the current document to 355 Internet Standard. 357 It may help the reader to understand that, to reduce the risk of 358 introducing errors, large parts of the document essentially merge the 359 earlier specifications listed in the bullet points above rather than 360 providing a completely rewritten, reorganized, and integrated 361 description of SMTP. An index is provided to assist in the quest for 362 information. 364 It obsoletes RFCs 5321 [47] (the earlier version of this 365 specification), 1846 [20] and incorporates the substance of 7504 366 [45]7504 (specification of reply codes), and 7505 [46] (the "Null MX" 367 specification). 369 // JcK: 202107219: does the text that follows need rewriting? See 370 // comment in Abstract. 371 Although SMTP was designed as a mail transport and delivery protocol, 372 this specification also contains information that is important to its 373 use as a "mail submission" protocol, as recommended for Post Office 374 Protocol (POP) (RFC 937 [14], RFC 1939 [23]) and IMAP (RFC 3501 375 [36]). In general, the separate mail submission protocol specified 376 in RFC 6409 [41] is now preferred to direct use of SMTP; more 377 discussion of that subject appears in that document. 379 Section 2.3 provides definitions of terms specific to this document. 380 Except when the historical terminology is necessary for clarity, this 381 document uses the current 'client' and 'server' terminology to 382 identify the sending and receiving SMTP processes, respectively. 384 A companion document, RFC 5322 [12], discusses message header 385 sections and bodies and specifies formats and structures for them. 386 Other relevant documents and their relationships are discussed in a 387 forthcoming Applicability Statement [48]. 389 1.3. Document Conventions 391 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 392 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 393 document are to be interpreted as described in RFC 2119 [1]. As each 394 of these terms was intentionally and carefully chosen to improve the 395 interoperability of email, each use of these terms is to be treated 396 as a conformance requirement. 398 Because this document has a long history and to avoid the risk of 399 various errors and of confusing readers and documents that point to 400 this one, most examples and the domain names they contain are 401 preserved from RFC 2821. Readers are cautioned that these are 402 illustrative examples that should not actually be used in either code 403 or configuration files. 405 2. The SMTP Model 407 2.1. Basic Structure 409 The SMTP design can be pictured as: 411 +----------+ +----------+ 412 +------+ | | | | 413 | User |<-->| | SMTP | | 414 +------+ | Client- |Commands/Replies| Server- | 415 +------+ | SMTP |<-------------->| SMTP | +------+ 416 | File |<-->| | and Mail | |<-->| File | 417 |System| | | | | |System| 418 +------+ +----------+ +----------+ +------+ 419 SMTP client SMTP server 421 When an SMTP client has a message to transmit, it establishes a two- 422 way transmission channel to an SMTP server. The responsibility of an 423 SMTP client is to transfer mail messages to one or more SMTP servers, 424 or report its failure to do so. 426 The means by which a mail message is presented to an SMTP client, and 427 how that client determines the identifier(s) ("names") of the 428 domain(s) to which mail messages are to be transferred, are local 429 matters. They are not addressed by this document. In some cases, 430 the designated domain(s), or those determined by an SMTP client, will 431 identify the final destination(s) of the mail message. In other 432 cases, common with SMTP clients associated with implementations of 433 the POP (RFC 937 [14], RFC 1939 [23]) or IMAP (RFC 3501 [36]) 434 protocols, or when the SMTP client is inside an isolated transport 435 service environment, the domain determined will identify an 436 intermediate destination through which all mail messages are to be 437 relayed. SMTP clients that transfer all traffic regardless of the 438 target domains associated with the individual messages, or that do 439 not maintain queues for retrying message transmissions that initially 440 cannot be completed, may otherwise conform to this specification but 441 are not considered fully-capable. Fully-capable SMTP 442 implementations, including the relays used by these less capable 443 ones, and their destinations, are expected to support all of the 444 queuing, retrying, and alternate address functions discussed in this 445 specification. In many situations and configurations, the less- 446 capable clients discussed above SHOULD be using the message 447 submission protocol (RFC 6409 [41]) rather than SMTP. 449 The means by which an SMTP client, once it has determined a target 450 domain, determines the identity of an SMTP server to which a copy of 451 a message is to be transferred, and then performs that transfer, are 452 covered by this document. To effect a mail transfer to an SMTP 453 server, an SMTP client establishes a two-way transmission channel to 454 that SMTP server. An SMTP client determines the address of an 455 appropriate host running an SMTP server by resolving a destination 456 domain name to either an intermediate Mail eXchanger host or a final 457 target host. 459 An SMTP server may be either the ultimate destination or an 460 intermediate "relay" (that is, it may assume the role of an SMTP 461 client after receiving the message) or "gateway" (that is, it may 462 transport the message further using some protocol other than SMTP). 463 SMTP commands are generated by the SMTP client and sent to the SMTP 464 server. SMTP replies are sent from the SMTP server to the SMTP 465 client in response to the commands. 467 In other words, message transfer can occur in a single connection 468 between the original SMTP-sender and the final SMTP-recipient, or can 469 occur in a series of hops through intermediary systems. In either 470 case, once the server has issued a success response at the end of the 471 mail data, a formal handoff of responsibility for the message occurs: 472 the protocol requires that a server MUST accept responsibility for 473 either delivering the message or properly reporting the failure to do 474 so (see Sections 6.1, 6.2, and 7.8, below). 476 Once the transmission channel is established and initial handshaking 477 is completed, the SMTP client normally initiates a mail transaction. 478 Such a transaction consists of a series of commands to specify the 479 originator and destination of the mail and transmission of the 480 message content (including any lines in the header section or other 481 structure) itself. When the same message is sent to multiple 482 recipients, this protocol encourages the transmission of only one 483 copy of the data for all recipients at the same destination (or 484 intermediate relay) host. 486 The server responds to each command with a reply; replies may 487 indicate that the command was accepted, that additional commands are 488 expected, or that a temporary or permanent error condition exists. 489 Commands specifying the sender or recipients may include server- 490 permitted SMTP service extension requests, as discussed in 491 Section 2.2. The dialog is purposely lock-step, one-at-a-time, 492 although this can be modified by mutually agreed upon extension 493 requests such as command pipelining (RFC 2920 [31]). 495 Once a given mail message has been transmitted, the client may either 496 request that the connection be shut down or may initiate other mail 497 transactions. In addition, an SMTP client may use a connection to an 498 SMTP server for ancillary services such as verification of email 499 addresses or retrieval of mailing list subscriber addresses. 501 As suggested above, this protocol provides mechanisms for the 502 transmission of mail. Historically, this transmission normally 503 occurred directly from the sending user's host to the receiving 504 user's host when the two hosts are connected to the same transport 505 service. When they are not connected to the same transport service, 506 transmission occurs via one or more relay SMTP servers. A very 507 common case in the Internet today involves submission of the original 508 message to an intermediate, "message submission" server, which is 509 similar to a relay but has some additional properties; such servers 510 are discussed in Section 2.3.10 and at some length in RFC 6409 [41]. 511 An intermediate host that acts as either an SMTP relay or as a 512 gateway into some other transmission environment is usually selected 513 through the use of the domain name service (DNS) Mail eXchanger 514 mechanism. 516 2.2. The Extension Model 518 2.2.1. Background 520 In an effort that started in 1990, approximately a decade after RFC 521 821 was completed, the protocol was modified with a "service 522 extensions" model that permits the client and server to agree to 523 utilize shared functionality beyond the original SMTP requirements. 524 The SMTP extension mechanism defines a means whereby an extended SMTP 525 client and server may recognize each other, and the server can inform 526 the client as to the service extensions that it supports. 528 Contemporary SMTP implementations MUST support the basic extension 529 mechanisms. For instance, servers MUST support the EHLO command even 530 if they do not implement any specific extensions and clients SHOULD 531 preferentially utilize EHLO rather than HELO. (However, for 532 compatibility with older conforming implementations, SMTP clients and 533 servers MUST support the original HELO mechanisms as a fallback.) 534 Unless the different characteristics of HELO must be identified for 535 interoperability purposes, this document discusses only EHLO. 537 SMTP is widely deployed and high-quality implementations have proven 538 to be very robust. However, the Internet community now considers 539 some services to be important that were not anticipated when the 540 protocol was first designed. If support for those services is to be 541 added, it must be done in a way that permits older implementations to 542 continue working acceptably. The extension framework consists of: 544 * The SMTP command EHLO, superseding the earlier HELO, 546 * a registry of SMTP service extensions, 548 * additional parameters to the SMTP MAIL and RCPT commands, and 550 * optional replacements for commands defined in this protocol, such 551 as for DATA in non-ASCII transmissions (RFC 3030 [33]). 553 SMTP's strength comes primarily from its simplicity. Experience with 554 many protocols has shown that protocols with few options tend towards 555 ubiquity, whereas protocols with many options tend towards obscurity. 557 Each and every extension, regardless of its benefits, must be 558 carefully scrutinized with respect to its implementation, deployment, 559 and interoperability costs. In many cases, the cost of extending the 560 SMTP service will likely outweigh the benefit. 562 2.2.2. Definition and Registration of Extensions 564 The IANA maintains a registry of SMTP service extensions [52]. A 565 corresponding EHLO keyword value is associated with each extension. 566 Each service extension registered with the IANA must be defined in a 567 formal Standards-Track or IESG-approved Experimental protocol 568 document. The definition must include: 570 * the textual name of the SMTP service extension; 572 * the EHLO keyword value associated with the extension; 574 * the syntax and possible values of parameters associated with the 575 EHLO keyword value; 577 * any additional SMTP verbs associated with the extension 578 (additional verbs will usually be, but are not required to be, the 579 same as the EHLO keyword value); 581 * any new parameters the extension associates with the MAIL or RCPT 582 verbs; 584 * a description of how support for the extension affects the 585 behavior of a server and client SMTP; and 587 * the increment by which the extension is increasing the maximum 588 length of the commands MAIL and/or RCPT, over that specified in 589 this Standard. 591 Any keyword value presented in the EHLO response MUST correspond to a 592 Standard, Standards-Track, or IESG-approved Experimental SMTP service 593 extension registered with IANA. A conforming server MUST NOT offer 594 keyword values that are not described in a registered extension. 596 2.2.3. Special Issues with Extensions 598 Extensions that change fairly basic properties of SMTP operation are 599 permitted. The text in other sections of this document must be 600 understood in that context. In particular, extensions can change the 601 minimum limits specified in Section 4.5.3, can change the ASCII 602 character set requirement as mentioned above, or can introduce some 603 optional modes of message handling. 605 In particular, if an extension implies that the delivery path 606 normally supports special features of that extension, and an 607 intermediate SMTP system finds a next hop that does not support the 608 required extension, it MAY choose, based on the specific extension 609 and circumstances, to requeue the message and try later and/or try an 610 alternate MX host. If this strategy is employed, the timeout to fall 611 back to an unextended format (if one is available) SHOULD be less 612 than the normal timeout for bouncing as undeliverable (e.g., if 613 normal timeout is three days, the requeue timeout before attempting 614 to transmit the mail without the extension might be one day). 616 2.3. SMTP Terminology 618 2.3.1. Mail Objects 620 SMTP transports a mail object. A mail object contains an envelope 621 and content. 623 The SMTP envelope is sent as a series of SMTP protocol units 624 (described in Section 3). It consists of an originator address (to 625 which error reports should be directed), one or more recipient 626 addresses, and optional protocol extension material. Historically, 627 variations on the reverse-path (originator) address specification 628 command (MAIL) could be used to specify alternate delivery modes, 629 such as immediate display; those variations have now been deprecated 630 (see Appendix F and Appendix F.6). 632 The SMTP content is sent in the SMTP DATA protocol unit and has two 633 parts: the header section and the body. If the content conforms to 634 other contemporary standards, the header section consists of a 635 collection of header fields, each consisting of a header name, a 636 colon, and data, structured as in the message format specification 637 (RFC 5322 [12]); the body, if structured, is defined according to 638 MIME (RFC 2045 [25]). The content is textual in nature, expressed 639 using the US-ASCII repertoire [2]. Although SMTP extensions (such as 640 "8BITMIME", RFC 6152 [44]) may relax this restriction for the content 641 body, the content header fields are always encoded using the US-ASCII 642 repertoire. Two MIME extensions (RFC 2047 [26] and RFC 2231 [29]) 643 define an algorithm for representing header values outside the US- 644 ASCII repertoire, while still encoding them using the US-ASCII 645 repertoire. 647 2.3.2. Senders and Receivers 649 In RFC 821, the two hosts participating in an SMTP transaction were 650 described as the "SMTP-sender" and "SMTP-receiver". This document 651 has been changed to reflect current industry terminology and hence 652 refers to them as the "SMTP client" (or sometimes just "the client") 653 and "SMTP server" (or just "the server"), respectively. Since a 654 given host may act both as server and client in a relay situation, 655 "receiver" and "sender" terminology is still used where needed for 656 clarity. 658 2.3.3. Mail Agents and Message Stores 660 Additional mail system terminology became common after RFC 821 was 661 published and, where convenient, is used in this specification. In 662 particular, SMTP servers and clients provide a mail transport service 663 and therefore act as "Mail Transfer Agents" (MTAs). "Mail User 664 Agents" (MUAs or UAs) are normally thought of as the sources and 665 targets of mail. At the source, an MUA might collect mail to be 666 transmitted from a user and hand it off to an MTA or, more commonly 667 in recent years, a specialized variation on an MTA called a 668 "Submission Server" (MSA) [41]. . At the other end of the process, 669 the final ("delivery") MTA would be thought of as handing the mail 670 off to an MUA (or at least transferring responsibility to it, e.g., 671 by depositing the message in a "message store"). However, while 672 these terms are used with at least the appearance of great precision 673 in other environments, the implied boundaries between MUAs and MTAs 674 often do not accurately match common, and conforming, practices with 675 Internet mail. Hence, the reader should be cautious about inferring 676 the strong relationships and responsibilities that might be implied 677 if these terms were used elsewhere 679 2.3.4. Host 681 For the purposes of this specification, a host is a computer system 682 attached to the Internet (or, in some cases, to a private TCP/IP 683 network) and supporting the SMTP protocol. Hosts are known by names 684 (see the next section); they SHOULD NOT be identified by numerical 685 addresses, i.e., by address literals as described in Section 4.1.2. 687 2.3.5. Domain Names 689 A domain name (or often just a "domain") consists of one or more 690 components, separated by dots if more than one appears. In the case 691 of a top-level domain used by itself in an email address, a single 692 string is used without any dots. This makes the requirement, 693 described in more detail below, that only fully-qualified domain 694 names appear in SMTP transactions on the public Internet, 695 particularly important where top-level domains are involved. These 696 components ("labels" in the DNS terminology of RFC 1035 [4]) are 697 restricted for purposes of SMTP as defined here to consist of a 698 sequence of letters, digits, and hyphens drawn from the ASCII 699 character set [2] and conforming to what RFC 1035 Section 2.3.1 calls 700 the "preferred name syntax". Domain names are used as names of hosts 701 and, except where additionally restricted in this document, of other 702 entities in the domain name hierarchy. For example, a domain may 703 refer to a host alias (label of a CNAME RR) or the label of Mail 704 eXchanger records to be used to deliver mail instead of representing 705 a host name. See RFC 1035 and Section 5 of this specification. 707 The domain name, as described in this document and in RFC 1035 [4], 708 MUST be the entire, fully-qualified name (often referred to as an 709 "FQDN"). Other than an address literal (see Section 4.1.3) where 710 those are permitted, any string that is not a domain name in FQDN 711 form is no more than a reference to be interpreted locally. Such 712 local references for domain names MUST NOT appear in any SMTP 713 transaction (Cf. Section 5). Mechanisms for inferring FQDNs from 714 local references (including partial names or local aliases) are 715 outside of this specification and normally the province of message 716 submission. Due to a history of problems, SMTP servers SHOULD NOT 717 make such inferences (Message Submission Servers [41] have somewhat 718 more flexibility) and intermediate (relay) SMTP servers MUST NOT make 719 them. 721 When domain names are used in SMTP, and unless further restricted in 722 this document, names that can be resolved to MX RRs or address (i.e., 723 A or AAAA) RRs (as discussed in Section 5) are permitted, as are 724 CNAME RRs whose targets can be resolved, in turn, to MX or address 725 RRs. There are two exceptions to the rule requiring FQDNs: 727 * The domain name given in the EHLO command MUST be either a primary 728 host name (a domain name that resolves to an address RR) or, if 729 the host has no name, an address literal, as described in 730 Section 4.1.3 and discussed further in the EHLO discussion of 731 Section 4.1.4. 733 * The reserved mailbox name "postmaster" MAY be used in a RCPT 734 command without domain qualification (see Section 4.1.1.3) and 735 MUST be accepted if so used. 737 2.3.6. Buffer and State Table 739 SMTP sessions are stateful, with both parties carefully maintaining a 740 common view of the current state. In this document, we model this 741 state by a virtual "buffer" and a "state table" on the server that 742 may be used by the client to, for example, "clear the buffer" or 743 "reset the state table", causing the information in the buffer to be 744 discarded and the state to be returned to some previous state. 746 2.3.7. Commands and Replies 748 SMTP commands and, unless altered by a service extension, message 749 data, are transmitted from the sender to the receiver via the 750 transmission channel in "lines". 752 An SMTP reply is an acknowledgment (positive or negative) sent in 753 "lines" from receiver to sender via the transmission channel in 754 response to a command. The general form of a reply is a numeric 755 completion code (indicating failure or success) usually followed by a 756 text string. The codes are for use by programs and the text is 757 usually intended for human users. RFC 3463 [7], specifies further 758 structuring of the reply strings, including the use of supplemental 759 and more specific completion codes (see also RFC 5248 [43]). 761 2.3.8. Lines 763 Lines consist of zero or more data characters terminated by the 764 sequence ASCII character "CR" (hex value 0D) followed immediately by 765 ASCII character "LF" (hex value 0A). This termination sequence is 766 denoted as in this document. Conforming implementations MUST 767 NOT recognize or generate any other character or character sequence 768 as a line terminator. Limits MAY be imposed on line lengths by 769 servers (see Section 4). 771 In addition, the appearance of "bare" "CR" or "LF" characters in text 772 (i.e., either without the other) has a long history of causing 773 problems in mail implementations and applications that use the mail 774 system as a tool. SMTP client implementations MUST NOT transmit 775 these characters except when they are intended as line terminators 776 and then MUST, as indicated above, transmit them only as a 777 sequence. 779 2.3.9. Message Content and Mail Data 781 The terms "message content" and "mail data" are used interchangeably 782 in this document to describe the material transmitted after the DATA 783 command is accepted and before the end of data indication is 784 transmitted. Message content includes the message header section and 785 the possibly structured message body. In the absence of extensions, 786 both are required to be ASCII (see Section 2.3.1). The MIME 787 specification (RFC 2045 [25]) provides the standard mechanisms for 788 structured message bodies. 790 2.3.10. Originator, Delivery, Relay, and Gateway Systems 792 This specification makes a distinction among four types of SMTP 793 systems, based on the role those systems play in transmitting 794 electronic mail. An "originating" system (sometimes called an SMTP 795 originator) introduces mail into the Internet or, more generally, 796 into a transport service environment. A "delivery" SMTP system is 797 one that receives mail from a transport service environment and 798 passes it to a mail user agent or deposits it in a message store that 799 a mail user agent is expected to subsequently access. A "relay" SMTP 800 system (usually referred to just as a "relay") receives mail from an 801 SMTP client and transmits it, without modification to the message 802 data other than adding trace information (see Section 4.4), to 803 another SMTP server for further relaying or for delivery. 805 A "gateway" SMTP system (usually referred to just as a "gateway") 806 receives mail from a client system in one transport environment and 807 transmits it to a server system in another transport environment. 808 Differences in protocols or message semantics between the transport 809 environments on either side of a gateway may require that the gateway 810 system perform transformations to the message that are not permitted 811 to SMTP relay systems. For the purposes of this specification, 812 firewalls that rewrite addresses should be considered as gateways, 813 even if SMTP is used on both sides of them (see RFC 2979 [32]). 815 2.3.11. Mailbox and Address 817 As used in this specification, an "address" is a character string 818 that identifies a user to whom mail will be sent or a location into 819 which mail will be deposited. The term "mailbox" refers to that 820 depository. The two terms are typically used interchangeably unless 821 the distinction between the location in which mail is placed (the 822 mailbox) and a reference to it (the address) is important. An 823 address normally consists of user and domain specifications. The 824 standard mailbox naming convention is defined to be "local- 825 part@domain"; contemporary usage permits a much broader set of 826 applications than simple "user names". Consequently, and due to a 827 long history of problems when intermediate hosts have attempted to 828 optimize transport by modifying them, the local-part MUST be 829 interpreted and assigned semantics only by the host specified in the 830 domain part of the address. 832 2.4. General Syntax Principles and Transaction Model 834 SMTP commands and replies have a rigid syntax. All commands begin 835 with a command verb. All replies begin with a three digit numeric 836 code. In some commands and replies, arguments are required following 837 the verb or reply code. Some commands do not accept arguments (after 838 the verb), and some reply codes are followed, sometimes optionally, 839 by free form text. In both cases, where text appears, it is 840 separated from the verb or reply code by a space character. Complete 841 definitions of commands and replies appear in Section 4. 843 Verbs and argument values (e.g., "TO:" or "to:" in the RCPT command 844 and extension name keywords) are not case sensitive, with the sole 845 exception in this specification of a mailbox local-part (SMTP 846 Extensions may explicitly specify case-sensitive elements). That is, 847 a command verb, an argument value other than a mailbox local-part, 848 and free form text MAY be encoded in upper case, lower case, or any 849 mixture of upper and lower case with no impact on its meaning. The 850 local-part of a mailbox MUST BE treated as case sensitive. 852 Therefore, SMTP implementations MUST take care to preserve the case 853 of mailbox local-parts. In particular, for some hosts, the user 854 "smith" is different from the user "Smith". However, exploiting the 855 case sensitivity of mailbox local-parts impedes interoperability and 856 is discouraged. Mailbox domains follow normal DNS rules and are 857 hence not case sensitive. 859 A few SMTP servers, in violation of this specification (and RFC 821) 860 require that command verbs be encoded by clients in upper case. 861 Implementations MAY wish to employ this encoding to accommodate those 862 servers. 864 The argument clause consists of a variable-length character string 865 ending with the end of the line, i.e., with the character sequence 866 . The receiver will take no action until this sequence is 867 received. 869 The syntax for each command is shown with the discussion of that 870 command. Common elements and parameters are shown in Section 4.1.2. 872 Commands and replies are composed of characters from the ASCII 873 character set [2]. When the transport service provides an 8-bit byte 874 (octet) transmission channel, each 7-bit character is transmitted, 875 right justified, in an octet with the high-order bit cleared to zero. 876 More specifically, the unextended SMTP service provides 7-bit 877 transport only. An originating SMTP client that has not successfully 878 negotiated an appropriate extension with a particular server (see the 879 next paragraph) MUST NOT transmit messages with information in the 880 high-order bit of octets. If such messages are transmitted in 881 violation of this rule, receiving SMTP servers MAY clear the high- 882 order bit or reject the message as invalid. In general, a relay SMTP 883 SHOULD assume that the message content it has received is valid and, 884 assuming that the envelope permits doing so, relay it without 885 inspecting that content. Of course, if the content is mislabeled and 886 the data path cannot accept the actual content, this may result in 887 the ultimate delivery of a severely garbled message to the recipient. 888 Delivery SMTP systems MAY reject such messages, or return them as 889 undeliverable, rather than deliver them. In the absence of a server- 890 offered extension explicitly permitting it, a sending SMTP system is 891 not permitted to send envelope commands in any character set other 892 than US-ASCII. Receiving systems SHOULD reject such commands, 893 normally using "500 syntax error - invalid character" replies. 895 8-bit message content transmission MAY be requested of the server by 896 a client using extended SMTP facilities, notably the "8BITMIME" 897 extension, RFC 6152 [44]. 8BITMIME SHOULD be supported by SMTP 898 servers. However, it MUST NOT be construed as authorization to 899 transmit unrestricted 8-bit material, nor does 8BITMIME authorize 900 transmission of any envelope material in other than ASCII. 8BITMIME 901 MUST NOT be requested by senders for material with the high bit on 902 that is not in MIME format with an appropriate content-transfer 903 encoding; servers MAY reject such messages. 905 The metalinguistic notation used in this document corresponds to the 906 "Augmented BNF" used in other Internet mail system documents. The 907 reader who is not familiar with that syntax should consult the ABNF 908 specification in RFC 5234 [11]. Metalanguage terms used in running 909 text are surrounded by pointed brackets (e.g., ) for clarity. 910 The reader is cautioned that the grammar expressed in the 911 metalanguage is not comprehensive. There are many instances in which 912 provisions in the text constrain or otherwise modify the syntax or 913 semantics implied by the grammar. 915 3. The SMTP Procedures: An Overview 917 This section contains descriptions of the procedures used in SMTP: 918 session initiation, mail transaction, forwarding mail, verifying 919 mailbox names and expanding mailing lists, and opening and closing 920 exchanges. Comments on relaying, a note on mail domains, and a 921 discussion of changing roles are included at the end of this section. 922 Several complete scenarios are presented in Appendix D. 924 3.1. Session Initiation 926 An SMTP session is initiated when a client opens a connection to a 927 server and the server responds with an opening message. 929 SMTP server implementations MAY include identification of their 930 software and version information in the connection greeting reply 931 after the 220 code, a practice that permits more efficient isolation 932 and repair of any problems. Implementations MAY make provision for 933 SMTP servers to disable the software and version announcement where 934 it causes security concerns. While some systems also identify their 935 contact point for mail problems, this is not a substitute for 936 maintaining the required "postmaster" address (see Section 4). 938 The SMTP protocol allows a server to formally reject a mail session 939 while still allowing the initial connection as follows: a 521 940 response MAY be given in the initial connection opening message 941 instead of the 220. A server taking this approach MUST still wait 942 for the client to send a QUIT (see Section 4.1.1.10) before closing 943 the connection and SHOULD respond to any intervening commands with 944 "503 bad sequence of commands". Since an attempt to make an SMTP 945 connection to such a system is probably in error, a server returning 946 a 521 947 // (or 554?) 948 response on connection opening SHOULD provide enough information in 949 the reply text to facilitate debugging of the sending system. See 950 Section 4.2.4.2. 952 3.2. Client Initiation 954 Once the server has sent the greeting (welcoming) message and the 955 client has received it, the client normally sends the EHLO command to 956 the server, indicating the client's identity. In addition to opening 957 the session, use of EHLO indicates that the client is able to process 958 service extensions and requests that the server provide a list of the 959 extensions it supports. Older SMTP systems that are unable to 960 support service extensions, and contemporary clients that do not 961 require service extensions in the mail session being initiated, MAY 962 use HELO instead of EHLO. Servers MUST NOT return the extended EHLO- 963 style response to a HELO command. For a particular connection 964 attempt, if the server returns a "command not recognized" response to 965 EHLO, the client SHOULD be able to fall back and send HELO. 967 In the EHLO command, the host sending the command identifies itself; 968 the command may be interpreted as saying "Hello, I am " (and, 969 in the case of EHLO, "and I support service extension requests"). 971 3.3. Mail Transactions 973 There are three steps to SMTP mail transactions. The transaction 974 starts with a MAIL command that gives the sender identification. (In 975 general, the MAIL command may be sent only when no mail transaction 976 is in progress; see Section 4.1.4.) A series of one or more RCPT 977 commands follows, giving the receiver information. Then, a DATA 978 command initiates transfer of the mail data and is terminated by the 979 "end of mail" data indicator, which also confirms (and terminates) 980 the transaction. 982 Mail transactions are also terminated by the RSET command 983 (Section 4.1.1.5), the sending of an EHLO command (Section 3.2), or 984 the sending of a QUIT command (Section 3.8) which terminates both any 985 active mail transaction and the SMTP connection. 987 The first step in the procedure is the MAIL command. 989 MAIL FROM: [SP ] 991 This command tells the SMTP-receiver that a new mail transaction is 992 starting and to reset all its state tables and buffers, including any 993 recipients or mail data. The portion of the first or 994 only argument contains the source mailbox (between "<" and ">" 995 brackets), which can be used to report errors (see Section 4.2 for a 996 discussion of error reporting). If accepted, the SMTP server returns 997 a "250 OK" reply. If the mailbox specification is not acceptable for 998 some reason, the server MUST return a reply indicating whether the 999 failure is permanent (i.e., will occur again if the client tries to 1000 send the same address again) or temporary (i.e., the address might be 1001 accepted if the client tries again later). Despite the apparent 1002 scope of this requirement, there are circumstances in which the 1003 acceptability of the reverse-path may not be determined until one or 1004 more forward-paths (in RCPT commands) can be examined. In those 1005 cases, the server MAY reasonably accept the reverse-path (with a 250 1006 reply) and then report problems after the forward-paths are received 1007 and examined. Normally, failures produce 550 or 553 replies. 1009 Historically, the was permitted to contain more than 1010 just a mailbox; however source routing is now deprecated (see 1011 Appendix F.2). 1013 The optional are associated with negotiated SMTP 1014 service extensions (see Section 2.2). 1016 The second step in the procedure is the RCPT command. This step of 1017 the procedure can be repeated any number of times. 1019 RCPT TO: [ SP ] 1021 The first or only argument to this command includes a forward-path 1022 (normally a mailbox local-part and domain, always surrounded by "<" 1023 and ">" brackets) identifying one recipient. If accepted, the SMTP 1024 server returns a "250 OK" reply and stores the forward-path. If the 1025 recipient is known not to be a deliverable address, the SMTP server 1026 returns a 550 reply, typically with a string such as "no such user - 1027 " and the mailbox name (other circumstances and reply codes are 1028 possible). 1030 Historically, the was permitted to contain a source 1031 routing list of hosts and the destination mailbox; however, source 1032 routes are now deprecated (see Appendix F.2). Restricted-capability 1033 clients MUST NOT assume that any SMTP server on the Internet can be 1034 used as their mail processing (relaying) site. If a RCPT command 1035 appears without a previous MAIL command, the server MUST return a 503 1036 "Bad sequence of commands" response. The optional 1037 are associated with negotiated SMTP service extensions (see 1038 Section 2.2). 1039 // [5321bis]: this section would be improved by being more specific 1040 // about where mail transactions begin and end and then talking about 1041 // "transaction state" here, rather than specific prior commands. 1042 // --JcK 1043 Since it has been a common source of errors, it is worth noting that 1044 spaces are not permitted on either side of the colon following FROM 1045 in the MAIL command or TO in the RCPT command. The syntax is exactly 1046 as given above. 1048 The third step in the procedure is the DATA command (or some 1049 alternative specified in a service extension). 1051 DATA 1053 If accepted, the SMTP server returns a 354 Intermediate reply and 1054 considers all succeeding lines up to but not including the end of 1055 mail data indicator to be the message text. When the end of text is 1056 successfully received and stored, the SMTP-receiver sends a "250 OK" 1057 reply. 1059 Since the mail data is sent on the transmission channel, the end of 1060 mail data must be indicated so that the command and reply dialog can 1061 be resumed. SMTP indicates the end of the mail data by sending a 1062 line containing only a "." (period or full stop, hex 2E). A 1063 transparency procedure is used to prevent this from interfering with 1064 the user's text (see Section 4.5.2). 1066 The end of mail data indicator also confirms the mail transaction and 1067 tells the SMTP server to now process the stored recipients and mail 1068 data. If accepted, the SMTP server returns a "250 OK" reply. The 1069 DATA command can fail at only two points in the protocol exchange: 1071 If there was no MAIL, or no RCPT, command, or all such commands were 1072 rejected, the server MAY return a "command out of sequence" (503) or 1073 "no valid recipients" (554) reply in response to the DATA command. 1074 If one of those replies (or any other 5yz reply) is received, the 1075 client MUST NOT send the message data; more generally, message data 1076 MUST NOT be sent unless a 354 reply is received. 1078 If the verb is initially accepted and the 354 reply issued, the DATA 1079 command should fail only if the mail transaction was incomplete (for 1080 example, no recipients), if resources were unavailable (including, of 1081 course, the server unexpectedly becoming unavailable), or if the 1082 server determines that the message should be rejected for policy or 1083 other reasons. 1085 However, in practice, some servers do not perform recipient 1086 verification until after the message text is received. These servers 1087 SHOULD treat a failure for one or more recipients as a "subsequent 1088 failure" and return a mail message as discussed in Section 6 and, in 1089 particular, in Section 6.1. Using a "550 mailbox not found" (or 1090 equivalent) reply code after the data are accepted makes it difficult 1091 or impossible for the client to determine which recipients failed. 1093 When the RFC 822 format ([13], [12]) is being used, the mail data 1094 include the header fields such as those named Date, Subject, To, Cc, 1095 and From. Server SMTP systems SHOULD NOT reject messages based on 1096 perceived defects in the RFC 822 or MIME (RFC 2045 [25]) message 1097 header section or message body. In particular, they MUST NOT reject 1098 messages in which the numbers of Resent-header fields do not match or 1099 Resent-to appears without Resent-from and/or Resent-date. 1101 Mail transaction commands MUST be used in the order discussed above. 1103 3.4. Address Modification and Expansion 1105 3.4.1. Forwarding for Address Correction or Updating 1107 Forwarding support is most often required to consolidate and simplify 1108 addresses within, or relative to, some enterprise and less frequently 1109 to establish addresses to link a person's prior address with a 1110 current one. Silent forwarding of messages (without server 1111 notification to the sender), for security or non-disclosure purposes, 1112 is common in the contemporary Internet. 1114 In both the enterprise and the "new address" cases, information 1115 hiding (and sometimes security) considerations argue against exposure 1116 of the "final" address through the SMTP protocol as a side effect of 1117 the forwarding activity. This may be especially important when the 1118 final address may not even be reachable by the sender. Consequently, 1119 the "forwarding" mechanisms described in Section 3.2 of RFC 821, and 1120 especially the 251 (corrected destination) and 551 reply codes from 1121 RCPT must be evaluated carefully by implementers and, when they are 1122 available, by those configuring systems (see also Section 7.4). 1124 In particular: 1126 * Servers MAY forward messages when they are aware of an address 1127 change. When they do so, they MAY either provide address-updating 1128 information with a 251 code, or may forward "silently" and return 1129 a 250 code. However, if a 251 code is used, they MUST NOT assume 1130 that the client will actually update address information or even 1131 return that information to the user. 1133 Alternately, 1135 * Servers MAY reject messages or return them as non-deliverable when 1136 they cannot be delivered precisely as addressed. When they do so, 1137 they MAY either provide address-updating information with a 551 1138 code, or may reject the message as undeliverable with a 550 code 1139 and no address-specific information. However, if a 551 code is 1140 used, they MUST NOT assume that the client will actually update 1141 address information or even return that information to the user. 1143 SMTP server implementations that support the 251 and/or 551 reply 1144 codes SHOULD provide configuration mechanisms so that sites that 1145 conclude that they would undesirably disclose information can disable 1146 or restrict their use. See Section 7.4 for further discussion of 1147 that issue. 1149 3.4.2. Aliases and Mailing Lists 1151 Many SMTP-capable hosts support address expansion for multiple 1152 delivery via one or both of the alias and the list models. When a 1153 message is delivered or forwarded to each address of an expanded list 1154 form, the return address in the envelope ("MAIL FROM:") MUST be 1155 changed to be the address of a person or other entity who administers 1156 the list. However, in this case, the message header section (RFC 1157 5322 [12]) MUST be left unchanged; in particular, the "From" field of 1158 the header section is unaffected. 1160 // Editor's Note (temporary, 2022-01-21): Discussion durng the 1161 // Interim Meeting this date indicated that the above sentence should 1162 // be removed. See mailing list. 1164 An important mail facility is a mechanism for multi-destination 1165 delivery of a single message, by transforming (or "expanding" or 1166 "exploding") a pseudo-mailbox address into a list of destination 1167 mailbox addresses. When a message is sent to such a pseudo-mailbox 1168 (sometimes called an "exploder"), copies are forwarded or 1169 redistributed to each mailbox in the expanded list. Servers SHOULD 1170 simply utilize the addresses on the list; application of heuristics 1171 or other matching rules to eliminate some addresses, such as that of 1172 the originator, is strongly discouraged. We classify such a pseudo- 1173 mailbox as an "alias" or a "list", depending upon the expansion 1174 rules. 1176 3.4.2.1. Simple Aliases 1178 To expand an alias, the recipient mailer simply replaces the pseudo- 1179 mailbox address in the envelope with each of the expanded addresses 1180 in turn; the rest of the envelope and the message body are left 1181 unchanged. The message is then delivered or forwarded to each 1182 expanded address. 1184 3.4.2.2. Mailing Lists 1186 Processing of a mailing list may be said to operate by 1187 "redistribution" rather than by "forwarding" (as in the simple alias 1188 case in the subsection above). To expand a list, the recipient 1189 mailer replaces the pseudo-mailbox address in the envelope with each 1190 of the expanded addresses in turn. The return (backward-pointing) 1191 address in the envelope is changed so that all error messages 1192 generated by the final deliveries will be returned to a list 1193 administrator, not to the message originator, who generally has no 1194 control over the contents of the list and will typically find error 1195 messages annoying. Note that the key difference between handling 1196 simple aliases Section 3.4.2.1 and redistribution (this subsection) 1197 is the change to the backward-pointing address. When a system 1198 managing a list constrains its processing to the very limited set of 1199 modifications and actions described here, it is acting as part of an 1200 MTA; such list processing, like alias processing, can be treated as a 1201 continuation of email transit. 1203 Mailing list management systems do exist that perform additional, 1204 sometimes extensive, modifications to a message and its envelope. 1205 Such mailing lists need to be viewed as MUAs that accept a message 1206 delivery and then submit a new message for multiple recipients. 1208 3.5. Commands for Debugging Addresses 1210 3.5.1. Overview 1212 SMTP provides commands to verify a user name or obtain the content of 1213 a mailing list. This is done with the VRFY and EXPN commands, which 1214 have character string arguments. Implementations SHOULD support VRFY 1215 and EXPN (however, see Section 3.5.2 and Section 7.3). 1217 For the VRFY command, the string is a user name or a user name and 1218 domain (see below). If a normal (i.e., 250) response is returned, 1219 the response MAY include the full name of the user and MUST include 1220 the mailbox of the user. It MUST be in either of the following 1221 forms: 1223 User Name 1224 local-part@domain 1226 When a name that is the argument to VRFY could identify more than one 1227 mailbox, the server MAY either note the ambiguity or identify the 1228 alternatives. In other words, any of the following are legitimate 1229 responses to VRFY: 1231 553 User ambiguous 1233 or 1235 553- Ambiguous; Possibilities are 1236 553-Joe Smith 1237 553-Harry Smith 1238 553 Melvin Smith 1240 or 1242 553-Ambiguous; Possibilities 1243 553- 1244 553- 1245 553 1247 Under normal circumstances, a client receiving a 553 reply would be 1248 expected to expose the result to the user. Use of exactly the forms 1249 given, and the "user ambiguous" or "ambiguous" keywords, possibly 1250 supplemented by extended reply codes, such as those described in RFC 1251 3463 [7], will facilitate automated translation into other languages 1252 as needed. Of course, a client that was highly automated or that was 1253 operating in another language than English might choose to try to 1254 translate the response to return some other indication to the user 1255 than the literal text of the reply, or to take some automated action 1256 such as consulting a directory service for additional information 1257 before reporting to the user. 1259 For the EXPN command, the string identifies a mailing list, and the 1260 successful (i.e., 250) multiline response MAY include the full name 1261 of the users and MUST give the mailboxes on the mailing list. 1263 In some hosts, the distinction between a mailing list and an alias 1264 for a single mailbox is a bit fuzzy, since a common data structure 1265 may hold both types of entries, and it is possible to have mailing 1266 lists containing only one mailbox. If a request is made to apply 1267 VRFY to a mailing list, a positive response MAY be given if a message 1268 so addressed would be delivered to everyone on the list, otherwise an 1269 error SHOULD be reported (e.g., "550 That is a mailing list, not a 1270 user" or "252 Unable to verify members of mailing list"). If a 1271 request is made to expand a user name, the server MAY return a 1272 positive response consisting of a list containing one name, or an 1273 error MAY be reported (e.g., "550 That is a user name, not a mailing 1274 list"). 1276 In the case of a successful multiline reply (normal for EXPN), 1277 exactly one mailbox is to be specified on each line of the reply. 1278 The case of an ambiguous request is discussed above. 1280 "User name" is a fuzzy term and has been used deliberately. An 1281 implementation of the VRFY or EXPN commands MUST include at least 1282 recognition of local mailboxes as "user names". However, since 1283 current Internet practice often results in a single host handling 1284 mail for multiple domains, hosts, especially hosts that provide this 1285 functionality, SHOULD accept the "local-part@domain" form as a "user 1286 name"; hosts MAY also choose to recognize other strings as "user 1287 names". 1289 The case of expanding a mailbox list requires a multiline reply, such 1290 as: 1292 C: EXPN Example-People 1293 S: 250-Jon Postel 1294 S: 250-Fred Fonebone 1295 S: 250 Sam Q. Smith 1297 or 1299 C: EXPN Executive-Washroom-List 1300 S: 550 Access Denied to You. 1302 The character string arguments of the VRFY and EXPN commands cannot 1303 be further restricted due to the variety of implementations of the 1304 user name and mailbox list concepts. On some systems, it may be 1305 appropriate for the argument of the EXPN command to be a file name 1306 for a file containing a mailing list, but again there are a variety 1307 of file naming conventions in the Internet. Similarly, historical 1308 variations in what is returned by these commands are such that the 1309 response SHOULD be interpreted very carefully, if at all, and SHOULD 1310 generally only be used for diagnostic purposes. 1312 3.5.2. VRFY Normal Response 1314 When normal (2yz or 551) responses are returned from a VRFY or EXPN 1315 request, the reply MUST include the name using a "" construction, where "domain" is a fully-qualified 1317 domain name. In circumstances exceptional enough to justify 1318 violating the intent of this specification, free-form text MAY be 1319 returned. In order to facilitate parsing by both computers and 1320 people, addresses SHOULD appear in pointed brackets. When addresses, 1321 rather than free-form debugging information, are returned, EXPN and 1322 VRFY MUST return only valid domain addresses that are usable in SMTP 1323 RCPT commands. Consequently, if an address implies delivery to a 1324 program or other system, the mailbox name used to reach that target 1325 MUST be given. Paths (explicit source routes) MUST NOT be returned 1326 by VRFY or EXPN. 1328 Server implementations SHOULD support both VRFY and EXPN. For 1329 security reasons, implementations MAY provide local installations a 1330 way to disable either or both of these commands through configuration 1331 options or the equivalent (see Section 7.3). When these commands are 1332 supported, they are not required to work across relays when relaying 1333 is supported. Since they were both optional in RFC 821, but VRFY was 1334 made mandatory in RFC 1123 [5], if EXPN is supported, it MUST be 1335 listed as a service extension in an EHLO response. VRFY MAY be 1336 listed as a convenience but, since support for it is required, SMTP 1337 clients are not required to check for its presence on the extension 1338 list before using it. 1340 3.5.3. Meaning of VRFY or EXPN Success Response 1342 A server MUST NOT return a 250 code in response to a VRFY or EXPN 1343 command unless it has actually verified the address. In particular, 1344 a server MUST NOT return 250 if all it has done is to verify that the 1345 syntax given is valid. In that case, 502 (Command not implemented) 1346 or 500 (Syntax error, command unrecognized) SHOULD be returned. As 1347 stated elsewhere, implementation (in the sense of actually validating 1348 addresses and returning information) of VRFY and EXPN are strongly 1349 recommended. Hence, implementations that return 500 or 502 for VRFY 1350 are not in full compliance with this specification. 1352 There may be circumstances where an address appears to be valid but 1353 cannot reasonably be verified in real time, particularly when a 1354 server is acting as a mail exchanger for another server or domain. 1355 "Apparent validity", in this case, would normally involve at least 1356 syntax checking and might involve verification that any domains 1357 specified were ones to which the host expected to be able to relay 1358 mail. In these situations, reply code 252 SHOULD be returned. These 1359 cases parallel the discussion of RCPT verification in Section 2.1. 1361 Similarly, the discussion in Section 3.4.1 applies to the use of 1362 reply codes 251 and 551 with VRFY (and EXPN) to indicate addresses 1363 that are recognized but that would be forwarded or rejected were mail 1364 received for them. Implementations generally SHOULD be more 1365 aggressive about address verification in the case of VRFY than in the 1366 case of RCPT, even if it takes a little longer to do so. 1368 3.5.4. Semantics and Applications of EXPN 1370 EXPN is often very useful in debugging and understanding problems 1371 with mailing lists and multiple-target-address aliases. Some systems 1372 have attempted to use source expansion of mailing lists as a means of 1373 eliminating duplicates. The propagation of aliasing systems with 1374 mail on the Internet for hosts (typically with MX and CNAME DNS 1375 records), for mailboxes (various types of local host aliases), and in 1376 various proxying arrangements has made it nearly impossible for these 1377 strategies to work consistently, and mail systems SHOULD NOT attempt 1378 them. 1380 3.6. Relaying and Mail Routing 1382 3.6.1. Mail eXchange Records and Relaying 1384 A relay SMTP server is usually the target of a DNS MX record that 1385 designates it, rather than the final delivery system. The relay 1386 server may accept or reject the task of relaying the mail in the same 1387 way it accepts or rejects mail for a local user. If it accepts the 1388 task, it then becomes an SMTP client, establishes a transmission 1389 channel to the next SMTP server specified in the DNS (according to 1390 the rules in Section 5), and sends it the mail. If it declines to 1391 relay mail to a particular address for policy reasons, a 550 response 1392 SHOULD be returned. 1394 This specification does not deal with the verification of return 1395 paths. Server efforts to verify a return path and actions to be 1396 taken under various circumstances are outside the scope of this 1397 specification. 1399 3.6.2. Message Submission Servers as Relays 1401 Many mail-sending clients exist, especially in conjunction with 1402 facilities that receive mail via POP3 or IMAP, that have limited 1403 capability to support some of the requirements of this specification, 1404 such as the ability to queue messages for subsequent delivery 1405 attempts. For these clients, it is common practice to make private 1406 arrangements to send all messages to a single server for processing 1407 and subsequent distribution. SMTP, as specified here, is not ideally 1408 suited for this role. A standardized mail submission protocol has 1409 been developed that is gradually superseding practices based on SMTP 1410 (see RFC 6409 [41]). In any event, because these arrangements are 1411 private and fall outside the scope of this specification, they are 1412 not described here. 1414 It is important to note that MX records can point to SMTP servers 1415 that act as gateways into other environments, not just SMTP relays 1416 and final delivery systems; see Sections 3.7 and 5. 1418 If an SMTP server has accepted the task of relaying the mail and 1419 later finds that the destination is incorrect or that the mail cannot 1420 be delivered for some other reason, then it MUST construct an 1421 "undeliverable mail" notification message and send it to the 1422 originator of the undeliverable mail (as indicated by the reverse- 1423 path). Formats specified for non-delivery reports by other standards 1424 (see, for example, RFC 3461 [34] and RFC 3464 [35]) SHOULD be used if 1425 possible. 1427 This notification message must be from the SMTP server at the relay 1428 host or the host that first determines that delivery cannot be 1429 accomplished. Of course, SMTP servers MUST NOT send notification 1430 messages about problems transporting notification messages. One way 1431 to prevent loops in error reporting is to specify a null reverse-path 1432 in the MAIL command of a notification message. When such a message 1433 is transmitted, the reverse-path MUST be set to null (see 1434 Section 4.5.5 for additional discussion). A MAIL command with a null 1435 reverse-path appears as follows: 1437 MAIL FROM:<> 1439 As discussed in Section 6.4, a relay SMTP has no need to inspect or 1440 act upon the header section or body of the message data and MUST NOT 1441 do so except to add its own "Received:" header field (Section 4.4.1 1442 and possibly other trace header fields) and, optionally, to attempt 1443 to detect looping in the mail system (see Section 6.3). Of course, 1444 this prohibition also applies to any modifications of these header 1445 fields or text (see also Section 7.9). 1447 3.7. Mail Gatewaying 1449 While the relay function discussed above operates within the Internet 1450 SMTP transport service environment, MX records or various forms of 1451 explicit routing may require that an intermediate SMTP server perform 1452 a translation function between one transport service and another. As 1453 discussed in Section 2.3.10, when such a system is at the boundary 1454 between two transport service environments, we refer to it as a 1455 "gateway" or "gateway SMTP". 1457 Gatewaying mail between different mail environments, such as 1458 different mail formats and protocols, is complex and does not easily 1459 yield to standardization. However, some general requirements may be 1460 given for a gateway between the Internet and another mail 1461 environment. 1463 3.7.1. Header Fields in Gatewaying 1465 Header fields MAY be rewritten when necessary as messages are 1466 gatewayed across mail environment boundaries. This may involve 1467 inspecting the message body or interpreting the local-part of the 1468 destination address in spite of the prohibitions in Section 6.4. 1470 Other mail systems gatewayed to the Internet often use a subset of 1471 the RFC 822 header section or provide similar functionality with a 1472 different syntax, but some of these mail systems do not have an 1473 equivalent to the SMTP envelope. Therefore, when a message leaves 1474 the Internet environment, it may be necessary to fold the SMTP 1475 envelope information into the message header section. A possible 1476 solution would be to create new header fields to carry the envelope 1477 information (e.g., "X-SMTP-MAIL:" and "X-SMTP-RCPT:"); however, this 1478 would require changes in mail programs in foreign environments and 1479 might risk disclosure of private information (see Section 7.2). 1481 3.7.2. Received Lines in Gatewaying 1483 When forwarding a message into or out of the Internet environment, a 1484 gateway MUST prepend a Received: line, but it MUST NOT alter in any 1485 way a Received: line that is already in the header section. 1487 "Received:" header fields of messages originating from other 1488 environments may not conform exactly to this specification. However, 1489 the most important use of Received: lines is for debugging mail 1490 faults, and this debugging can be severely hampered by well-meaning 1491 gateways that try to "fix" a Received: line. As another consequence 1492 of trace header fields arising in non-SMTP environments, receiving 1493 systems MUST NOT reject mail based on the format of a trace header 1494 field and SHOULD be extremely robust in the light of unexpected 1495 information or formats in those header fields. 1497 The gateway SHOULD indicate the environment and protocol in the "via" 1498 clauses of Received header field(s) that it supplies. 1500 3.7.3. Addresses in Gatewaying 1502 From the Internet side, the gateway SHOULD accept all valid address 1503 formats in SMTP commands and in the RFC 822 header section, and all 1504 valid RFC 822 messages. Addresses and header fields generated by 1505 gateways MUST conform to applicable standards (including this one and 1506 RFC 5322 [12]). Gateways are, of course, subject to the same rules 1507 for handling source routes as those described for other SMTP systems 1508 in Section 3.3. 1510 3.7.4. Other Header Fields in Gatewaying 1512 The gateway MUST ensure that all header fields of a message that it 1513 forwards into the Internet mail environment meet the requirements for 1514 Internet mail. In particular, all addresses in "From:", "To:", 1515 "Cc:", etc., header fields MUST be transformed (if necessary) to 1516 satisfy the standard header syntax of RFC 5322 [12], MUST reference 1517 only fully-qualified domain names, and MUST be effective and useful 1518 for sending replies. The translation algorithm used to convert mail 1519 from the Internet protocols to another environment's protocol SHOULD 1520 ensure that error messages from the foreign mail environment are 1521 delivered to the reverse-path from the SMTP envelope, not to an 1522 address in the "From:", "Sender:", or similar header fields of the 1523 message. 1525 3.7.5. Envelopes in Gatewaying 1527 Similarly, when forwarding a message from another environment into 1528 the Internet, the gateway SHOULD set the envelope return path in 1529 accordance with an error message return address, if supplied by the 1530 foreign environment. If the foreign environment has no equivalent 1531 concept, the gateway must select and use a best approximation, with 1532 the message originator's address as the default of last resort. 1534 3.8. Terminating Sessions and Connections 1536 An SMTP connection is terminated when the client sends a QUIT 1537 command. The server responds with a positive reply code, after which 1538 it closes the connection. 1540 An SMTP server MUST NOT intentionally close the connection under 1541 normal operational circumstances (see Section 7.8) except: 1543 * After receiving a QUIT command and responding with a 221 reply. 1545 * After detecting the need to shut down the SMTP service and 1546 returning a 421 reply code. This reply code can be issued after 1547 the server receives any command or, if necessary, asynchronously 1548 from command receipt (on the assumption that the client will 1549 receive it after the next command is issued). 1551 * After a timeout, as specified in Section 4.5.3.2, occurs waiting 1552 for the client to send a command or data. 1554 In particular, a server that closes connections in response to 1555 commands that are not understood is in violation of this 1556 specification. Servers are expected to be tolerant of unknown 1557 commands, issuing a 500 reply and awaiting further instructions from 1558 the client. 1560 An SMTP server that is forcibly shut down via external means SHOULD 1561 attempt to send a line containing a 421 reply code to the SMTP client 1562 before exiting. The SMTP client will normally read the 421 reply 1563 code after sending its next command. 1565 SMTP clients that experience a connection close, reset, or other 1566 communications failure due to circumstances not under their control 1567 (in violation of the intent of this specification but sometimes 1568 unavoidable) SHOULD, to maintain the robustness of the mail system, 1569 treat the mail transaction as if a 421 response had been received and 1570 act accordingly. 1572 There are circumstances, contrary to the intent of this 1573 specification, in which an SMTP server may receive an indication that 1574 the underlying TCP connection has been closed or reset. To preserve 1575 the robustness of the mail system, SMTP servers SHOULD be prepared 1576 for this condition and SHOULD treat it as if a QUIT had been received 1577 before the connection disappeared. 1579 4. The SMTP Specifications 1581 4.1. SMTP Commands 1583 4.1.1. Command Semantics and Syntax 1585 The SMTP commands define the mail transfer or the mail system 1586 function requested by the user. SMTP commands are character strings 1587 terminated by . The commands themselves are alphabetic 1588 characters terminated by if parameters follow and 1589 otherwise. (In the interest of improved interoperability, SMTP 1590 receivers SHOULD tolerate trailing white space before the terminating 1591 .) The syntax of the local part of a mailbox MUST conform to 1592 receiver site conventions and the syntax specified in Section 4.1.2. 1594 The SMTP commands are discussed below. The SMTP replies are 1595 discussed in Section 4.2. 1597 A mail transaction involves several data objects that are 1598 communicated as arguments to different commands. The reverse-path is 1599 the argument of the MAIL command, the forward-path is the argument of 1600 the RCPT command, and the mail data is the argument of the DATA 1601 command. These arguments or data objects must be transmitted and 1602 held, pending the confirmation communicated by the end of mail data 1603 indication that finalizes the transaction. The model for this is 1604 that distinct buffers are provided to hold the types of data objects; 1605 that is, there is a reverse-path buffer, a forward-path buffer, and a 1606 mail data buffer. Specific commands cause information to be appended 1607 to a specific buffer, or cause one or more buffers to be cleared. 1609 Several commands (RSET, DATA, QUIT) are specified as not permitting 1610 parameters. In the absence of specific extensions offered by the 1611 server and accepted by the client, clients MUST NOT send such 1612 parameters and servers SHOULD reject commands containing them as 1613 having invalid syntax. 1615 4.1.1.1. Extended HELLO (EHLO) or HELLO (HELO) 1617 These commands are used to identify the SMTP client to the SMTP 1618 server. The argument clause contains the fully-qualified domain name 1619 of the SMTP client, if one is available. In situations in which the 1620 SMTP client system does not have a meaningful domain name (e.g., when 1621 its address is dynamically allocated and no reverse mapping record is 1622 available), the client SHOULD send an address literal (see 1623 Section 4.1.3). Additional discussion of domain names in SMTP 1624 commands appears in Section 2.3.5. 1626 RFC 2821, and some earlier informal practices, encouraged following 1627 the literal by information that would help to identify the client 1628 system. That convention was not widely supported, and many SMTP 1629 servers considered it an error. In the interest of interoperability, 1630 it is probably wise for servers to be prepared for this string to 1631 occur, but SMTP clients SHOULD NOT send it. 1633 The SMTP server identifies itself to the SMTP client in the 1634 connection greeting reply and in the response to this command. 1636 A client SMTP SHOULD start an SMTP session by issuing the EHLO 1637 command. If the SMTP server supports the SMTP service extensions, it 1638 will give a successful response, a failure response, or an error 1639 response. If the SMTP server, in violation of this specification, 1640 does not support any SMTP service extensions, it will generate an 1641 error response. Older client SMTP systems MAY, as discussed above, 1642 use HELO (as specified in RFC 821) instead of EHLO, and servers MUST 1643 support the HELO command and reply properly to it. In any event, a 1644 client MUST issue HELO or EHLO before starting a mail transaction. 1646 These commands, and a "250 OK" reply to one of them, confirm that 1647 both the SMTP client and the SMTP server are in the initial state, 1648 that is, there is no transaction in progress and all state tables and 1649 buffers are cleared. 1651 Syntax: 1653 ehlo = "EHLO" SP ( Domain / address-literal ) CRLF 1655 helo = "HELO" SP Domain CRLF 1657 Normally, the response to EHLO will be a multiline reply. Each line 1658 of the response contains a keyword and, optionally, one or more 1659 parameters. Following the normal syntax for multiline replies, these 1660 keywords follow the code (250) and a hyphen for all but the last 1661 line, and the code and a space for the last line. The syntax for a 1662 positive response, using the ABNF notation and terminal symbols of 1663 RFC 5234 [11], is: 1665 ehlo-ok-rsp = ( "250" SP Domain [ SP ehlo-greet ] CRLF ) 1666 / ( "250-" Domain [ SP ehlo-greet ] CRLF 1667 *( "250-" ehlo-line CRLF ) 1668 "250" SP ehlo-line CRLF ) 1670 ehlo-greet = 1*(%d0-9 / %d11-12 / %d14-127) 1671 ; string of any characters other than CR or LF 1673 ehlo-line = ehlo-keyword *( SP ehlo-param ) 1675 ehlo-keyword = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-") 1676 ; additional syntax of ehlo-params depends on 1677 ; ehlo-keyword 1679 ehlo-param = 1*(%d33-126) 1680 ; any CHAR excluding and all 1681 ; control characters (US-ASCII 0-31 and 127 1682 ; inclusive) 1684 Although EHLO keywords may be specified in upper, lower, or mixed 1685 case, they MUST always be recognized and processed in a case- 1686 insensitive manner. This is simply an extension of practices 1687 specified in RFC 821 and Section 2.4. 1689 The EHLO response MUST contain keywords (and associated parameters if 1690 required) for all commands not listed as "required" in Section 4.5.1. 1692 4.1.1.2. MAIL (MAIL) 1694 This command is used to initiate a mail transaction in which the mail 1695 data is delivered to an SMTP server that may, in turn, deliver it to 1696 one or more mailboxes or pass it on to another system (possibly using 1697 SMTP). The argument clause contains a reverse-path and may contain 1698 optional parameters. In general, the MAIL command may be sent only 1699 when no mail transaction is in progress, see Section 4.1.4. 1701 The reverse-path consists of the sender mailbox. Historically, that 1702 mailbox might optionally have been preceded by a list of hosts, but 1703 that behavior is now deprecated (see Appendix F.2). In some types of 1704 reporting messages for which a reply is likely to cause a mail loop 1705 (for example, mail delivery and non-delivery notifications), the 1706 reverse-path may be null (see Section 3.6). 1708 This command clears the reverse-path buffer, the forward-path buffer, 1709 and the mail data buffer, and it inserts the reverse-path information 1710 from its argument clause into the reverse-path buffer. 1712 If service extensions were negotiated, the MAIL command may also 1713 carry parameters associated with a particular service extension. 1715 Syntax: 1717 mail = "MAIL FROM:" Reverse-path 1718 [SP Mail-parameters] CRLF 1720 4.1.1.3. RECIPIENT (RCPT) 1722 This command is used to identify an individual recipient of the mail 1723 data; multiple recipients are specified by multiple uses of this 1724 command. The argument clause contains a forward-path and may contain 1725 optional parameters. 1727 The forward-path consists of the required destination mailbox. When 1728 mail reaches its ultimate destination, the SMTP server inserts it 1729 into the destination mailbox in accordance with its host mail 1730 conventions. 1732 Prior versions of the SMTP specification included text and examples 1733 in this section of use of the deprecated source route construct. If 1734 desired, see Appendix F.2 for discussion of that mechanism. 1736 This command appends its forward-path argument to the forward-path 1737 buffer; it does not change the reverse-path buffer nor the mail data 1738 buffer. 1740 For example, mail received at relay host xyz.com with envelope 1741 commands 1743 MAIL FROM: 1744 RCPT TO: 1746 will result in a DNS lookup for d.bar.org and transmission to the 1747 host specified in the most-preferred MX record that is available (or 1748 by the address record if there are no MX records). It will use 1749 envelope commands identical to the above, i.e., 1751 MAIL FROM: 1752 RCPT TO: 1754 Since hosts are not required to relay mail at all, xyz.com MAY also 1755 reject the message entirely when the RCPT command is received, using 1756 a 550 code (since this is a "policy reason"). 1758 If the SMTP server determines that a message sent to the mailbox in 1759 the forward-path is not deliverable, it MUST either return an 1760 appropriate response code (see Section 4.2.2) or generate a non- 1761 delivery notification. 1763 // Editor's Note: Following text moved from Section 4.4.1, per 2021 1764 // November discussion, and rewritten slightly (in -09). With the 1765 // current structure of the document, this seemed the least-bad place 1766 // to put it. Other plausible alternatives would be to put a "Non- 1767 // delivery Notifications" section into either Section 3 or 1768 // Section 6. 1770 If there were multiple failed recipients, either a single 1771 notification listing all of the failed recipients or separate 1772 notification messages MUST be sent for each failed recipient. For 1773 economy of processing by the sender, the former SHOULD be used when 1774 possible. All notification messages about undeliverable mail MUST be 1775 sent using the MAIL command and MUST use a null return path as 1776 discussed in Section 3.6. 1778 If service extensions were negotiated, the RCPT command may also 1779 carry parameters associated with a particular service extension 1780 offered by the server. The client MUST NOT transmit parameters other 1781 than those associated with a service extension offered by the server 1782 in its EHLO response. 1784 Syntax: 1786 rcpt = "RCPT TO:" ( "" / "" / 1787 Forward-path ) [SP Rcpt-parameters] CRLF 1788 Note that, in a departure from the usual rules for 1789 local-parts, the "Postmaster" string shown above is 1790 treated as case-insensitive. 1792 4.1.1.4. DATA (DATA) 1794 The receiver normally sends a 354 response to DATA, and then treats 1795 the lines (strings ending in sequences, as described in 1796 Section 2.3.7) following the command as mail data from the sender. 1797 This command causes the mail data to be appended to the mail data 1798 buffer. The mail data may contain any of the 128 ASCII character 1799 codes, although experience has indicated that use of control 1800 characters other than SP, HT, CR, and LF may cause problems and 1801 SHOULD be avoided when possible. 1803 The mail data are terminated by a line containing only a period, that 1804 is, the character sequence ".", where the first is 1805 actually the terminator of the previous line (see Section 4.5.2). 1806 This is the end of mail data indication. The first of this 1807 terminating sequence is also the that ends the final line of 1808 the data (message text) or, if there was no mail data, ends the DATA 1809 command itself (the "no mail data" case does not conform to this 1810 specification since it would require that neither the trace header 1811 fields required by this specification nor the message header section 1812 required by RFC 5322 [12] be transmitted). An extra MUST NOT 1813 be added, as that would cause an empty line to be added to the 1814 message. The only exception to this rule would arise if the message 1815 body were passed to the originating SMTP-sender with a final "line" 1816 that did not end in ; in that case, the originating SMTP system 1817 MUST either reject the message as invalid or add in order to 1818 have the receiving SMTP server recognize the "end of data" condition. 1820 The custom of accepting lines ending only in , as a concession to 1821 non-conforming behavior on the part of some UNIX systems, has proven 1822 to cause more interoperability problems than it solves, and SMTP 1823 server systems MUST NOT do this, even in the name of improved 1824 robustness. In particular, the sequence "." (bare line 1825 feeds, without carriage returns) MUST NOT be treated as equivalent to 1826 . as the end of mail data indication. 1828 Receipt of the end of mail data indication requires the server to 1829 process the stored mail transaction information. This processing 1830 consumes the information in the reverse-path buffer, the forward-path 1831 buffer, and the mail data buffer, and on the completion of this 1832 command these buffers are cleared. If the processing is successful, 1833 the receiver MUST send an OK reply. If the processing fails, the 1834 receiver MUST send a failure reply. The SMTP model does not allow 1835 for partial failures at this point: either the message is accepted by 1836 the server for delivery and a positive response is returned or it is 1837 not accepted and a failure reply is returned. In sending a positive 1838 "250 OK" completion reply to the end of data indication, the receiver 1839 takes full responsibility for the message (see Section 6.1). Errors 1840 that are diagnosed subsequently MUST be reported in a mail message, 1841 as discussed in Section 4.4. 1843 When the SMTP server accepts a message either for relaying or for 1844 final delivery, it inserts a trace record (also referred to 1845 interchangeably as a "time stamp line" or "Received" line) at the top 1846 of the mail data. This trace record indicates the identity of the 1847 host that sent the message, the identity of the host that received 1848 the message (and is inserting this time stamp), and the date and time 1849 the message was received. Relayed messages will have multiple time 1850 stamp lines. Details for formation of these lines, including their 1851 syntax, is specified in Section 4.4. 1853 Additional discussion about the operation of the DATA command appears 1854 in Section 3.3. 1856 Syntax: 1858 data = "DATA" CRLF 1860 4.1.1.5. RESET (RSET) 1862 This command specifies that the current mail transaction will be 1863 aborted. Any stored sender, recipients, and mail data MUST be 1864 discarded, and all buffers and state tables cleared. The receiver 1865 MUST send a "250 OK" reply to a RSET command with no arguments. A 1866 reset command may be issued by the client at any time. It is 1867 effectively equivalent to a NOOP (i.e., it has no effect) if issued 1868 immediately after EHLO, before EHLO is issued in the session, after 1869 an end of data indicator has been sent and acknowledged, or 1870 immediately before a QUIT. An SMTP server MUST NOT close the 1871 connection as the result of receiving a RSET; that action is reserved 1872 for QUIT (see Section 4.1.1.10). 1874 Since EHLO implies some additional processing and response by the 1875 server, RSET will normally be more efficient than reissuing that 1876 command, even though the formal semantics are the same. 1878 Syntax: 1880 rset = "RSET" CRLF 1882 4.1.1.6. VERIFY (VRFY) 1884 This command asks the receiver to confirm that the argument 1885 identifies a user or mailbox. If it is a user name, information is 1886 returned as specified in Section 3.5. 1888 This command has no effect on the reverse-path buffer, the forward- 1889 path buffer, or the mail data buffer. 1891 Syntax: 1893 vrfy = "VRFY" SP String CRLF 1895 4.1.1.7. EXPAND (EXPN) 1897 This command asks the receiver to confirm that the argument 1898 identifies a mailing list, and if so, to return the membership of 1899 that list. If the command is successful, a reply is returned 1900 containing information as described in Section 3.5. This reply will 1901 have multiple lines except in the trivial case of a one-member list. 1903 This command has no effect on the reverse-path buffer, the forward- 1904 path buffer, or the mail data buffer, and it may be issued at any 1905 time. 1907 Syntax: 1909 expn = "EXPN" SP String CRLF 1911 4.1.1.8. HELP (HELP) 1913 This command causes the server to send helpful information to the 1914 client. The command MAY take an argument (e.g., any command name) 1915 and return more specific information as a response. 1917 This command has no effect on the reverse-path buffer, the forward- 1918 path buffer, or the mail data buffer, and it may be issued at any 1919 time. 1921 SMTP servers SHOULD support HELP without arguments and MAY support it 1922 with arguments. 1924 Syntax: 1926 help = "HELP" [ SP String ] CRLF 1928 4.1.1.9. NOOP (NOOP) 1930 This command does not affect any parameters or previously entered 1931 commands. It specifies no action other than that the receiver send a 1932 "250 OK" reply. 1934 This command has no effect on the reverse-path buffer, the forward- 1935 path buffer, or the mail data buffer, and it may be issued at any 1936 time. If a parameter string is specified, servers SHOULD ignore it. 1938 Syntax: 1940 noop = "NOOP" [ SP String ] CRLF 1942 4.1.1.10. QUIT (QUIT) 1944 This command specifies that the receiver MUST send a "221 OK" reply, 1945 and then close the transmission channel. 1947 The receiver MUST NOT intentionally close the transmission channel 1948 until it receives and replies to a QUIT command (even if there was an 1949 error). The sender MUST NOT intentionally close the transmission 1950 channel until it sends a QUIT command, and it SHOULD wait until it 1951 receives the reply (even if there was an error response to a previous 1952 command). If the connection is closed prematurely due to violations 1953 of the above or system or network failure, the server MUST cancel any 1954 pending transaction, but not undo any previously completed 1955 transaction, and generally MUST act as if the command or transaction 1956 in progress had received a temporary error (i.e., a 4yz response). 1958 The QUIT command may be issued at any time. Any current uncompleted 1959 mail transaction will be aborted. 1961 Syntax: 1963 quit = "QUIT" CRLF 1965 4.1.1.11. Mail-Parameter and Rcpt-Parameter Error Responses 1967 If the server SMTP does not recognize or cannot implement one or more 1968 of the parameters associated with a particular MAIL or RCPT command, 1969 it will return code 555. 1971 If, for some reason, the server is temporarily unable to accommodate 1972 one or more of the parameters associated with a MAIL or RCPT command, 1973 and if the definition of the specific parameter does not mandate the 1974 use of another code, it should return code 455. 1976 Errors specific to particular parameters and their values will be 1977 specified in the parameter's defining RFC. 1979 4.1.2. Command Argument Syntax 1981 The syntax of the argument clauses of the above commands (using the 1982 syntax specified in RFC 5234 [11] where applicable) is given below. 1983 Some terminals not defined in this document, but are defined 1984 elsewhere, specifically: 1986 * In the "core" syntax in Appendix B of RFC 5234 [11]: ALPHA , CRLF 1987 , DIGIT , HEXDIG , and SP 1989 * In the message format syntax in RFC 5322 [12]: atext , CFWS , and 1990 FWS . 1992 Reverse-path = Path / "<>" 1994 Forward-path = Path 1996 Path = "<" Mailbox ">" 1998 Mail-parameters = esmtp-param *(SP esmtp-param) 2000 Rcpt-parameters = esmtp-param *(SP esmtp-param) 2002 esmtp-param = esmtp-keyword ["=" esmtp-value] 2004 esmtp-keyword = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-") 2006 esmtp-value = 1*(%d33-60 / %d62-126) 2007 ; any CHAR excluding "=", SP, and control 2008 ; characters. If this string is an email address, 2009 ; i.e., a Mailbox, then the "xtext" syntax [34] 2010 ; SHOULD be used. 2012 Keyword = Ldh-str 2014 Argument = Atom 2016 Domain = sub-domain *("." sub-domain) 2017 sub-domain = Let-dig [Ldh-str] 2019 Let-dig = ALPHA / DIGIT 2021 Ldh-str = *( ALPHA / DIGIT / "-" ) Let-dig 2023 address-literal = "[" ( IPv4-address-literal / 2024 IPv6-address-literal / 2025 General-address-literal ) "]" 2026 ; See Section 4.1.3 2028 Mailbox = Local-part "@" ( Domain / address-literal ) 2030 Local-part = Dot-string / Quoted-string 2031 ; MAY be case-sensitive 2033 Dot-string = Atom *("." Atom) 2035 Atom = 1*atext 2037 Quoted-string = DQUOTE 1*QcontentSMTP DQUOTE 2039 QcontentSMTP = qtextSMTP / quoted-pairSMTP 2041 quoted-pairSMTP = %d92 %d32-126 2042 ; i.e., backslash followed by any ASCII 2043 ; graphic (including itself) or SPace 2045 qtextSMTP = %d32-33 / %d35-91 / %d93-126 2046 ; i.e., within a quoted string, any 2047 ; ASCII graphic or space is permitted 2048 ; without backslash-quoting except 2049 ; double-quote and the backslash itself. 2051 String = Atom / Quoted-string 2053 Note that the backslash, "\", is a quote character, which is used to 2054 indicate that the next character is to be used literally (instead of 2055 its normal interpretation). For example, "Joe\,Smith" indicates a 2056 single nine-character user name string with the comma being the 2057 fourth character of that string. 2059 While the above definition for Local-part is relatively permissive, 2060 for maximum interoperability, a mailbox SHOULD NOT be defined with 2061 Local-part requiring (or using) the Quoted-string form or with the 2062 Local-part being case-sensitive. Further, when comparing a Local- 2063 part (e.g., to a specific mailbox name), all quoting MUST be treated 2064 as equivalent. A sending system SHOULD transmit the form that uses 2065 the minimum quoting possible. 2067 For example, the following 3 local-parts are equivalent and MUST 2068 compare equal: "ab cd ef", "ab\ cd ef" and "ab\ \cd ef". 2069 Similarly, "fred" and fred must compare equal. White space 2070 reduction MUST NOT be applied to Local-part by intermediate 2071 systems. 2073 Systems MUST NOT define mailboxes in such a way as to require the use 2074 in SMTP of non-ASCII characters (octets with the high order bit set 2075 to one) or ASCII "control characters" (decimal value 0-31 and 127). 2076 These characters MUST NOT be used in MAIL or RCPT commands or other 2077 commands that require mailbox names. 2079 To promote interoperability and consistent with long-standing 2080 guidance about conservative use of the DNS in naming and applications 2081 (e.g., see Section 2.3.1 of the base DNS document, RFC 1035 [4]), 2082 characters outside the set of alphabetic characters, digits, and 2083 hyphen MUST NOT appear in domain name labels for SMTP clients or 2084 servers. In particular, the underscore character is not permitted. 2085 SMTP servers that receive a command in which invalid character codes 2086 have been employed, and for which there are no other reasons for 2087 rejection, MUST reject that command with a 501 response (this rule, 2088 like others, could be overridden by appropriate SMTP extensions). 2090 4.1.3. Address Literals 2092 Sometimes a host is not known to the domain name system and 2093 communication (and, in particular, communication to report and repair 2094 the error) is blocked. To bypass this barrier, a special literal 2095 form of the address is allowed as an alternative to a domain name. 2096 For IPv4 addresses, this form uses four small decimal integers 2097 separated by dots and enclosed by brackets such as [123.255.37.2], 2098 which indicates an (IPv4) Internet Address in sequence-of-octets 2099 form. For IPv6 and other forms of addressing that might eventually 2100 be standardized, the form consists of a standardized "tag" that 2101 identifies the address syntax, a colon, and the address itself, in a 2102 format specified as part of the relevant standards (i.e., RFC 4291 2103 [10] for IPv6). 2105 // [5321bis] Proposed erratum 4315 (2015-03-27) suggests yet another 2106 // modification to the IPv6 address literal syntax, based on part on 2107 // RFC 5952. We should consider whether those, or other, 2108 // modifications are appropriate and/or whether, given both the 2109 // issues of spam/malware and servers supporting multiple domains, it 2110 // it time to deprecate mailboxes containing address literals 2111 // entirely (EHLO fields may be a different issue). If we are going 2112 // to allow IPv6 address literals, it may be time to incorporate 2113 // something by reference rather than including specific syntax here 2114 // (RFC 5952 is 14 pages long and does not contain any ABNF). 2116 Specifically: 2118 IPv4-address-literal = Snum 3("." Snum) 2120 IPv6-address-literal = "IPv6:" IPv6-addr 2122 General-address-literal = Standardized-tag ":" 1*dcontent 2124 Standardized-tag = Ldh-str 2125 ; Standardized-tag MUST be specified in a 2126 ; Standards-Track RFC and registered with IANA 2128 dcontent = %d33-90 / ; Printable US-ASCII 2129 %d94-126 ; excl. "[", "\", "]" 2131 Snum = 1*3DIGIT 2132 ; representing a decimal integer 2133 ; value in the range 0 through 255 2135 IPv6-addr = 6( h16 ":" ) ls32 2136 / "::" 5( h16 ":" ) ls32 2137 / [ h16 ] "::" 4( h16 ":" ) ls32 2138 / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32 2139 / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32 2140 / [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32 2141 / [ *4( h16 ":" ) h16 ] "::" ls32 2142 / [ *5( h16 ":" ) h16 ] "::" h16 2143 / [ *6( h16 ":" ) h16 ] "::" 2144 ; This definition is consistent with the one for 2145 ; URIs [40]. 2147 ls32 = ( h16 ":" h16 ) / IPv4address 2148 ; least-significant 32 bits of address 2150 h16 = 1*4HEXDIG 2151 ; 16 bits of address represented in hexadecimal 2153 4.1.4. Order of Commands 2155 There are restrictions on the order in which these commands may be 2156 used. 2158 A session that will contain mail transactions MUST first be 2159 initialized by the use of the EHLO command. An SMTP server SHOULD 2160 accept commands for non-mail transactions (e.g., VRFY, EXPN, or NOOP) 2161 without this initialization. 2163 An EHLO command MAY be issued by a client later in the session. If 2164 it is issued after the session begins and the EHLO command is 2165 acceptable to the SMTP server, the SMTP server MUST clear all buffers 2166 and reset the state exactly as if a RSET command had been issued 2167 (specifically, it terminates any mail transaction that was in 2168 progress, see Section 3.3). In other words, the sequence of RSET 2169 followed immediately by EHLO is redundant, but not harmful other than 2170 in the performance cost of executing unnecessary commands. However 2171 the response to an additional EHLO command MAY be different from that 2172 from prior ones; the client MUST rely only on the responses from the 2173 most recent EHLO command. 2175 If the EHLO command is not acceptable to the SMTP server, 501, 500, 2176 502, or 550 failure replies MUST be returned as appropriate. The 2177 SMTP server MUST stay in the same state after transmitting these 2178 replies that it was in before the EHLO was received. 2180 The SMTP client MUST, if possible, ensure that the domain parameter 2181 to the EHLO command is a primary host name as specified for this 2182 command in Section 2.3.5. If this is not possible (e.g., when the 2183 client's address is dynamically assigned and the client does not have 2184 an obvious name), an address literal SHOULD be substituted for the 2185 domain name. 2187 An SMTP server MAY verify that the domain name argument in the EHLO 2188 command has an address record matching the IP address of the client. 2190 // JcK 20211022: Note that Alessandro's email of 2021-10-13 proposes 2191 // adding "See [A/S] for further discussion." after that sentence. 2192 // Noting that phrasing could get us in trouble if the A/S takes a 2193 // long time to complete, can the WG please make a decision? 2195 // JcK 20211022: Additional question: should we be clear that this 2196 // refers to a forward lookup of the domain name, not a reverse 2197 // lookup of the address? 2198 The NOOP, HELP, EXPN, VRFY, and RSET commands can be used at any time 2199 during a session, or without previously initializing a session. SMTP 2200 servers SHOULD process these normally (that is, not return a 503 2201 code) even if no EHLO command has yet been received; clients SHOULD 2202 open a session with EHLO before sending these commands. 2204 If these rules are followed, the example in RFC 821 that shows "550 2205 access denied to you" in response to an EXPN command is incorrect 2206 unless an EHLO command precedes the EXPN or the denial of access is 2207 based on the client's IP address or other authentication or 2208 authorization-determining mechanisms. 2210 A mail transaction begins with a MAIL command and then consists of 2211 one or more RCPT commands, and a DATA command, in that order. A mail 2212 transaction may be aborted by the RSET, a new EHLO, or the QUIT 2213 command. 2215 SMTP extensions (see Section 2.2) may create additional commands that 2216 initiate, abort, or end the transaction.More generally, any new 2217 command MUST clearly document any effect it has on the transaction 2218 state. 2220 There may be zero or more transactions in a session. MAIL MUST NOT 2221 be sent if a mail transaction is already open, i.e., it should be 2222 sent only if no mail transaction had been started in the session, or 2223 if the previous one successfully concluded with a successful DATA 2224 command, or if the previous one was aborted, e.g., with a RSET or new 2225 EHLO. 2226 // [5321bis] See comment about changing this convoluted discussion to 2227 // talk about 'mail transaction' above. --JcK (and see Ticket #11 2228 // correspondence with Alexey 2021-07-06) 2230 If the transaction beginning command argument is not acceptable, a 2231 501 failure reply MUST be returned and the SMTP server MUST stay in 2232 the same state. If the commands in a transaction are out of order to 2233 the degree that they cannot be processed by the server, a 503 failure 2234 reply MUST be returned and the SMTP server MUST stay in the same 2235 state. 2237 The last command in a session MUST be the QUIT command. The QUIT 2238 command SHOULD be used by the client SMTP to request connection 2239 closure, even when no session opening command was sent and accepted. 2241 4.2. SMTP Replies 2243 Replies to SMTP commands serve to ensure the synchronization of 2244 requests and actions in the process of mail transfer and to guarantee 2245 that the SMTP client always knows the state of the SMTP server. 2246 Every command MUST generate exactly one reply. 2248 The details of the command-reply sequence are described in 2249 Section 4.3. 2251 An SMTP reply consists of a three digit number (transmitted as three 2252 numeric characters) followed by some text unless specified otherwise 2253 in this document. The number is for use by automata to determine 2254 what state to enter next; the text is for the human user. The three 2255 digits contain enough encoded information that the SMTP client need 2256 not examine the text and may either discard it or pass it on to the 2257 user, as appropriate. Exceptions are as noted elsewhere in this 2258 document. In particular, the 220, 221, 251, 421, and 551 reply codes 2259 are associated with message text that must be parsed and interpreted 2260 by machines. In the general case, the text may be receiver dependent 2261 and context dependent, so there are likely to be varying texts for 2262 each reply code. A discussion of the theory of reply codes is given 2263 in Section 4.2.1. Formally, a reply is defined to be the sequence: a 2264 three-digit code, , one line of text, and , or a multiline 2265 reply (as defined in the same section). Since, in violation of this 2266 specification, the text is sometimes not sent, clients that do not 2267 receive it SHOULD be prepared to process the code alone (with or 2268 without a trailing space character). Only the EHLO, EXPN, and HELP 2269 commands are expected to result in multiline replies in normal 2270 circumstances; however, multiline replies are allowed for any 2271 command. 2273 In ABNF, server responses are: 2275 Greeting = ( "220 " (Domain / address-literal) 2276 [ SP textstring ] CRLF ) / 2277 ( "220-" (Domain / address-literal) 2278 [ SP textstring ] CRLF 2279 *( "220-" [ textstring ] CRLF ) 2280 "220" [ SP textstring ] CRLF ) 2282 textstring = 1*(%d09 / %d32-126) ; HT, SP, Printable US-ASCII 2284 Reply-line = *( Reply-code "-" [ textstring ] CRLF ) 2285 Reply-code [ SP textstring ] CRLF 2287 Reply-code = %x32-35 %x30-35 %x30-39 2288 where "Greeting" appears only in the 220 response that announces that 2289 the server is opening its part of the connection. (Other possible 2290 server responses upon connection follow the syntax of Reply-line.) 2292 An SMTP server SHOULD send only the reply codes listed in this 2293 document or additions to the list as discussed below. An SMTP server 2294 SHOULD use the text shown in the examples whenever appropriate. 2296 An SMTP client MUST determine its actions only by the reply code, not 2297 by the text (except for the "change of address" 251 and 551 and, if 2298 necessary, 220, 221, and 421 replies); in the general case, any text, 2299 including no text at all (although senders SHOULD NOT send bare 2300 codes), MUST be acceptable. The space (blank) following the reply 2301 code is considered part of the text. A Sender-SMTP MUST first test 2302 the whole 3 digit reply code it receives, as well as any accompanying 2303 supplemental codes or information (see RFC 3463 [7] and RFC 5248 2304 [43]). If the full reply code is not recognized, and the additional 2305 information is not recognized or missing, the Sender-SMTP MUST use 2306 the first digit (severity indication) of a reply code it receives. 2308 The list of codes that appears below MUST NOT be construed as 2309 permanent. While the addition of new codes should be a rare and 2310 significant activity, with supplemental information in the textual 2311 part of the response (including enhanced status codes [7] and the 2312 successors to that specification) being preferred, new codes may be 2313 added as the result of new Standards or Standards-Track 2314 specifications. Consequently, a sender-SMTP MUST be prepared to 2315 handle codes not specified in this document and MUST do so by 2316 interpreting the first digit only. 2318 In the absence of extensions negotiated with the client, SMTP servers 2319 MUST NOT send reply codes whose first digits are other than 2, 3, 4, 2320 or 5. Clients that receive such out-of-range codes SHOULD normally 2321 treat them as fatal errors and terminate the mail transaction. 2323 4.2.1. Reply Code Severities and Theory 2325 The three digits of the reply each have a special significance. The 2326 first digit denotes whether the response is good, bad, or incomplete. 2327 An unsophisticated SMTP client, or one that receives an unexpected 2328 code, will be able to determine its next action (proceed as planned, 2329 redo, retrench, etc.) by examining this first digit. An SMTP client 2330 that wants to know approximately what kind of error occurred (e.g., 2331 mail system error, command syntax error) may examine the second 2332 digit. The third digit and any supplemental information that may be 2333 present is reserved for the finest gradation of information. 2335 There are four values for the first digit of the reply code: 2337 2yz Positive Completion reply 2338 The requested action has been successfully completed. A new 2339 request may be initiated. 2341 3yz Positive Intermediate reply 2342 The command has been accepted, but the requested action is being 2343 held in abeyance, pending receipt of further information. The 2344 SMTP client should send another command specifying this 2345 information. This reply is used in command sequence groups (i.e., 2346 in DATA). 2348 4yz Transient Negative Completion reply 2349 The command was not accepted, and the requested action did not 2350 occur. However, the error condition is temporary, and the action 2351 may be requested again. The sender should return to the beginning 2352 of the command sequence (if any). It is difficult to assign a 2353 meaning to "transient" when two different sites (receiver- and 2354 sender-SMTP agents) must agree on the interpretation. Each reply 2355 in this category might have a different time value, but the SMTP 2356 client SHOULD try again. A rule of thumb to determine whether a 2357 reply fits into the 4yz or the 5yz category (see below) is that 2358 replies are 4yz if they can be successful if repeated without any 2359 change in command form or in properties of the sender or receiver 2360 (that is, the command is repeated identically and the receiver 2361 does not put up a new implementation). 2363 5yz Permanent Negative Completion reply 2364 The command was not accepted and the requested action did not 2365 occur. The SMTP client SHOULD NOT repeat the exact request (in 2366 the same sequence). Even some "permanent" error conditions can be 2367 corrected, so the human user may want to direct the SMTP client to 2368 reinitiate the command sequence by direct action at some point in 2369 the future (e.g., after the spelling has been changed, or the user 2370 has altered the account status). 2372 It is worth noting that the file transfer protocol (FTP) [15] uses a 2373 very similar code architecture and that the SMTP codes are based on 2374 the FTP model. However, SMTP uses a one-command, one-response model 2375 (while FTP is asynchronous) and FTP's 1yz codes are not part of the 2376 SMTP model. 2378 The second digit encodes responses in specific categories: 2380 x0z Syntax: These replies refer to syntax errors, syntactically 2381 correct commands that do not fit any functional category, and 2382 unimplemented or superfluous commands. 2384 x1z Information: These are replies to requests for information, such 2385 as status or help. 2387 x2z Connections: These are replies referring to the transmission 2388 channel. 2390 x3z Unspecified. 2392 x4z Unspecified. 2394 x5z Mail system: These replies indicate the status of the receiver 2395 mail system vis-a-vis the requested transfer or other mail system 2396 action. 2398 The third digit gives a finer gradation of meaning in each category 2399 specified by the second digit. The list of replies illustrates this. 2400 Each reply text is recommended rather than mandatory, and may even 2401 change according to the command with which it is associated. On the 2402 other hand, the reply codes must strictly follow the specifications 2403 in this section. Receiver implementations should not invent new 2404 codes for slightly different situations from the ones described here, 2405 but rather adapt codes already defined. 2407 For example, a command such as NOOP, whose successful execution does 2408 not offer the SMTP client any new information, will return a 250 2409 reply. The reply is 502 when the command requests an unimplemented 2410 non-site-specific action. A refinement of that is the 504 reply for 2411 a command that is implemented, but that requests an unimplemented 2412 parameter. 2414 The reply text may be longer than a single line; in these cases the 2415 complete text must be marked so the SMTP client knows when it can 2416 stop reading the reply. This requires a special format to indicate a 2417 multiple line reply. 2419 The format for multiline replies requires that every line, except the 2420 last, begin with the reply code, followed immediately by a hyphen, 2421 "-" (also known as minus), followed by text. The last line will 2422 begin with the reply code, followed immediately by , optionally 2423 some text, and . As noted above, servers SHOULD send the 2424 if subsequent text is not sent, but clients MUST be prepared for it 2425 to be omitted. 2427 For example: 2429 250-First line 2430 250-Second line 2431 250-234 Text beginning with numbers 2432 250 The last line 2434 In a multiline reply, the reply code on each of the lines MUST be the 2435 same. It is reasonable for the client to rely on this, so it can 2436 make processing decisions based on the code in any line, assuming 2437 that all others will be the same. In a few cases, there is important 2438 data for the client in the reply "text". The client will be able to 2439 identify these cases from the current context. 2441 4.2.2. Reply Codes by Function Groups 2443 500 Syntax error, command unrecognized (This may include errors such 2444 as command line too long) 2446 501 Syntax error in parameters or arguments 2448 502 Command not implemented (see Section 4.2.4.1) 2450 503 Bad sequence of commands 2452 504 Command parameter not implemented 2454 211 System status, or system help reply 2456 214 Help message (Information on how to use the receiver or the 2457 meaning of a particular non-standard command; this reply is useful 2458 only to the human user) 2460 220 Service ready 2462 221 Service closing transmission channel 2464 421 Service not available, closing transmission channel 2465 (This may be a reply to any command if the service knows it must 2466 shut down) 2468 521 No mail service here. 2470 556 No mail service at this domain. 2472 250 Requested mail action okay, completed 2474 251 User not local; will forward to (See 2475 Section 3.4.1) 2477 252 Cannot VRFY user, but will accept message and attempt delivery 2478 (See Section 3.5.3) 2480 455 Server unable to accommodate parameters 2481 555 MAIL FROM/RCPT TO parameters not recognized or not implemented 2483 450 Requested mail action not taken: mailbox unavailable (e.g., 2484 mailbox busy or temporarily blocked for policy reasons) 2486 550 Requested action not taken: mailbox unavailable (e.g., mailbox 2487 not found, no access, or command rejected for policy reasons) 2489 451 Requested action aborted: error in processing 2491 551 User not local; please try (See Section 3.4.1) 2493 452 Requested action not taken: insufficient system storage 2494 (preferred code for "too many recipients", see Section 4.5.3.1.10) 2496 552 Requested mail action aborted: exceeded storage allocation. 2498 553 Requested action not taken: mailbox name not allowed (e.g., 2499 mailbox syntax incorrect) 2501 354 Start mail input; end with . 2503 554 Transaction failed (Or, historically in the case of a 2504 connection-opening response, "No SMTP service here". 521 is now 2505 preferred for that function at connection-opening if the server 2506 never accepts mail.) 2508 4.2.3. Reply Codes in Numeric Order 2510 211 System status, or system help reply 2512 214 Help message (Information on how to use the receiver or the 2513 meaning of a particular non-standard command; this reply is useful 2514 only to the human user) 2516 220 Service ready 2518 221 Service closing transmission channel 2520 250 Requested mail action okay, completed 2522 251 User not local; will forward to (See 2523 Section 3.4.1) 2525 252 Cannot VRFY user, but will accept message and attempt delivery 2526 (See Section 3.5.3) 2528 354 Start mail input; end with . 2529 421 Service not available, closing transmission channel 2530 (This may be a reply to any command if the service knows it must 2531 shut down) 2533 450 Requested mail action not taken: mailbox unavailable (e.g., 2534 mailbox busy or temporarily blocked for policy reasons) 2536 451 Requested action aborted: local error in processing 2538 452 Requested action not taken: insufficient system storage (also 2539 preferred code for "too many recipients", see Section 4.5.3.1.10) 2541 455 Server unable to accommodate parameters 2543 500 Syntax error, command unrecognized (This may include errors such 2544 as command line too long) 2546 501 Syntax error in parameters or arguments 2548 502 Command not implemented (see Section 4.2.4.1) 2550 503 Bad sequence of commands 2552 504 Command parameter not implemented 2554 521 No mail service (See Section 4.2.4.2.) 2556 550 Requested action not taken: mailbox unavailable (e.g., mailbox 2557 not found, no access, or command rejected for policy reasons) 2559 551 User not local; please try (See Section 3.4.1) 2561 552 Requested mail action aborted: exceeded storage allocation. 2563 553 Requested action not taken: mailbox name not allowed (e.g., 2564 mailbox syntax incorrect) 2566 554 Transaction failed (Or, in the case of a connection-opening 2567 response, "No SMTP service here" although 521 is now preferred for 2568 the latter. See Section 4.2.4.2.) 2570 555 MAIL FROM/RCPT TO parameters not recognized or not implemented 2572 556 No mail service at this domain. (See Section 4.2.4.2.) 2574 4.2.4. Some specific code situations and relationships 2576 4.2.4.1. Reply Code 502 2578 Questions have been raised as to when reply code 502 (Command not 2579 implemented) SHOULD be returned in preference to other codes. 502 2580 SHOULD be used when the command is actually recognized by the SMTP 2581 server, but not implemented. If the command is not recognized, code 2582 500 SHOULD be returned. Extended SMTP systems MUST NOT list 2583 capabilities in response to EHLO for which they will return 502 (or 2584 500) replies. 2586 4.2.4.2. "No mail accepted" situations and the 521, 554, and 556 codes 2588 Codes 521, 554, and 556 are all used to report different types of "no 2589 mail accepted" situations. They differ as follows. 521 is an 2590 indication from a system answering on the SMTP port that it does not 2591 support SMTP service (a so-called "dummy server" as discussed in RFC 2592 7504 [45] and elsewhere). Obviously, it requires that system exist 2593 and that a connection can be made successfully to it. Because a 2594 system that does not accept any mail cannot meaningfully accept a 2595 RCPT command, any commands (other than QUIT) issued after an SMTP 2596 server has issued a 521 reply are client (sender) errors. 2598 When a domain does not intend to accept mail and wishes to publish 2599 that fact rather than being subjected to connection attempts, the 2600 best way to accomplish that is to use the "Null MX" convention. This 2601 is done by advertising a single MX RR (see Section 3.3.9 of (RFC 1035 2602 [4]) with an RDATA section consisting of preference number 0 and a 2603 zero-length label, written in master files as ".", as the exchange 2604 domain, to denote that there exists no mail exchanger for that 2605 domain. Reply code 556 is then used by a message submission or 2606 intermediate SMTP system (see Section 1.1) to report that it cannot 2607 forward the message further because it knows from the DNS entry that 2608 the recipient domain does not accept mail. If, despite publishing 2609 the DNS entry, the server domain chooses to respond on the SMTP port, 2610 it SHOULD respond with the 556 code as well. The details of the Null 2611 MX convention were first defined in RFC 7505 [46]; see that document 2612 for additional discussion of the rationale for that convention. 2614 Reply code 554 would normally be used in response to a RCPT command 2615 (or extension command with similar intent) when the SMTP system 2616 identifies a domain that it can (or has) determined never accepts 2617 mail. Other codes, including 554 and the temporary 450, are used for 2618 more transient situations and situations in which an SMTP server 2619 cannot or will not deliver to (or accept mail for) a particular 2620 system or mailbox for policy reasons rather than ones directly 2621 related to SMTP processing. 2623 // [JcK 20210904]: do we want/need to discuss temporary server 2624 // outages? And is the discussion above sufficient to obsolete RFC 2625 // 7505 or do we need either more text or some pretense to claim to 2626 // update it. 2628 4.2.4.3. Reply Codes after DATA and the Subsequent . 2630 When an SMTP server returns a positive completion status (2yz code) 2631 after the DATA command is completed with ., it accepts 2632 responsibility for: 2634 * delivering the message (if the recipient mailbox exists), or 2636 * if attempts to deliver the message fail due to transient 2637 conditions, retrying delivery some reasonable number of times at 2638 intervals as specified in Section 4.5.4. 2640 * if attempts to deliver the message fail due to permanent 2641 conditions, or if repeated attempts to deliver the message fail 2642 due to transient conditions, returning appropriate notification to 2643 the sender of the original message (using the address in the SMTP 2644 MAIL command). 2646 When an SMTP server returns a temporary error status (4yz) code after 2647 the DATA command is completed with ., it MUST NOT make a 2648 subsequent attempt to deliver that message. The SMTP client retains 2649 responsibility for the delivery of that message and may either return 2650 it to the user or requeue it for a subsequent attempt (see 2651 Section 4.5.4.1). 2653 The user who originated the message SHOULD be able to interpret the 2654 return of a transient failure status (by mail message or otherwise) 2655 as a non-delivery indication, just as a permanent failure would be 2656 interpreted. If the client SMTP successfully handles these 2657 conditions, the user will not receive such a reply. 2659 When an SMTP server returns a permanent error status (5yz) code after 2660 the DATA command is completed with ., it MUST NOT make 2661 any subsequent attempt to deliver the message. As with temporary 2662 error status codes, the SMTP client retains responsibility for the 2663 message, but SHOULD NOT again attempt delivery to the same server 2664 without user review of the message and response and appropriate 2665 intervention. 2667 4.3. Sequencing of Commands and Replies 2668 4.3.1. Sequencing Overview 2670 The communication between the sender and receiver is an alternating 2671 dialogue, controlled by the sender. As such, the sender issues a 2672 command and the receiver responds with a reply. Unless other 2673 arrangements are negotiated through service extensions, the sender 2674 MUST wait for this response before sending further commands. One 2675 important reply is the connection greeting. Normally, a receiver 2676 will send a 220 "Service ready" reply when the connection is 2677 completed. The sender SHOULD wait for this greeting message before 2678 sending any commands. 2680 Note: all the greeting-type replies have the official name (the 2681 fully-qualified primary domain name) of the server host as the first 2682 word following the reply code. Sometimes the host will have no 2683 meaningful name. See Section 4.1.3 for a discussion of alternatives 2684 in these situations. 2686 For example, 2688 220 ISIF.USC.EDU Service ready 2690 or 2692 220 mail.example.com SuperSMTP v 6.1.2 Service ready 2694 or 2696 220 [10.0.0.1] Clueless host service ready 2698 The table below lists alternative success and failure replies for 2699 each command. These SHOULD be strictly adhered to. A receiver MAY 2700 substitute text in the replies, but the meanings and actions implied 2701 by the code numbers and by the specific command reply sequence MUST 2702 be preserved. However, in order to provide robustness as SMTP is 2703 extended and evolves, the discussion in Section 4.2.1 still applies: 2704 all SMTP clients MUST be prepared to accept any code that conforms to 2705 the discussion in that section and MUST be prepared to interpret it 2706 on the basis of its first digit only. 2708 4.3.2. Command-Reply Sequences 2710 Each command is listed with its usual possible replies. The prefixes 2711 used before the possible replies are "I" for intermediate, "S" for 2712 success, and "E" for error. Since some servers may generate other 2713 replies under special circumstances, and to allow for future 2714 extension, SMTP clients SHOULD, when possible, interpret only the 2715 first digit of the reply and MUST be prepared to deal with 2716 unrecognized reply codes by interpreting the first digit only. 2717 Unless extended using the mechanisms described in Section 2.2, SMTP 2718 servers MUST NOT transmit reply codes to an SMTP client that are 2719 other than three digits or that do not start in a digit between 2 and 2720 5 inclusive. 2722 These sequencing rules and, in principle, the codes themselves, can 2723 be extended or modified by SMTP extensions offered by the server and 2724 accepted (requested) by the client. However, if the target is more 2725 precise granularity in the codes, rather than codes for completely 2726 new purposes, the system described in RFC 3463 [7] SHOULD be used in 2727 preference to the invention of new codes. 2729 In addition to the codes listed below, any SMTP command can return 2730 any of the following codes if the corresponding unusual circumstances 2731 are encountered: 2733 500 For the "command line too long" case or if the command name was 2734 not recognized. Note that producing a "command not recognized" 2735 error in response to the required subset of these commands is a 2736 violation of this specification. Similarly, producing a "command 2737 too long" message for a command line shorter than 512 characters 2738 would violate the provisions of Section 4.5.3.1.4. 2740 501 Syntax error in command or arguments. In order to provide for 2741 future extensions, commands that are specified in this document as 2742 not accepting arguments (DATA, RSET, QUIT) SHOULD return a 501 2743 message if arguments are supplied in the absence of EHLO- 2744 advertised extensions. 2746 421 Service shutting down and closing transmission channel 2748 Specific sequences are: 2750 CONNECTION ESTABLISHMENT 2752 - S: 220 2753 E: 521, 554, 556 2755 EHLO or HELO 2757 - S: 250 2758 E: 504 (a conforming implementation could return this code only 2759 in fairly obscure cases), 550, 502 (permitted only with an old- 2760 style server that does not support EHLO) 2762 MAIL 2763 - S: 250 2764 E: 552, 451, 452, 550, 553, 503, 455, 555 2766 RCPT 2768 - S: 250, 251 (but see Section 3.4.1 for discussion of 251 and 2769 551) 2770 E: 550, 551, 552 (obsolete for "too many recipients; see 2771 Section 4.5.3.1.10, 553, 450, 451, 452, 503, 455, 555 2773 DATA 2775 - I: 354 -> data -> S: 250 2777 o E: 552, 554, 451, 452 2779 o E: 450, 550 (rejections for policy reasons) 2781 - E: 503, 554 2783 RSET 2785 - S: 250 2787 VRFY 2789 - S: 250, 251, 252 2790 E: 550, 551, 553, 502, 504 2792 EXPN 2794 - S: 250, 252 2795 E: 550, 500, 502, 504 2797 HELP 2799 - S: 211, 214 2800 E: 502, 504 2802 NOOP 2804 - S: 250 2806 QUIT 2808 - S: 221 2810 4.4. Trace Information 2812 Trace information is used to provide an audit trail of message 2813 handling. In addition, it indicates a route back to the sender of 2814 the message. 2816 4.4.1. Received Header Field 2818 When an SMTP server receives a message for delivery or further 2819 processing, it MUST insert trace (often referred to as "time stamp" 2820 or "Received" information) at the beginning of the message content, 2821 as discussed in Section 4.1.1.4. 2823 This line MUST be structured as follows: 2825 * The FROM clause, which MUST be supplied in an SMTP environment, 2826 SHOULD contain both (1) the name of the source host as presented 2827 in the EHLO command and (2) an address literal containing the IP 2828 address of the source, determined from the TCP connection. 2830 * The ID clause MAY contain an "@" as suggested in RFC 822, but this 2831 is not required. 2833 * If the FOR clause appears, it MUST contain exactly one 2834 entry, even when multiple RCPT commands have been given. Multiple 2835 s raise some security issues and have been deprecated, see 2836 Section 7.2. 2838 An Internet mail program MUST NOT change or delete a Received: line 2839 that was previously added to the message header section. SMTP 2840 servers MUST prepend Received lines to messages; they MUST NOT change 2841 the order of existing lines or insert Received lines in any other 2842 location. 2844 As the Internet grows, comparability of Received header fields is 2845 important for detecting problems, especially slow relays. SMTP 2846 servers that create Received header fields SHOULD use explicit 2847 offsets in the dates (e.g., -0800), rather than time zone names of 2848 any type. Local time (with an offset) SHOULD be used rather than UT 2849 when feasible. This formulation allows slightly more information 2850 about local circumstances to be specified. If UT is needed, the 2851 receiver need merely do some simple arithmetic to convert the values. 2852 Use of UT loses information about the time zone-location of the 2853 server. If it is desired to supply a time zone name, it SHOULD be 2854 included in a comment. 2856 When the delivery SMTP server makes the "final delivery" of a 2857 message, it inserts a return-path line at the beginning of the mail 2858 data. This use of return-path is required; mail systems MUST support 2859 it. The return-path line preserves the information in the from the MAIL command. Here, final delivery means the message 2861 has left the SMTP environment. Normally, this would mean it had been 2862 delivered to the destination user or an associated mail drop, but in 2863 some cases it may be further processed and transmitted by another 2864 mail system. 2866 It is possible for the mailbox in the return path to be different 2867 from the actual sender's mailbox, for example, if error responses are 2868 to be delivered to a special error handling mailbox rather than to 2869 the message sender. When mailing lists are involved, this 2870 arrangement is common and useful as a means of directing errors to 2871 the list maintainer rather than the message originator. 2873 The text above implies that the final mail data will begin with a 2874 return path line, followed by one or more time stamp lines. These 2875 lines will be followed by the rest of the mail data: first the 2876 balance of the mail header section and then the body (RFC 5322 [12]). 2878 It is sometimes difficult for an SMTP server to determine whether or 2879 not it is making final delivery since forwarding or other operations 2880 may occur after the message is accepted for delivery. Consequently, 2881 any further (forwarding, gateway, or relay) systems MAY remove the 2882 return path and rebuild the MAIL command as needed to ensure that 2883 exactly one such line appears in a delivered message. 2885 A message-originating SMTP system SHOULD NOT send a message that 2886 already contains a Return-path header field. SMTP servers performing 2887 a relay function MUST NOT inspect the message data, and especially 2888 not to the extent needed to determine if Return-path header fields 2889 are present. SMTP servers making final delivery MAY remove Return- 2890 path header fields before adding their own. 2892 The primary purpose of the Return-path is to designate the address to 2893 which messages indicating non-delivery or other mail system failures 2894 are to be sent. For this to be unambiguous, exactly one return path 2895 SHOULD be present when the message is delivered. Systems using RFC 2896 822 syntax with non-SMTP transports SHOULD designate an unambiguous 2897 address, associated with the transport envelope, to which error 2898 reports (e.g., non-delivery messages) should be sent. 2900 Historical note: Text in RFC 822 that appears to contraindicate the 2901 use of the Return-path header field (or the envelope reverse-path 2902 address from the MAIL command) if the destination for error messages 2903 is not applicable on the Internet. The reverse-path address (as 2904 copied into the Return-path) MUST be used as the target of any mail 2905 containing delivery error messages. 2907 In particular: 2909 * a gateway from SMTP -> elsewhere SHOULD insert a return-path 2910 header field, unless it is known that the "elsewhere" transport 2911 also uses Internet domain addresses and maintains the envelope 2912 sender address separately. 2914 * a gateway from elsewhere -> SMTP SHOULD delete any return-path 2915 header field present in the message, and either copy that 2916 information to the SMTP envelope or combine it with information 2917 present in the envelope of the other transport system to construct 2918 the reverse-path argument to the MAIL command in the SMTP 2919 envelope. 2921 The server must give special treatment to cases in which the 2922 processing following the end of mail data indication is only 2923 partially successful. This could happen if, after accepting several 2924 recipients and the mail data, the SMTP server finds that the mail 2925 data could be successfully delivered to some, but not all, of the 2926 recipients. In such cases, the response to the DATA command MUST be 2927 an OK reply. However, the SMTP server MUST compose and send an 2928 "undeliverable mail" notification message to the originator of the 2929 message. 2931 The time stamp line and the return path line are formally defined as 2932 follows (the definitions for "FWS" and "CFWS" appear in RFC 5322 2933 [12]): 2935 Return-path-line = "Return-Path:" FWS Reverse-path 2937 Time-stamp-line = "Received:" FWS Stamp 2939 Stamp = From-domain By-domain Opt-info [CFWS] ";" 2940 FWS date-time 2941 ; where "date-time" is as defined in RFC 5322 [12] 2942 ; but the "obs-" forms, especially two-digit 2943 ; years, are prohibited in SMTP and MUST NOT be used. 2945 From-domain = "FROM" FWS Extended-Domain 2947 By-domain = CFWS "BY" FWS Extended-Domain 2948 Extended-Domain = Domain / 2949 ( Domain FWS "(" TCP-info ")" ) / 2950 ( address-literal FWS "(" TCP-info ")" ) 2952 TCP-info = address-literal / ( Domain FWS address-literal ) 2953 ; Information derived by server from TCP connection 2954 ; not client EHLO. 2956 Opt-info = [Via] [With] [ID] [For] 2957 [Additional-Registered-Clauses] 2959 Via = CFWS "VIA" FWS Link 2961 With = CFWS "WITH" FWS Protocol 2963 ID = CFWS "ID" FWS ( Atom / msg-id ) 2964 ; msg-id is defined in RFC 5322 [12] 2966 For = CFWS "FOR" FWS ( Path / Mailbox ) 2968 Additional-Registered-Clauses = 1* (CFWS Atom FWS String) 2970 // [5321bis] 5321 errata #1683, 20090215, 2971 ; Additional standard clauses may be added in this 2972 ; location by future standards and registration with 2973 ; IANA. SMTP servers SHOULD NOT use unregistered 2974 ; names. See Section 8. 2976 Link = "TCP" / Addtl-Link 2978 Addtl-Link = Atom 2979 ; Additional standard names for links are 2980 ; registered with the Internet Assigned Numbers 2981 ; Authority (IANA). "Via" is primarily of value 2982 ; with non-Internet transports. SMTP servers 2983 ; SHOULD NOT use unregistered names. 2985 Protocol = "ESMTP" / "SMTP" / Attdl-Protocol 2987 Addtl-Protocol = Atom 2988 ; Additional standard names for protocols are 2989 ; registered with the Internet Assigned Numbers 2990 ; Authority (IANA) in the "mail parameters" 2991 ; registry [8]. SMTP servers SHOULD NOT 2992 ; use unregistered names. 2994 4.5. Additional Implementation Issues 2996 4.5.1. Minimum Implementation 2998 In order to make SMTP workable, the following minimum implementation 2999 MUST be provided by all receivers. The following commands MUST be 3000 supported to conform to this specification: 3002 EHLO 3003 HELO 3004 MAIL 3005 RCPT 3006 DATA 3007 RSET 3008 NOOP 3009 QUIT 3010 VRFY 3012 Any system that includes an SMTP server supporting mail relaying or 3013 delivery MUST support the reserved mailbox "postmaster" as a case- 3014 insensitive local name. This postmaster address is not strictly 3015 necessary if the server always returns 554 on connection opening (as 3016 described in Section 3.1). The requirement to accept mail for 3017 postmaster implies that RCPT commands that specify a mailbox for 3018 postmaster at any of the domains for which the SMTP server provides 3019 mail service, as well as the special case of "RCPT TO:" 3020 (with no domain specification), MUST be supported. 3022 SMTP systems are expected to make every reasonable effort to accept 3023 mail directed to Postmaster from any other system on the Internet. 3024 In extreme cases -- such as to contain a denial of service attack or 3025 other breach of security -- an SMTP server may block mail directed to 3026 Postmaster. However, such arrangements SHOULD be narrowly tailored 3027 so as to avoid blocking messages that are not part of such attacks. 3029 4.5.2. Transparency 3031 Without some provision for data transparency, the character sequence 3032 "." ends the mail text and cannot be sent by the user. 3033 In general, users are not aware of such "forbidden" sequences. To 3034 allow all user composed text to be transmitted transparently, the 3035 following procedures are used: 3037 * Before sending a line of mail text, the SMTP client checks the 3038 first character of the line. If it is a period, one additional 3039 period is inserted at the beginning of the line. 3041 * When a line of mail text is received by the SMTP server, it checks 3042 the line. If the line is composed of a single period, it is 3043 treated as the end of mail indicator. If the first character is a 3044 period and there are other characters on the line, the first 3045 character is deleted. 3047 The mail data may contain any of the 128 ASCII characters. All 3048 characters are to be delivered to the recipient's mailbox, including 3049 spaces, vertical and horizontal tabs, and other control characters. 3050 If the transmission channel provides an 8-bit byte (octet) data 3051 stream, the 7-bit ASCII codes are transmitted, right justified, in 3052 the octets, with the high-order bits cleared to zero. See 3053 Section 3.6 for special treatment of these conditions in SMTP systems 3054 serving a relay function. 3056 In some systems, it may be necessary to transform the data as it is 3057 received and stored. This may be necessary for hosts that use a 3058 different character set than ASCII as their local character set, that 3059 store data in records rather than strings, or which use special 3060 character sequences as delimiters inside mailboxes. If such 3061 transformations are necessary, they MUST be reversible, especially if 3062 they are applied to mail being relayed. 3064 4.5.3. Sizes and Timeouts 3066 4.5.3.1. Size Limits and Minimums 3068 There are several objects that have required minimum/maximum sizes. 3069 Every implementation MUST be able to receive objects of at least 3070 these sizes. Objects larger than these sizes SHOULD be avoided when 3071 possible. However, some Internet mail constructs such as encoded 3072 X.400 addresses (RFC 2156 [27]) will often require larger objects. 3073 Clients MAY attempt to transmit these, but MUST be prepared for a 3074 server to reject them if they cannot be handled by it. To the 3075 maximum extent possible, implementation techniques that impose no 3076 limits on the length of these objects should be used. 3078 Extensions to SMTP may involve the use of characters that occupy more 3079 than a single octet each. This section therefore specifies lengths 3080 in octets where absolute lengths, rather than character counts, are 3081 intended. 3083 4.5.3.1.1. Local-part 3085 The maximum total length of a user name or other local-part is 64 3086 octets. 3088 4.5.3.1.2. Domain 3090 The maximum total length of a domain name or number is 255 octets. 3092 4.5.3.1.3. Path 3094 The maximum total length of a reverse-path or forward-path is 256 3095 octets (including the punctuation and element separators). 3097 4.5.3.1.4. Command Line 3099 The maximum total length of a command line including the command word 3100 and the is 512 octets. SMTP extensions may be used to 3101 increase this limit. 3103 4.5.3.1.5. Reply Line 3105 The maximum total length of a reply line including the reply code and 3106 the is 512 octets. More information may be conveyed through 3107 multiple-line replies. 3109 4.5.3.1.6. Text Line 3111 The maximum total length of a text line including the is 1000 3112 octets (not counting the leading dot duplicated for transparency). 3113 This number may be increased by the use of SMTP Service Extensions. 3115 4.5.3.1.7. Message Content 3117 The maximum total length of a message content (including any message 3118 header section as well as the message body) MUST BE at least 64K 3119 octets. Since the introduction of Internet Standards for multimedia 3120 mail (RFC 2045 [25]), message lengths on the Internet have grown 3121 dramatically, and message size restrictions should be avoided if at 3122 all possible. SMTP server systems that must impose restrictions 3123 SHOULD implement the "SIZE" service extension of RFC 1870 [6], and 3124 SMTP client systems that will send large messages SHOULD utilize it 3125 when possible. 3127 4.5.3.1.8. Recipient Buffer 3129 The minimum total number of recipients that MUST be buffered is 100 3130 recipients. Rejection of messages (for excessive recipients) with 3131 fewer than 100 RCPT commands is a violation of this specification. 3132 The general principle that relaying SMTP server MUST NOT, and 3133 delivery SMTP servers SHOULD NOT, perform validation tests on message 3134 header fields suggests that messages SHOULD NOT be rejected based on 3135 the total number of recipients shown in header fields. A server that 3136 imposes a limit on the number of recipients MUST behave in an orderly 3137 fashion, such as rejecting additional addresses over its limit rather 3138 than silently discarding addresses previously accepted. A client 3139 that needs to deliver a message containing over 100 RCPT commands 3140 SHOULD be prepared to transmit in 100-recipient "chunks" if the 3141 server declines to accept more than 100 recipients in a single 3142 message. 3144 4.5.3.1.9. Treatment When Limits Exceeded 3146 Errors due to exceeding these limits may be reported by using the 3147 reply codes. Some examples of reply codes are: 3149 500 Line too long. 3151 or 3153 501 Path too long 3155 or 3157 452 Too many recipients (see below) 3159 or 3161 552 Too much mail data (historically also used for too many 3162 recipients (see below). 3164 4.5.3.1.10. Too Many Recipients Code 3166 RFC 821 [3] incorrectly listed the error where an SMTP server 3167 exhausts its implementation limit on the number of RCPT commands 3168 ("too many recipients") as having reply code 552. The correct reply 3169 code for this condition is 452. At the time RFC 5321 was written, 3170 the use of response code 552 by servers was sufficiently common that 3171 client implementation were advised to simply treat it as if 452 had 3172 been sent. That advice is no longer necessary or useful. 3174 When a conforming SMTP server encounters this condition, it has at 3175 least 100 successful RCPT commands in its recipient buffer. If the 3176 server is able to accept the message, then at least these 100 3177 addresses will be removed from the SMTP client's queue. When the 3178 client attempts retransmission of those addresses that received 452 3179 responses, at least 100 of these will be able to fit in the SMTP 3180 server's recipient buffer. Each retransmission attempt that is able 3181 to deliver anything will be able to dispose of at least 100 of these 3182 recipients. 3184 If an SMTP server has an implementation limit on the number of RCPT 3185 commands and this limit is exhausted, it MUST use a response code of 3186 452. If the server has a configured site-policy limitation on the 3187 number of RCPT commands, it MAY instead use a 5yz response code. In 3188 particular, if the intent is to prohibit messages with more than a 3189 site-specified number of recipients, rather than merely limit the 3190 number of recipients in a given mail transaction, it would be 3191 reasonable to return a 503 response to any DATA command received 3192 subsequent to the 452 code or to simply return the 503 after DATA 3193 without returning any previous negative response. 3195 4.5.3.2. Timeouts 3197 An SMTP client MUST provide a timeout mechanism. It MUST use per- 3198 command timeouts rather than somehow trying to time the entire mail 3199 transaction. Timeouts SHOULD be easily reconfigurable, preferably 3200 without recompiling the SMTP code. To implement this, a timer is set 3201 for each SMTP command and for each buffer of the data transfer. The 3202 latter means that the overall timeout is inherently proportional to 3203 the size of the message. 3205 Based on extensive experience with busy mail-relay hosts, the minimum 3206 per-command timeout values SHOULD be as follows: 3208 4.5.3.2.1. Initial 220 Message: 5 Minutes 3210 An SMTP client process needs to distinguish between a failed TCP 3211 connection and a delay in receiving the initial 220 greeting message. 3212 Many SMTP servers accept a TCP connection but delay delivery of the 3213 220 message until their system load permits more mail to be 3214 processed. 3216 4.5.3.2.2. MAIL Command: 5 Minutes 3218 4.5.3.2.3. RCPT Command: 5 Minutes 3220 A longer timeout is required if processing of mailing lists and 3221 aliases is not deferred until after the message was accepted. 3223 4.5.3.2.4. DATA Initiation: 2 Minutes 3225 This is while awaiting the "354 Start Input" reply to a DATA command. 3227 4.5.3.2.5. Data Block: 3 Minutes 3229 This is while awaiting the completion of each TCP SEND call 3230 transmitting a chunk of data. 3232 4.5.3.2.6. DATA Termination: 10 Minutes. 3234 This is while awaiting the "250 OK" reply. When the receiver gets 3235 the final period terminating the message data, it typically performs 3236 processing to deliver the message to a user mailbox. A spurious 3237 timeout at this point would be very wasteful and would typically 3238 result in delivery of multiple copies of the message, since it has 3239 been successfully sent and the server has accepted responsibility for 3240 delivery. See Section 6.1 for additional discussion. 3242 4.5.3.2.7. Server Timeout: 5 Minutes. 3244 An SMTP server SHOULD have a timeout of at least 5 minutes while it 3245 is awaiting the next command from the sender. 3247 4.5.4. Retry Strategies 3249 The common structure of a host SMTP implementation includes user 3250 mailboxes, one or more areas for queuing messages in transit, and one 3251 or more daemon processes for sending and receiving mail. The exact 3252 structure will vary depending on the needs of the users on the host 3253 and the number and size of mailing lists supported by the host. We 3254 describe several optimizations that have proved helpful, particularly 3255 for mailers supporting high traffic levels. 3257 Any queuing strategy MUST include timeouts on all activities on a 3258 per-command basis. A queuing strategy MUST NOT send error messages 3259 in response to error messages under any circumstances. 3261 4.5.4.1. Sending Strategy 3263 The general model for an SMTP client is one or more processes that 3264 periodically attempt to transmit outgoing mail. In a typical system, 3265 the program that composes a message has some method for requesting 3266 immediate attention for a new piece of outgoing mail, while mail that 3267 cannot be transmitted immediately MUST be queued and periodically 3268 retried by the sender. A mail queue entry will include not only the 3269 message itself but also the envelope information. 3271 The sender MUST delay retrying a particular destination after one 3272 attempt has failed. In general, the retry interval SHOULD be at 3273 least 30 minutes; however, more sophisticated and variable strategies 3274 will be beneficial when the SMTP client can determine the reason for 3275 non-delivery. 3277 Retries continue until the message is transmitted or the sender gives 3278 up; the give-up time generally needs to be at least 4-5 days. It MAY 3279 be appropriate to set a shorter maximum number of retries for non- 3280 delivery notifications and equivalent error messages than for 3281 standard messages. The parameters to the retry algorithm MUST be 3282 configurable. 3284 A client SHOULD keep a list of hosts it cannot reach and 3285 corresponding connection timeouts, rather than just retrying queued 3286 mail items. 3288 Experience suggests that failures are typically transient (the target 3289 system or its connection has crashed), favoring a policy of two 3290 connection attempts in the first hour the message is in the queue, 3291 and then backing off to one every two or three hours. 3293 The SMTP client can shorten the queuing delay in cooperation with the 3294 SMTP server. For example, if mail is received from a particular 3295 address, it is likely that mail queued for that host can now be sent. 3296 Application of this principle may, in many cases, eliminate the 3297 requirement for an explicit "send queues now" function such as ETRN, 3298 RFC 1985 [24]. 3300 The strategy may be further modified as a result of multiple 3301 addresses per host (see below) to optimize delivery time versus 3302 resource usage. 3304 An SMTP client may have a large queue of messages for each 3305 unavailable destination host. If all of these messages were retried 3306 in every retry cycle, there would be excessive Internet overhead and 3307 the sending system would be blocked for a long period. Note that an 3308 SMTP client can generally determine that a delivery attempt has 3309 failed only after a timeout of several minutes, and even a one-minute 3310 timeout per connection will result in a very large delay if retries 3311 are repeated for dozens, or even hundreds, of queued messages to the 3312 same host. 3314 At the same time, SMTP clients SHOULD use great care in caching 3315 negative responses from servers. In an extreme case, if EHLO is 3316 issued multiple times during the same SMTP connection, different 3317 answers may be returned by the server. More significantly, 5yz 3318 responses to the MAIL command MUST NOT be cached. 3320 When a mail message is to be delivered to multiple recipients, and 3321 the SMTP server to which a copy of the message is to be sent is the 3322 same for multiple recipients, then only one copy of the message 3323 SHOULD be transmitted. That is, the SMTP client SHOULD use the 3324 command sequence: MAIL, RCPT, RCPT, ..., RCPT, DATA instead of the 3325 sequence: MAIL, RCPT, DATA, ..., MAIL, RCPT, DATA. However, if there 3326 are very many addresses, a limit on the number of RCPT commands per 3327 MAIL command MAY be imposed. This efficiency feature SHOULD be 3328 implemented. 3330 Similarly, to achieve timely delivery, the SMTP client MAY support 3331 multiple concurrent outgoing mail transactions. However, some limit 3332 may be appropriate to protect the host from devoting all its 3333 resources to mail. 3335 4.5.4.2. Receiving Strategy 3337 The SMTP server SHOULD attempt to keep a pending listen on the SMTP 3338 port (specified by IANA as port 25) at all times. This requires the 3339 support of multiple incoming TCP connections for SMTP. Some limit 3340 MAY be imposed, but servers that cannot handle more than one SMTP 3341 transaction at a time are not in conformance with the intent of this 3342 specification. 3344 As discussed above, when the SMTP server receives mail from a 3345 particular host address, it could activate its own SMTP queuing 3346 mechanisms to retry any mail pending for that host address. 3348 4.5.5. Messages with a Null Reverse-Path 3350 There are several types of notification messages that are required by 3351 existing and proposed Standards to be sent with a null reverse-path, 3352 namely non-delivery notifications as discussed in Section 3.6.1 and 3353 Section 3.6.2, other kinds of Delivery Status Notifications (DSNs, 3354 RFC 3461 [34]), and Message Disposition Notifications (MDNs, RFC 8098 3355 [37]). All of these kinds of messages are notifications about a 3356 previous message, and they are sent to the reverse-path of the 3357 previous mail message. (If the delivery of such a notification 3358 message fails, that usually indicates a problem with the mail system 3359 of the host to which the notification message is addressed. For this 3360 reason, at some hosts the MTA is set up to forward such failed 3361 notification messages to someone who is able to fix problems with the 3362 mail system, e.g., via the postmaster alias.) 3364 All other types of messages (i.e., any message which is not required 3365 by a Standards-Track RFC to have a null reverse-path) SHOULD be sent 3366 with a valid, non-null reverse-path. 3368 Implementers of automated email processors should be careful to make 3369 sure that the various kinds of messages with a null reverse-path are 3370 handled correctly. In particular, such systems SHOULD NOT reply to 3371 messages with a null reverse-path, and they SHOULD NOT add a non-null 3372 reverse-path, or change a null reverse-path to a non-null one, to 3373 such messages when forwarding. 3375 5. Address Resolution and Mail Handling 3377 5.1. Locating the Target Host 3379 Once an SMTP client lexically identifies a domain to which mail will 3380 be delivered for processing (as described in Sections 2.3.5 and 3.6), 3381 a DNS lookup MUST be performed to resolve the domain name (RFC 1035 3382 [4]. The names are required to be fully-qualified domain names 3383 (FQDNs) as discussed in Section 2.3.5. 3385 The lookup first attempts to locate an MX record associated with the 3386 name. If a CNAME record is found, the resulting name is processed as 3387 if it were the initial name. If a non-existent domain error is 3388 returned, this situation MUST be reported as an error. If a 3389 temporary error is returned, the message MUST be queued and retried 3390 later (see Section 4.5.4.1). If an empty list of MXs is returned, 3391 the address is treated as if it was associated with an implicit MX RR 3392 with a preference of 0, pointing to that host. If MX records are 3393 present, but none of them are usable, or the implicit MX is unusable, 3394 this situation MUST be reported as an error. 3396 When the lookup succeeds, the mapping can result in a list of 3397 alternative delivery addresses rather than a single address. This 3398 can be due to multiple MX records, multihoming, or both. To provide 3399 reliable mail transmission, the SMTP client MUST be able to try (and 3400 be prepared to retry) each of the relevant addresses in this list in 3401 order (see below), until a delivery attempt succeeds. However, as 3402 discussed more generally in Section 7.8 there MAY also be a 3403 configurable limit on the number of alternate addresses that can be 3404 tried. In any case, the SMTP client SHOULD try at least two 3405 addresses. 3407 If one or more MX RRs are found for a given name, SMTP systems MUST 3408 NOT utilize any address RRs associated with that name unless they are 3409 located using the MX RRs; the "implicit MX" rule above applies only 3410 if there are no MX records present. If MX records are present, but 3411 none of them are usable, this situation MUST be reported as an error. 3413 When a domain name associated with an MX RR is looked up and the 3414 associated data field obtained, the data field of that response MUST 3415 contain a domain name that conforms to the specifications of 3416 Section 2.3.5. 3418 // [[5321bis Editor's Note: Depending on how the "null MX" discussion 3419 // unfolds, some additional text may be in order here (20140718)]] 3420 That domain name, when queried, MUST return at least one address 3421 record (e.g., A or AAAA RR) that gives the IP address of the SMTP 3422 server to which the message should be directed. Any other response, 3423 specifically including a value that will return a CNAME record when 3424 queried, lies outside the scope of this Standard. The prohibition on 3425 labels in the data that resolve to CNAMEs is discussed in more detail 3426 in RFC 2181, Section 10.3 [28]. 3428 Two types of information are used to rank the host addresses: 3429 multiple MX records, and multihomed hosts. 3431 MX records contain a numerical preference indication that MUST be 3432 used in sorting if more than one such record appears. Lower numbers 3433 are more preferred than higher ones. The sender-SMTP MUST inspect 3434 the list for any of the names or addresses by which it might be known 3435 in mail transactions. If a matching record is found, all records at 3436 that preference level and higher-numbered ones MUST be discarded from 3437 consideration. If there are no records left at that point, it is an 3438 error condition, and a 5yz reply code generated (terminating the mail 3439 transaction) or the message MUST be returned as undeliverable. If 3440 there is a single MX record at the most-preferred preference label, 3441 the data field associated with that record is used as the next 3442 destination. Otherwise, if there are multiple records with the same 3443 preference and there is no clear reason to favor one (e.g., by 3444 recognition of an easily reached address), then the sender-SMTP MUST 3445 randomize them to spread the load across multiple mail exchangers for 3446 a specific organization. 3448 The destination host (from either the data field of the preferred MX 3449 record of from an address records fount in an implicit MX) may be 3450 multihomed. In those cases the domain name resolver will return a 3451 list of alternative IP addresses. It is the responsibility of the 3452 domain name resolver interface to have ordered this list by 3453 decreasing preference if necessary, and the SMTP sender MUST try them 3454 in the order presented. 3456 Although the capability to try multiple alternative addresses is 3457 required, specific installations may want to limit or disable the use 3458 of alternative addresses. The question of whether a sender should 3459 attempt retries using the different addresses of a multihomed host 3460 has been controversial. The main argument for using the multiple 3461 addresses is that it maximizes the likelihood of timely delivery, and 3462 indeed sometimes the likelihood of any delivery; the counter-argument 3463 is that it may result in unnecessary resource use. Note that 3464 resource use is also strongly determined by the sending strategy 3465 discussed in Section 4.5.4.1. 3467 If an SMTP server receives a message with a destination for which it 3468 is a designated Mail eXchanger, it MAY relay the message (potentially 3469 after having rewritten the MAIL FROM and/or RCPT TO addresses), make 3470 final delivery of the message, or hand it off using some mechanism 3471 outside the SMTP-provided transport environment. Of course, neither 3472 of the latter require that the list of MX records be examined 3473 further. 3475 If it determines that it should relay the message without rewriting 3476 the address, it MUST process the MX records as described above to 3477 determine candidates for delivery. 3479 5.2. IPv6 and MX Records 3481 In the contemporary Internet, SMTP clients and servers may be hosted 3482 on IPv4 systems, IPv6 systems, or dual-stack systems that are 3483 compatible with either version of the Internet Protocol. The host 3484 domains to which MX records point may, consequently, contain "A RR"s 3485 (IPv4), "AAAA RR"s (IPv6), or any combination of them. While RFC 3486 3974 [39] discusses some operational experience in mixed 3487 environments, it was not comprehensive enough to justify 3488 standardization, and some of its recommendations appear to be 3489 inconsistent with this specification. The appropriate actions to be 3490 taken either will depend on local circumstances, such as performance 3491 of the relevant networks and any conversions that might be necessary, 3492 or will be obvious (e.g., an IPv6-only client need not attempt to 3493 look up A RRs or attempt to reach IPv4-only servers). Designers of 3494 SMTP implementations that might run in IPv6 or dual-stack 3495 environments should study the procedures above, especially the 3496 comments about multihomed hosts, and, preferably, provide mechanisms 3497 to facilitate operational tuning and mail interoperability between 3498 IPv4 and IPv6 systems while considering local circumstances. 3500 6. Problem Detection and Handling 3501 6.1. Reliable Delivery and Replies by Email 3503 When the receiver-SMTP accepts a piece of mail (by sending a "250 OK" 3504 message in response to DATA), it is accepting responsibility for 3505 delivering or relaying the message. It must take this responsibility 3506 seriously. It MUST NOT lose the message for frivolous reasons, such 3507 as because the host later crashes or because of a predictable 3508 resource shortage. Some reasons that are not considered frivolous 3509 are discussed in the next subsection and in Section 7.8. 3511 If there is a delivery failure after acceptance of a message, the 3512 receiver-SMTP MUST formulate and mail a notification message. This 3513 notification MUST be sent using a null ("<>") reverse-path in the 3514 envelope. The recipient of this notification MUST be the address 3515 from the envelope return path (or the Return-Path: line). However, 3516 if this address is null ("<>"), the receiver-SMTP MUST NOT send a 3517 notification. Obviously, nothing in this section can or should 3518 prohibit local decisions (i.e., as part of the same system 3519 environment as the receiver-SMTP) to log or otherwise transmit 3520 information about null address events locally if that is desired. 3522 Some delivery failures after the message is accepted by SMTP will be 3523 unavoidable. For example, it may be impossible for the receiving 3524 SMTP server to validate all the delivery addresses in RCPT command(s) 3525 due to a "soft" domain system error, because the target is a mailing 3526 list (see earlier discussion of RCPT), or because the server is 3527 acting as a relay and has no immediate access to the delivering 3528 system. 3530 To avoid receiving duplicate messages as the result of timeouts, a 3531 receiver-SMTP MUST seek to minimize the time required to respond to 3532 the final . end of data indicator. See RFC 1047 [17] for 3533 a discussion of this problem. 3535 6.2. Unwanted, Unsolicited, and "Attack" Messages 3537 Utility and predictability of the Internet mail system requires that 3538 messages that can be delivered should be delivered, regardless of any 3539 syntax or other faults associated with those messages and regardless 3540 of their content. If they cannot be delivered, and cannot be 3541 rejected by the SMTP server during the SMTP transaction, they should 3542 be "bounced" (returned with non-delivery notification messages) as 3543 described above. In today's world, in which many SMTP server 3544 operators have discovered that the quantity of undesirable bulk email 3545 vastly exceeds the quantity of desired mail and in which accepting a 3546 message may trigger additional undesirable traffic by providing 3547 verification of the address, those principles may not be practical. 3549 As discussed in Section 7.8 and Section 7.9 below, dropping mail 3550 without notification of the sender is permitted in practice. 3551 However, it is extremely dangerous and violates a long tradition and 3552 community expectations that mail is either delivered or returned. If 3553 silent message-dropping is misused, it could easily undermine 3554 confidence in the reliability of the Internet's mail systems. So 3555 silent dropping of messages should be considered only in those cases 3556 where there is very high confidence that the messages are seriously 3557 fraudulent or otherwise inappropriate. 3559 To stretch the principle of delivery if possible even further, it may 3560 be a rational policy to not deliver mail that has an invalid return 3561 address, although the history of the network is that users are 3562 typically better served by delivering any message that can be 3563 delivered. Reliably determining that a return address is invalid can 3564 be a difficult and time-consuming process, especially if the putative 3565 sending system is not directly accessible or does not fully and 3566 accurately support VRFY and, even if a "drop messages with invalid 3567 return addresses" policy is adopted, it SHOULD be applied only when 3568 there is near-certainty that the return addresses are, in fact, 3569 invalid. 3571 Conversely, if a message is rejected because it is found to contain 3572 hostile content (a decision that is outside the scope of an SMTP 3573 server as defined in this document), rejection ("bounce") messages 3574 SHOULD NOT be sent unless the receiving site is confident that those 3575 messages will be usefully delivered. The preference and default in 3576 these cases is to avoid sending non-delivery messages when the 3577 incoming message is determined to contain hostile content. 3579 6.3. Loop Detection 3581 Simple counting of the number of "Received:" header fields in a 3582 message has proven to be an effective, although rarely optimal, 3583 method of detecting loops in mail systems. SMTP servers using this 3584 technique SHOULD use a large rejection threshold, normally at least 3585 100 Received entries. Whatever mechanisms are used, servers MUST 3586 contain provisions for detecting and stopping trivial loops. 3588 6.4. Compensating for Irregularities 3590 Unfortunately, variations, creative interpretations, and outright 3591 violations of Internet mail protocols do occur; some would suggest 3592 that they occur quite frequently. The debate as to whether a well- 3593 behaved SMTP receiver or relay should reject a malformed message, 3594 attempt to pass it on unchanged, or attempt to repair it to increase 3595 the odds of successful delivery (or subsequent reply) began almost 3596 with the dawn of structured network mail and shows no signs of 3597 abating. Advocates of rejection claim that attempted repairs are 3598 rarely completely adequate and that rejection of bad messages is the 3599 only way to get the offending software repaired. Advocates of 3600 "repair" or "deliver no matter what" argue that users prefer that 3601 mail go through it if at all possible and that there are significant 3602 market pressures in that direction. In practice, these market 3603 pressures may be more important to particular vendors than strict 3604 conformance to the standards, regardless of the preference of the 3605 actual developers. 3607 The problems associated with ill-formed messages were exacerbated by 3608 the introduction of the split-UA mail reading protocols (Post Office 3609 Protocol (POP) version 2 [14], Post Office Protocol (POP) version 3 3610 [23], IMAP version 2 [19], and PCMAIL [18]). These protocols 3611 encouraged the use of SMTP as a posting (message submission) 3612 protocol, and SMTP servers as relay systems for these client hosts 3613 (which are often only intermittently connected to the Internet). 3614 Historically, many of those client machines lacked some of the 3615 mechanisms and information assumed by SMTP (and indeed, by the mail 3616 format protocol, RFC 822 [13]). Some could not keep adequate track 3617 of time; others had no concept of time zones; still others could not 3618 identify their own names or addresses; and, of course, none could 3619 satisfy the assumptions that underlay RFC 822's conception of 3620 authenticated addresses. 3622 In response to these weak SMTP clients, many SMTP systems now 3623 complete messages that are delivered to them in incomplete or 3624 incorrect form. This strategy is generally considered appropriate 3625 when the server can identify or authenticate the client, and there 3626 are prior agreements between them. By contrast, there is at best 3627 great concern about fixes applied by a relay or delivery SMTP server 3628 that has little or no knowledge of the user or client machine. Many 3629 of these issues are addressed by using a separate protocol, such as 3630 that defined in RFC 6409 [41], for message submission, rather than 3631 using originating SMTP servers for that purpose. 3633 The following changes to a message being processed MAY be applied 3634 when necessary by an originating SMTP server, or one used as the 3635 target of SMTP as an initial posting (message submission) protocol: 3637 * Addition of a message-id field when none appears 3639 * Addition of a date, time, or time zone when none appears 3641 * Correction of addresses to proper FQDN format 3642 The less information the server has about the client, the less likely 3643 these changes are to be correct and the more caution and conservatism 3644 should be applied when considering whether or not to perform fixes 3645 and how. These changes MUST NOT be applied by an SMTP server that 3646 provides an intermediate relay function. 3648 In all cases, properly operating clients supplying correct 3649 information are preferred to corrections by the SMTP server. In all 3650 cases, documentation SHOULD be provided in trace header fields and/or 3651 header field comments for actions performed by the servers. 3653 7. Security Considerations 3655 7.1. Mail Security and Spoofing 3657 SMTP mail is inherently insecure in that it is feasible for even 3658 fairly casual users to negotiate directly with receiving and relaying 3659 SMTP servers and create messages that will trick a naive recipient 3660 into believing that they came from somewhere else. Constructing such 3661 a message so that the "spoofed" behavior cannot be detected by an 3662 expert is somewhat more difficult, but not sufficiently so as to be a 3663 deterrent to someone who is determined and knowledgeable. 3664 Consequently, as knowledge of Internet mail increases, so does the 3665 knowledge that SMTP mail inherently cannot be authenticated, or 3666 integrity checks provided, at the transport level. Real mail 3667 security lies only in end-to-end methods involving the message 3668 bodies, such as those that use digital signatures (see RFC 1847 [21] 3669 and, e.g., Pretty Good Privacy (PGP) in RFC 4880 [42] or Secure/ 3670 Multipurpose Internet Mail Extensions (S/MIME) in RFC 8551 [38]). 3672 Various protocol extensions and configuration options that provide 3673 authentication at the transport level (e.g., from an SMTP client to 3674 an SMTP server) improve somewhat on the traditional situation 3675 described above. However, in general, they only authenticate one 3676 server to another rather than a chain of relays and servers, much 3677 less authenticating users or user machines. Consequently, unless 3678 they are accompanied by careful handoffs of responsibility in a 3679 carefully designed trust environment, they remain inherently weaker 3680 than end-to-end mechanisms that use digitally signed messages rather 3681 than depending on the integrity of the transport system. 3683 Efforts to make it more difficult for users to set envelope return 3684 path and header "From" fields to point to valid addresses other than 3685 their own are largely misguided: they frustrate legitimate 3686 applications in which mail is sent by one user on behalf of another, 3687 in which error (or normal) replies should be directed to a special 3688 address, or in which a single message is sent to multiple recipients 3689 on different hosts. (Systems that provide convenient ways for users 3690 to alter these header fields on a per-message basis should attempt to 3691 establish a primary and permanent mailbox address for the user so 3692 that Sender header fields within the message data can be generated 3693 sensibly.) 3695 This specification does not further address the authentication issues 3696 associated with SMTP other than to advocate that useful functionality 3697 not be disabled in the hope of providing some small margin of 3698 protection against a user who is trying to fake mail. 3700 7.2. "Blind" Copies 3702 Addresses that do not appear in the message header section may appear 3703 in the RCPT commands to an SMTP server for a number of reasons. The 3704 two most common involve the use of a mailing address as a "list 3705 exploder" (a single address that resolves into multiple addresses) 3706 and the appearance of "blind copies". Especially when more than one 3707 RCPT command is present, and in order to avoid defeating some of the 3708 purpose of these mechanisms, SMTP clients and servers SHOULD NOT copy 3709 the full set of RCPT command arguments into the header section, 3710 either as part of trace header fields or as informational or private- 3711 extension header fields. 3712 // [rfc5321bis] [[Note in draft - Suggestion from 20070124 that got 3713 // lost: delete "especially" and "the full set of" -- copying the 3714 // first one can be as harmful as copying all of them, at least 3715 // without verifying that the addresses do appear in the headers. 3716 // See G.7.9 and ticket #15. 3717 Because this rule is often violated in practice, and cannot be 3718 enforced, sending SMTP systems that are aware of "bcc" use MAY find 3719 it helpful to send each blind copy as a separate message transaction 3720 containing only a single RCPT command. 3722 There is no inherent relationship between either "reverse" (from the 3723 MAIL command) or "forward" (RCPT) addresses in the SMTP transaction 3724 ("envelope") and the addresses in the header section. Receiving 3725 systems SHOULD NOT attempt to deduce such relationships and use them 3726 to alter the header section of the message for delivery. The popular 3727 "Apparently-to" header field is a violation of this principle as well 3728 as a common source of unintended information disclosure and SHOULD 3729 NOT be used. 3731 7.3. VRFY, EXPN, and Security 3733 As discussed in Section 3.5, individual sites may want to disable 3734 either or both of VRFY or EXPN for security reasons (see below). As 3735 a corollary to the above, implementations that permit this MUST NOT 3736 appear to have verified addresses that are not, in fact, verified. 3737 If a site disables these commands for security reasons, the SMTP 3738 server MUST return a 252 response, rather than a code that could be 3739 confused with successful or unsuccessful verification. 3741 Returning a 250 reply code with the address listed in the VRFY 3742 command after having checked it only for syntax violates this rule. 3743 Of course, an implementation that "supports" VRFY by always returning 3744 550 whether or not the address is valid is equally not in 3745 conformance. 3747 On the public Internet, the contents of mailing lists have become 3748 popular as an address information source for so-called "spammers." 3749 The use of EXPN to "harvest" addresses has increased as list 3750 administrators have installed protections against inappropriate uses 3751 of the lists themselves. However, VRFY and EXPN are still useful for 3752 authenticated users and within an administrative domain. For 3753 example, VRFY and EXPN are useful for performing internal audits of 3754 how email gets routed to check and to make sure no one is 3755 automatically forwarding sensitive mail outside the organization. 3756 Sites implementing SMTP authentication may choose to make VRFY and 3757 EXPN available only to authenticated requestors. Implementations 3758 SHOULD still provide support for EXPN, but sites SHOULD carefully 3759 evaluate the tradeoffs. 3761 Whether disabling VRFY provides any real marginal security depends on 3762 a series of other conditions. In many cases, RCPT commands can be 3763 used to obtain the same information about address validity. On the 3764 other hand, especially in situations where determination of address 3765 validity for RCPT commands is deferred until after the DATA command 3766 is received, RCPT may return no information at all, while VRFY is 3767 expected to make a serious attempt to determine validity before 3768 generating a response code (see discussion above). 3770 7.4. Mail Rerouting Based on the 251 and 551 Response Codes 3772 Before a client uses the 251 or 551 reply codes from a RCPT command 3773 to automatically update its future behavior (e.g., updating the 3774 user's address book), it should be certain of the server's 3775 authenticity. If it does not, it may be subject to a man in the 3776 middle attack. 3778 7.5. Information Disclosure in Announcements 3780 There has been an ongoing debate about the tradeoffs between the 3781 debugging advantages of announcing server type and version (and, 3782 sometimes, even server domain name) in the greeting response or in 3783 response to the HELP command and the disadvantages of exposing 3784 information that might be useful in a potential hostile attack. The 3785 utility of the debugging information is beyond doubt. Those who 3786 argue for making it available point out that it is far better to 3787 actually secure an SMTP server rather than hope that trying to 3788 conceal known vulnerabilities by hiding the server's precise identity 3789 will provide more protection. Sites are encouraged to evaluate the 3790 tradeoff with that issue in mind; implementations SHOULD minimally 3791 provide for making type and version information available in some way 3792 to other network hosts. 3794 7.6. Information Disclosure in Trace Fields 3796 In some circumstances, such as when mail originates from within a LAN 3797 whose hosts are not directly on the public Internet, trace (e.g., 3798 "Received") header fields produced in conformance with this 3799 specification may disclose host names and similar information that 3800 would not normally be available. This ordinarily does not pose a 3801 problem, but sites with special concerns about name disclosure should 3802 be aware of it. Also, the optional FOR clause should be supplied 3803 with caution or not at all when multiple recipients are involved lest 3804 it inadvertently disclose the identities of "blind copy" recipients 3805 to others. 3807 7.7. Information Disclosure in Message Forwarding 3809 As discussed in Section 3.4.1, use of the 251 or 551 reply codes to 3810 identify the replacement address associated with a mailbox may 3811 inadvertently disclose sensitive information. Sites that are 3812 concerned about those issues should ensure that they select and 3813 configure servers appropriately. 3815 7.8. Local Operational Requirements and Resistance to Attacks 3817 In recent years, there has been an increase of attacks on SMTP 3818 servers, either in conjunction with attempts to discover addresses 3819 for sending unsolicited messages or simply to make the servers 3820 inaccessible to others (i.e., as an application-level denial of 3821 service attack). There may also be important local circumstances 3822 that justify departures from some of the limits specified in this 3823 documents especially ones involving maximums or minimums. While the 3824 means of doing so are beyond the scope of this Standard, rational 3825 operational behavior requires that servers be permitted to detect 3826 such attacks and take action to defend themselves. For example, if a 3827 server determines that a large number of RCPT commands are being 3828 sent, most or all with invalid addresses, as part of such an attack, 3829 it would be reasonable for the server to close the connection after 3830 generating an appropriate number of 5yz (normally 550) replies. 3832 7.9. Scope of Operation of SMTP Servers 3834 It is a well-established principle that an SMTP server may refuse to 3835 accept mail for any operational or technical reason that makes sense 3836 to the site providing the server. However, cooperation among sites 3837 and installations makes the Internet possible. If sites take 3838 excessive advantage of the right to reject traffic, the ubiquity of 3839 email availability (one of the strengths of the Internet) will be 3840 threatened; considerable care should be taken and balance maintained 3841 if a site decides to be selective about the traffic it will accept 3842 and process. 3844 In recent years, use of the relay function through arbitrary sites 3845 has been used as part of hostile efforts to hide the actual origins 3846 of mail. Some sites have decided to limit the use of the relay 3847 function to known or identifiable sources, and implementations SHOULD 3848 provide the capability to perform this type of filtering. When mail 3849 is rejected for these or other policy reasons, a 550 code SHOULD be 3850 used in response to EHLO (or HELO), MAIL, or RCPT as appropriate. 3852 8. IANA Considerations 3854 IANA maintains three registries in support of this specification, all 3855 of which were created for RFC 2821 or earlier. This document expands 3856 the third one as specified below. The registry references listed are 3857 as of the time of publication; IANA does not guarantee the locations 3858 associated with the URLs. The registries are as follows: 3860 * The first, "Simple Mail Transfer Protocol (SMTP) Service 3861 Extensions" [49], consists of SMTP service extensions with the 3862 associated keywords, and, as needed, parameters and verbs. 3863 Entries may be made only for service extensions (and associated 3864 keywords, parameters, or verbs) that are defined in Standards- 3865 Track or Experimental RFCs specifically approved by the IESG for 3866 this purpose. 3868 * The second registry, "Address Literal Tags" [50], consists of 3869 "tags" that identify forms of domain literals other than those for 3870 IPv4 addresses (specified in RFC 821 and in this document). The 3871 initial entry in that registry is for IPv6 addresses (specified in 3872 this document). Additional literal types require standardization 3873 before being used; none are anticipated at this time. 3875 * The third, "Mail Transmission Types" [49], established by RFC 821 3876 and renewed by this specification, is a registry of link and 3877 protocol identifiers to be used with the "via" and "with" 3878 subclauses of the time stamp ("Received:" header field) described 3879 in Section 4.4. Link and protocol identifiers in addition to 3880 those specified in this document may be registered only by 3881 standardization or by way of an RFC-documented, IESG-approved, 3882 Experimental protocol extension. This name space is for 3883 identification and not limited in size: the IESG is encouraged to 3884 approve on the basis of clear documentation and a distinct method 3885 rather than preferences about the properties of the method itself. 3886 An additional subsection has been added to the "VIA link types" 3887 and "WITH protocol types" subsections of this registry to contain 3888 registrations of "Additional-registered-clauses" as described 3889 above. The registry will contain clause names, a description, a 3890 summary of the syntax of the associated String, and a reference. 3891 As new clauses are defined, they may, in principle, specify 3892 creation of their own registries if the Strings consist of 3893 reserved terms or keywords rather than less restricted strings. 3894 As with link and protocol identifiers, additional clauses may be 3895 registered only by standardization or by way of an RFC-documented, 3896 IESG-approved, Experimental protocol extension. The additional 3897 clause name space is for identification and is not limited in 3898 size: the IESG is encouraged to approve on the basis of clear 3899 documentation, actual use or strong signs that the clause will be 3900 used, and a distinct requirement rather than preferences about the 3901 properties of the clause itself. 3903 In addition, if additional trace header fields (i.e., in addition to 3904 Return-path and Received) are ever created, those trace fields MUST 3905 be added to the IANA registry established by BCP 90 (RFC 3864) [9] 3906 for use with RFC 5322 [12]. 3908 9. Acknowledgments 3910 Many people contributed to the development of RFCs 2821 and 5321. 3911 Those documents should be consulted for those acknowledgments. 3913 Neither this document nor RFCs 2821 or 5321 would have been possible 3914 without the many contribution and insights of the late Jon Postel. 3915 Those contributions of course include the original specification of 3916 SMTP in RFC 821. A considerable quantity of text from RFC 821 still 3917 appears in this document as do several of Jon's original examples 3918 that have been updated only as needed to reflect other changes in the 3919 specification. 3921 The following filed errata against RFC 5321 that were not rejected at 3922 the time of submission: Jasen Betts, Adrien de Croy Guillaume Fortin- 3923 Debigare Roberto Javier Godoy, David Romerstein, Dominic Sayers, 3924 Rodrigo Speller, Alessandro Vesely, and Brett Watson. Some of those 3925 individuals made additional suggestions after the EMAILCORE WG was 3926 initiated. In addition to the above, several of whom continued to 3927 make other suggestions, specific suggestions that led to corrections 3928 and improvements in early versions of the current specification were 3929 received from Dave Crocker, Ned Freed, Arnt Gulbrandsen, Tony Hansen, 3930 Barry Leiba, Ivar Lumi, Pete Resnick, Hector Santos, Paul Smith and 3931 others. 3933 chetti contributed an analysis that clarified the ABNF productions 3934 that implicitly reference other documents. 3936 The EMAILCORE Working Group was chartered in September 2020 with 3937 Alexey Melnikov and Seth Blank as co-chairs. Todd Herr replaced Seth 3938 Blank early in 2021. Without their leadership and technical 3939 contributions, this document would never have been completed. 3941 10. References 3943 10.1. Normative References 3945 [1] Bradner, S., "Key words for use in RFCs to Indicate 3946 Requirement Levels", BCP 14, RFC 2119, 3947 DOI 10.17487/RFC2119, March 1997, 3948 . 3950 [2] American National Standards Institute (formerly United 3951 States of America Standards Institute), "USA Code for 3952 Information Interchange", ANSI X3.4-1968, 1968. ANSI 3953 X3.4-1968 has been replaced by newer versions with slight 3954 modifications, but the 1968 version remains definitive for 3955 the Internet. 3957 [3] Postel, J., "Simple Mail Transfer Protocol", STD 10, 3958 RFC 821, DOI 10.17487/RFC0821, August 1982, 3959 . 3961 [4] Mockapetris, P., "Domain names - implementation and 3962 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 3963 November 1987, . 3965 [5] Braden, R., Ed., "Requirements for Internet Hosts - 3966 Application and Support", STD 3, RFC 1123, 3967 DOI 10.17487/RFC1123, October 1989, 3968 . 3970 [6] Klensin, J., Freed, N., and K. Moore, "SMTP Service 3971 Extension for Message Size Declaration", STD 10, RFC 1870, 3972 DOI 10.17487/RFC1870, November 1995, 3973 . 3975 [7] Vaudreuil, G., "Enhanced Mail System Status Codes", 3976 RFC 3463, DOI 10.17487/RFC3463, January 2003, 3977 . 3979 [8] Newman, C., "ESMTP and LMTP Transmission Types 3980 Registration", RFC 3848, DOI 10.17487/RFC3848, July 2004, 3981 . 3983 [9] Klyne, G., Nottingham, M., and J. Mogul, "Registration 3984 Procedures for Message Header Fields", BCP 90, RFC 3864, 3985 DOI 10.17487/RFC3864, September 2004, 3986 . 3988 [10] Hinden, R. and S. Deering, "IP Version 6 Addressing 3989 Architecture", RFC 4291, DOI 10.17487/RFC4291, February 3990 2006, . 3992 [11] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 3993 Specifications: ABNF", STD 68, RFC 5234, 3994 DOI 10.17487/RFC5234, January 2008, 3995 . 3997 [12] Resnick, P., Ed., "Internet Message Format", RFC 5322, 3998 DOI 10.17487/RFC5322, October 2008, 3999 . 4001 10.2. Informative References 4003 [13] Crocker, D., "STANDARD FOR THE FORMAT OF ARPA INTERNET 4004 TEXT MESSAGES", STD 11, RFC 822, DOI 10.17487/RFC0822, 4005 August 1982, . 4007 [14] Butler, M., Postel, J., Chase, D., Goldberger, J., and J. 4008 Reynolds, "Post Office Protocol: Version 2", RFC 937, 4009 DOI 10.17487/RFC0937, February 1985, 4010 . 4012 [15] Postel, J. and J. Reynolds, "File Transfer Protocol", 4013 STD 9, RFC 959, DOI 10.17487/RFC0959, October 1985, 4014 . 4016 [16] Partridge, C., "Mail routing and the domain system", 4017 STD 10, RFC 974, DOI 10.17487/RFC0974, January 1986, 4018 . 4020 [17] Partridge, C., "Duplicate messages and SMTP", RFC 1047, 4021 DOI 10.17487/RFC1047, February 1988, 4022 . 4024 [18] Lambert, M., "PCMAIL: A distributed mail system for 4025 personal computers", RFC 1056, DOI 10.17487/RFC1056, June 4026 1988, . 4028 [19] Crispin, M., "Interactive Mail Access Protocol: Version 4029 2", RFC 1176, DOI 10.17487/RFC1176, August 1990, 4030 . 4032 [20] Durand, A. and F. Dupont, "SMTP 521 Reply Code", RFC 1846, 4033 DOI 10.17487/RFC1846, September 1995, 4034 . 4036 [21] Galvin, J., Murphy, S., Crocker, S., and N. Freed, 4037 "Security Multiparts for MIME: Multipart/Signed and 4038 Multipart/Encrypted", RFC 1847, DOI 10.17487/RFC1847, 4039 October 1995, . 4041 [22] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D. 4042 Crocker, "SMTP Service Extensions", STD 10, RFC 1869, 4043 DOI 10.17487/RFC1869, November 1995, 4044 . 4046 [23] Myers, J. and M. Rose, "Post Office Protocol - Version 3", 4047 STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996, 4048 . 4050 [24] De Winter, J., "SMTP Service Extension for Remote Message 4051 Queue Starting", RFC 1985, DOI 10.17487/RFC1985, August 4052 1996, . 4054 [25] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 4055 Extensions (MIME) Part One: Format of Internet Message 4056 Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996, 4057 . 4059 [26] Moore, K., "MIME (Multipurpose Internet Mail Extensions) 4060 Part Three: Message Header Extensions for Non-ASCII Text", 4061 RFC 2047, DOI 10.17487/RFC2047, November 1996, 4062 . 4064 [27] Kille, S., "MIXER (Mime Internet X.400 Enhanced Relay): 4065 Mapping between X.400 and RFC 822/MIME", RFC 2156, 4066 DOI 10.17487/RFC2156, January 1998, 4067 . 4069 [28] Elz, R. and R. Bush, "Clarifications to the DNS 4070 Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997, 4071 . 4073 [29] Freed, N. and K. Moore, "MIME Parameter Value and Encoded 4074 Word Extensions: Character Sets, Languages, and 4075 Continuations", RFC 2231, DOI 10.17487/RFC2231, November 4076 1997, . 4078 [30] Klensin, J., Ed., "Simple Mail Transfer Protocol", 4079 RFC 2821, DOI 10.17487/RFC2821, April 2001, 4080 . 4082 [31] Freed, N., "SMTP Service Extension for Command 4083 Pipelining", STD 60, RFC 2920, DOI 10.17487/RFC2920, 4084 September 2000, . 4086 [32] Freed, N., "Behavior of and Requirements for Internet 4087 Firewalls", RFC 2979, DOI 10.17487/RFC2979, October 2000, 4088 . 4090 [33] Vaudreuil, G., "SMTP Service Extensions for Transmission 4091 of Large and Binary MIME Messages", RFC 3030, 4092 DOI 10.17487/RFC3030, December 2000, 4093 . 4095 [34] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service 4096 Extension for Delivery Status Notifications (DSNs)", 4097 RFC 3461, DOI 10.17487/RFC3461, January 2003, 4098 . 4100 [35] Moore, K. and G. Vaudreuil, "An Extensible Message Format 4101 for Delivery Status Notifications", RFC 3464, 4102 DOI 10.17487/RFC3464, January 2003, 4103 . 4105 [36] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4106 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, 4107 . 4109 [37] Hansen, T., Ed. and A. Melnikov, Ed., "Message Disposition 4110 Notification", STD 85, RFC 8098, DOI 10.17487/RFC8098, 4111 February 2017, . 4113 [38] Schaad, J., Ramsdell, B., and S. Turner, "Secure/ 4114 Multipurpose Internet Mail Extensions (S/MIME) Version 4.0 4115 Message Specification", RFC 8551, DOI 10.17487/RFC8551, 4116 April 2019, . 4118 [39] Nakamura, M. and J. Hagino, "SMTP Operational Experience 4119 in Mixed IPv4/v6 Environments", RFC 3974, 4120 DOI 10.17487/RFC3974, January 2005, 4121 . 4123 [40] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 4124 Resource Identifier (URI): Generic Syntax", STD 66, 4125 RFC 3986, DOI 10.17487/RFC3986, January 2005, 4126 . 4128 [41] Gellens, R. and J. Klensin, "Message Submission for Mail", 4129 STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011, 4130 . 4132 [42] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. 4133 Thayer, "OpenPGP Message Format", RFC 4880, 4134 DOI 10.17487/RFC4880, November 2007, 4135 . 4137 [43] Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced 4138 Mail System Status Codes", BCP 138, RFC 5248, 4139 DOI 10.17487/RFC5248, June 2008, 4140 . 4142 [44] Klensin, J., Freed, N., Rose, M., and D. Crocker, Ed., 4143 "SMTP Service Extension for 8-bit MIME Transport", STD 71, 4144 RFC 6152, DOI 10.17487/RFC6152, March 2011, 4145 . 4147 [45] Klensin, J., "SMTP 521 and 556 Reply Codes", RFC 7504, 4148 DOI 10.17487/RFC7504, June 2015, 4149 . 4151 [46] Levine, J. and M. Delany, "A "Null MX" No Service Resource 4152 Record for Domains That Accept No Mail", RFC 7505, 4153 DOI 10.17487/RFC7505, June 2015, 4154 . 4156 [47] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 4157 DOI 10.17487/RFC5321, October 2008, 4158 . 4160 [48] Klensin, J.C., Ed., Murchison, K., Ed., and E. Sam, Ed., 4161 "Applicability Statement for IETF Core Email Protocols", 6 4162 August 2021, . 4165 [49] Internet Assigned Number Authority (IANA), "IANA Mail 4166 Parameters", 2007, 4167 . 4169 [50] Internet Assigned Number Authority (IANA), "Address 4170 Literal Tags", 2007, 4171 . 4173 [51] RFC Editor, "RFC Errata - RFC 5321", 2019, 4174 . Captured 4175 2019-11-19 4177 [52] IANA, "SMTP Service Extensions", 2021, 4178 . Notes in draft: RFC 4180 Editor: Please adjust date field to reflect whatever you 4181 want for a registry that is updated periodically. IANA: 4182 Please determine if the above URL is a sufficiently stable 4183 reference and adjust as appropriate if it is not. 4185 Appendix A. TCP Transport Service 4187 The TCP connection supports the transmission of 8-bit bytes. The 4188 SMTP data is 7-bit ASCII characters. Each character is transmitted 4189 as an 8-bit byte with the high-order bit cleared to zero. Service 4190 extensions may modify this rule to permit transmission of full 8-bit 4191 data bytes as part of the message body, or, if specifically designed 4192 to do so, in SMTP commands or responses. 4194 Appendix B. Generating SMTP Commands from RFC 822 Header Fields 4196 Some systems use an RFC 822 header section (only) in a mail 4197 submission protocol, or otherwise generate SMTP commands from RFC 822 4198 header fields when such a message is handed to an MTA from a UA. 4199 While the MTA-UA protocol is a private matter, not covered by any 4200 Internet Standard, there are problems with this approach. For 4201 example, there have been repeated problems with proper handling of 4202 "bcc" copies and redistribution lists when information that 4203 conceptually belongs to the mail envelope is not separated early in 4204 processing from header field information (and kept separate). 4206 It is recommended that the UA provide its initial ("submission 4207 client") MTA with an envelope separate from the message itself. 4208 However, if the envelope is not supplied, SMTP commands SHOULD be 4209 generated as follows: 4211 1. Each recipient address from a TO, CC, or BCC header field SHOULD 4212 be copied to a RCPT command (generating multiple message copies 4213 if that is required for queuing or delivery). This includes any 4214 addresses listed in a RFC 822 "group". Any BCC header fields 4215 SHOULD then be removed from the header section. Once this 4216 process is completed, the remaining header fields SHOULD be 4217 checked to verify that at least one TO, CC, or BCC header field 4218 remains. If none do, then a BCC header field with no additional 4219 information SHOULD be inserted as specified in [12]. 4221 2. The return address in the MAIL command SHOULD, if possible, be 4222 derived from the system's identity for the submitting (local) 4223 user, and the "From:" header field otherwise. If there is a 4224 system identity available, it SHOULD also be copied to the Sender 4225 header field if it is different from the address in the From 4226 header field. (Any Sender header field that was already there 4227 SHOULD be removed.) Systems may provide a way for submitters to 4228 override the envelope return address, but may want to restrict 4229 its use to privileged users. This will not prevent mail forgery, 4230 but may lessen its incidence; see Section 7.1. 4232 When an MTA is being used in this way, it bears responsibility for 4233 ensuring that the message being transmitted is valid. The mechanisms 4234 for checking that validity, and for handling (or returning) messages 4235 that are not valid at the time of arrival, are part of the MUA-MTA 4236 interface and not covered by this specification. 4238 A submission protocol based on Standard RFC 822 information alone 4239 MUST NOT be used to gateway a message from a foreign (non-SMTP) mail 4240 system into an SMTP environment. Additional information to construct 4241 an envelope must come from some source in the other environment, 4242 whether supplemental header fields or the foreign system's envelope. 4244 Attempts to gateway messages using only their header "To" and "Cc" 4245 fields have repeatedly caused mail loops and other behavior adverse 4246 to the proper functioning of the Internet mail environment. These 4247 problems have been especially common when the message originates from 4248 an Internet mailing list and is distributed into the foreign 4249 environment using envelope information. When these messages are then 4250 processed by a header-section-only remailer, loops back to the 4251 Internet environment (and the mailing list) are almost inevitable. 4253 Appendix C. Placeholder (formerly Source Routes) 4255 // This entire section has been removed, with some material moved 4256 // into Appendix F.2. This comment is retained as a temporary 4257 // placeholder because the WG, the Ticket list, and various email 4258 // threads refer to Appendix letters and it would not be good to 4259 // create confusion about that while rfc5321bis is under development. 4261 Appendix D. Scenarios 4263 This section presents complete scenarios of several types of SMTP 4264 sessions. In the examples, "C:" indicates what is said by the SMTP 4265 client, and "S:" indicates what is said by the SMTP server. 4267 D.1. A Typical SMTP Transaction Scenario 4269 This SMTP example shows mail sent by Smith at host bar.com, and to 4270 Jones, Green, and Brown at host foo.com. Here we assume that host 4271 bar.com contacts host foo.com directly. The mail is accepted for 4272 Jones and Brown. Green does not have a mailbox at host foo.com. 4274 S: 220 foo.com Simple Mail Transfer Service Ready 4275 C: EHLO bar.com 4276 S: 250-foo.com greets bar.com 4277 S: 250-8BITMIME 4278 S: 250-SIZE 4279 S: 250-DSN 4280 S: 250 HELP 4281 C: MAIL FROM: 4282 S: 250 OK 4283 C: RCPT TO: 4284 S: 250 OK 4285 C: RCPT TO: 4286 S: 550 No such user here 4287 C: RCPT TO: 4288 S: 250 OK 4289 C: DATA 4290 S: 354 Start mail input; end with . 4291 C: Blah blah blah... 4292 C: ...etc. etc. etc. 4293 C: . 4294 S: 250 OK 4295 C: QUIT 4296 S: 221 foo.com Service closing transmission channel 4298 D.2. Aborted SMTP Transaction Scenario 4299 S: 220 foo.com Simple Mail Transfer Service Ready 4300 C: EHLO bar.com 4301 S: 250-foo.com greets bar.com 4302 S: 250-8BITMIME 4303 S: 250-SIZE 4304 S: 250-DSN 4305 S: 250 HELP 4306 C: MAIL FROM: 4307 S: 250 OK 4308 C: RCPT TO: 4309 S: 250 OK 4310 C: RCPT TO: 4311 S: 550 No such user here 4312 C: RSET 4313 S: 250 OK 4314 C: QUIT 4315 S: 221 foo.com Service closing transmission channel 4317 D.3. Relayed Mail Scenario 4319 Step 1 -- Source Host to Relay Host 4320 The source host performs a DNS lookup on XYZ.COM (the destination 4321 address) and finds DNS MX records specifying xyz.com as the best 4322 preference and foo.com as a lower preference. It attempts to open a 4323 connection to xyz.com and fails. It then opens a connection to 4324 foo.com, with the following dialogue: 4326 S: 220 foo.com Simple Mail Transfer Service Ready 4327 C: EHLO bar.com 4328 S: 250-foo.com greets bar.com 4329 S: 250-8BITMIME 4330 S: 250-SIZE 4331 S: 250-DSN 4332 S: 250 HELP 4333 C: MAIL FROM: 4334 S: 250 OK 4335 C: RCPT TO: 4336 S: 250 OK 4337 C: DATA 4338 S: 354 Start mail input; end with . 4339 C: Date: Thu, 21 May 1998 05:33:29 -0700 4340 C: From: John Q. Public 4341 C: Subject: The Next Meeting of the Board 4342 C: To: Jones@xyz.com 4343 C: 4344 C: Bill: 4345 C: The next meeting of the board of directors will be 4346 C: on Tuesday. 4347 C: John. 4348 C: . 4349 S: 250 OK 4350 C: QUIT 4351 S: 221 foo.com Service closing transmission channel 4353 Step 2 -- Relay Host to Destination Host 4354 foo.com, having received the message, now does a DNS lookup on 4355 xyz.com. It finds the same set of MX records, but cannot use the one 4356 that points to itself (or to any other host as a worse preference). 4357 It tries to open a connection to xyz.com itself and succeeds. Then 4358 we have: 4360 S: 220 xyz.com Simple Mail Transfer Service Ready 4361 C: EHLO foo.com 4362 S: 250 xyz.com is on the air 4363 C: MAIL FROM: 4364 S: 250 OK 4365 C: RCPT TO: 4366 S: 250 OK 4367 C: DATA 4368 S: 354 Start mail input; end with . 4369 C: Received: from bar.com by foo.com ; Thu, 21 May 1998 4370 C: 05:33:29 -0700 4371 C: Date: Thu, 21 May 1998 05:33:29 -0700 4372 C: From: John Q. Public 4373 C: Subject: The Next Meeting of the Board 4374 C: To: Jones@xyz.com 4375 C: 4376 C: Bill: 4377 C: The next meeting of the board of directors will be 4378 C: on Tuesday. 4379 C: John. 4380 C: . 4381 S: 250 OK 4382 C: QUIT 4383 S: 221 xyz.com Service closing transmission channel 4385 D.4. Verifying and Sending Scenario 4386 S: 220 foo.com Simple Mail Transfer Service Ready 4387 C: EHLO bar.com 4388 S: 250-foo.com greets bar.com 4389 S: 250-8BITMIME 4390 S: 250-SIZE 4391 S: 250-DSN 4392 S: 250-VRFY 4393 S: 250 HELP 4394 C: VRFY Crispin 4395 S: 250 Mark Crispin 4396 C: MAIL FROM: 4397 S: 250 OK 4398 C: RCPT TO: 4399 S: 250 OK 4400 C: DATA 4401 S: 354 Start mail input; end with . 4402 C: Blah blah blah... 4403 C: ...etc. etc. etc. 4404 C: . 4405 S: 250 OK 4406 C: QUIT 4407 S: 221 foo.com Service closing transmission channel 4409 Appendix E. Other Gateway Issues 4411 In general, gateways between the Internet and other mail systems 4412 SHOULD attempt to preserve any layering semantics across the 4413 boundaries between the two mail systems involved. Gateway- 4414 translation approaches that attempt to take shortcuts by mapping 4415 (such as mapping envelope information from one system to the message 4416 header section or body of another) have generally proven to be 4417 inadequate in important ways. Systems translating between 4418 environments that do not support both envelopes and a header section 4419 and Internet mail must be written with the understanding that some 4420 information loss is almost inevitable. 4422 Appendix F. Deprecated Features of RFC 821 4424 A few features of RFC 821 have proven to be problematic and SHOULD 4425 NOT be used in Internet mail. Some of these features were deprecated 4426 in RFC 2821 in 2001; source routing and two-digit years in dates were 4427 deprecated even earlier, by RFC 1123 in 1989. Of the domain literal 4428 forms, RFC 1123 required support only for the dotted decimal form. 4429 With the possible exception of old, hardware-embedded, applications, 4430 there is no longer any excuse for these features to appear on the 4431 contemporary Internet. 4433 F.1. TURN 4435 This command, described in RFC 821, raises important security issues 4436 since, in the absence of strong authentication of the host requesting 4437 that the client and server switch roles, it can easily be used to 4438 divert mail from its correct destination. Its use is deprecated; 4439 SMTP systems SHOULD NOT use it unless the server can authenticate the 4440 client. 4442 F.2. Source Routing 4444 RFC 821 utilized the concept of explicit source routing to get mail 4445 from one host to another via a series of relays. Source routes could 4446 appear in either the or to show the 4447 hosts through which mail would be routed to reach the destination. 4448 The requirement to utilize source routes in regular mail traffic was 4449 eliminated by the introduction of the domain name system "MX" record 4450 by RFC 974 in early 1986 and the last significant justification for 4451 them was eliminated by the introduction, in RFC 1123, of a clear 4452 requirement that addresses following an "@" must all be fully- 4453 qualified domain names. Issues involving local aliases for mailboxes 4454 were addressed by the introduction of a separate specification for 4455 mail submission [41]. Consequently, there are no remaining 4456 justifications for the use of source routes other than support for 4457 very old SMTP clients. Even use in mail system debugging is unlikely 4458 to work because almost all contemporary systems either ignore or 4459 reject them. 4461 Historically, for relay purposes, the forward-path may have been a 4462 source route of the form "@ONE,@TWO:JOE@THREE", where ONE, TWO, and 4463 THREE MUST be fully-qualified domain names. This form was used to 4464 emphasize the distinction between an address and a route. The 4465 mailbox (here, JOE@THREE) is an absolute address, and the route is 4466 information about how to get there. The two concepts should not be 4467 confused. 4469 SMTP servers MAY continue to accept source route syntax as specified 4470 in this appendix. If they do so, they SHOULD ignore the routes and 4471 utilize only the target domain in the address. If they do utilize 4472 the source route, the message MUST be sent to the first domain shown 4473 in the address. 4475 In particular, a server MUST NOT guess at shortcuts within the source 4476 route. 4478 SMTP clients MUST NOT attempt to utilize explicit source routing. 4480 If source routes appear in mail received by an SMTP server contrary 4481 to the requirements and recommendations in this specification, RFC 4482 821 and the text below should be consulted for the mechanisms for 4483 constructing and updating the forward-path. A server that is reached 4484 by means of a source route (e.g., its domain name appears first in 4485 the list in the forward-path) MUST remove its domain name from any 4486 forward-paths in which that domain name appears before forwarding the 4487 message and MAY remove all other source routing information. Any 4488 source route information in the reverse-path SHOULD be removed by 4489 servers conforming to this specification. 4491 The following information is provided for historical information so 4492 that the source route syntax and application can be understood if 4493 needed. 4495 Syntax: 4496 The original form of the production in Section 4.1.2 was: 4498 Path = "<" [ A-d-l ":" ] Mailbox ">" 4500 A-d-l = At-domain *( "," At-domain ) 4502 At-domain = "@" Domain 4504 For example, suppose that a delivery service notification must be 4505 sent for a message that arrived with: 4506 MAIL FROM:<@a.example,@b.example:user@d.example> 4507 The notification message MUST be sent using: 4508 RCPT TO: 4510 F.3. HELO 4512 As discussed in Sections 3.1 and 4.1.1, EHLO SHOULD be used rather 4513 than HELO when the server will accept the former. Servers MUST 4514 continue to accept and process HELO in order to support older 4515 clients. 4517 F.4. #-literals 4519 RFC 821 provided for specifying an Internet address as a decimal 4520 integer host number prefixed by a pound sign, "#". In practice, that 4521 form has been obsolete since the introduction of TCP/IP. It is 4522 deprecated and MUST NOT be used. 4524 F.5. Dates and Years 4526 When dates are inserted into messages by SMTP clients or servers 4527 (e.g., in trace header fields), four-digit years MUST BE used. Two- 4528 digit years are deprecated; three-digit years were never permitted in 4529 the Internet mail system. 4531 F.6. Sending versus Mailing 4533 In addition to specifying a mechanism for delivering messages to 4534 user's mailboxes, RFC 821 provided additional, optional, commands to 4535 deliver messages directly to the user's terminal screen. These 4536 commands (SEND, SAML, SOML) were rarely implemented, and changes in 4537 workstation technology and the introduction of other protocols may 4538 have rendered them obsolete even where they are implemented. 4540 Clients SHOULD NOT use SEND, SAML, or SOML commands. If a server 4541 implements them, the implementation model specified in RFC 821 [3] 4542 MUST be used and the command names MUST be published in the response 4543 to the EHLO command. 4545 Appendix G. Other Outstanding Issues 4547 [[RFC Editor: Please remove this section before publication.]] 4549 In December 2019, an issue was raised on the ietf-smtp@ietf.org list 4550 that led to a broad discussion of ways in which existing practice had 4551 diverged from the specifications and recommendations of RFC 5321 in 4552 the more than eleven years since it was published (some of those 4553 issues probably affect the boundary between RFC 5321 and 5322 and 4554 hence the latter as well). In most cases, those divergences call for 4555 revision of the Technical Specification to match the practice, 4556 clarification of the specification text in other ways, or a more 4557 comprehensive explanation of why the practices recommended by the 4558 specification should really be followed. 4560 Those discussions raised two other issues, which were that 4562 * The publication of the Submission Server specification of RFC 6409 4563 in November 2011 may not have been fully reflected in RFC 5321 4564 (despite the even earlier publication of RFC 4409) and 4566 * There may be inconsistencies between the July 2009 Internet Mail 4567 Architecture description of RFC 5598 and the model described in 4568 RFC 5321. The issue called out in Appendix G.3 below may be an 4569 example of one of those inconsistencies. 4571 Those discrepancies should be identified and discussed and decisions 4572 made to fix them (and where) or to ignore them and let them continue. 4574 There has also been discussion on the mailing list, perhaps amounting 4575 to very rough consensus, that any revision of RFC 5321 and/or 5322 4576 should be accompanied by a separate Applicability Statement document 4577 that would make recommendations about applicability or best practices 4578 in particular areas rather than trying to get everything into the two 4579 technical specifications. This appendix does not attempt to identify 4580 which issues should get which treatment. 4582 This work is now (starting in the last half of 2020) being considered 4583 in the EMAILCORE WG. This appendix will act as a temporary record of 4584 issues that should be discussed and decided upon before a revised 4585 SMTP specification (or a related Applicability Statement) is 4586 published, issues that have not been reflected in errata (see 4587 Appendix H.1 below for those covered by errata). 4589 Ticket numbers listed below reference the list in 4590 https://trac.ietf.org/trac/emailcore/report/1 . 4592 G.1. IP Address literals 4594 The specification is unclear about whether IP address literals, 4595 particularly IP address literals used as arguments to the EHLO 4596 command, are required to be accepted or whether they are allowed to 4597 be rejected as part of the general "operational necessity" exception. 4598 Some have suggested that rejection of them is so common as an anti- 4599 spam measure that the use of such literals should be deprecated 4600 entirely in the specification, others that the are still useful and 4601 used and/or that, whatever is said about IP address literals within 4602 an SMTP session (e.g., in MAIL or RCPT commands), they should 4603 continue to be allowed (and required) in EHLO. 4604 Ticket #1 (issue for A/S). 4606 G.2. Repeated Use of EHLO (closed) 4608 While the specification says that an SMTP client's sending EHLO again 4609 after it has been issued (starting an SMTP session and treats it as 4610 if RSET had been sent (closing the session) followed by EHLO, there 4611 are apparently applications, at least some of them involving setting 4612 up of secure connections, in which the second EHLO is required and 4613 does not imply RSET. Does the specification need to be adjusted to 4614 reflect or call out those cases? 4616 After extended discussion in October 2020, it appears that the 4617 easiest fix to these problems is to clarify the conditions for 4618 termination of a mail transaction in Section 3.3 and to clearly 4619 specify the effect of a second (or subsequent) EHLO command in 4620 Section 4.1.4. 4621 See also Appendix G.7.4. 4622 Ticket #2. (closed - Both changes have been made in draft-ietf- 4623 emailcore-rfc5321bis-01). 4625 G.3. Meaning of "MTA" and Related Terminology 4627 A terminology issue has come up about what the term "MTA" actually 4628 refers to, a question that became at least slightly more complicated 4629 when we formalized RFC 6409 Submission Servers. Does the document 4630 need to be adjusted to be more clear about this topic? Note that the 4631 answer may interact with the question asked in Section 2 above. 4632 Possibly along the same lines, RFC 2821 changed the RFC 821 4633 terminology from "sender-SMTP" and "receiver-SMTP" to "SMTP client" 4634 and "SMTP server" respectively. As things have evolved, it is 4635 possible that newer terminology is a source of confusion and that the 4636 terminology should be changed back, something that also needs 4637 discussion. 4638 Ticket #3. 4640 G.4. Originator, or Originating System, Authentication 4642 Should RFC 5321bis address authentication and related issues or 4643 should Section 3.4.2 or other text be reshaped (in addition to or 4644 instead of the comment on that section) to lay a better foundation 4645 for such work, either in the context of mailing lists or more 4646 generally? 4647 This may interact with Erratum 4055 and Ticket #30 below. 4649 G.5. Remove or deprecate the work-around from code 552 to 452 (closed) 4651 The suggestion in Section 4.5.3.1.10 may have outlived its usefulness 4652 and/or be inconsistent with current practice. Should it be removed 4653 and/or explicitly deprecated? 4654 Ticket #5 (fixed and closed). 4656 SHOULD requirement removed. 4658 G.6. Clarify where the protocol stands with respect to submission and 4659 TLS issues 4661 1. submission on port 587 4663 2. submission on port 465 4665 3. TLS relay on a port different from 25 (whenever) 4666 4. Recommendations about general use of transport layer (hop by hop) 4667 security, particularly encryption including consideration of RFC 4668 8314. 4670 G.7. Probably-substantive Discussion Topics Identified in Other Ways 4672 The following issues were identified as a group in the opening Note 4673 but called out specifically only in embedded CREF comments in 4674 versions of this draft prior to the first EMAILCORE version. 4676 G.7.1. Issues with 521, 554, and 556 codes (closed) 4678 See new Section 4.2.4.2. More text may be needed, there or 4679 elsewhere, about choices of codes in response to initial opening and 4680 to EHLO, especially to deal with selective policy rejections. In 4681 particular, should we more strongly discourage the use of 554 on 4682 initial opening. And should we make up a 421 code (or a new 4yz 4683 code, perhaps 454) code for situations where the server is 4684 temporarily out of service? 4685 Ticket #6 (closed). 4687 G.7.2. SMTP Model, terminology, and relationship to RFC 5598 4689 CREF comment in Section 2, CREF comment in Section 2.3.10, and 4690 comments in the introductory portion of Appendix G. 4692 G.7.3. Resolvable FQDNs and private domain names 4694 Multiple CREF comments in Section 2.3.5 4695 Tickets #9 (definition of domain name), #10 (meaning of "resolvable 4696 domain name"; closed 2021-06-12), and #41 (closed -- no change 4697 2021-04-05). 4699 G.7.4. Possible clarification about mail transactions and transaction 4700 state 4702 CREF comment in Section 3.3 and also reference in Section 4.1.4 4703 Ticket #11. 4705 // See correspondence on this ticket 2021-07-06 through 2021-07-09. 4707 G.7.5. Issues with mailing lists, aliases, and forwarding 4709 CREF comment in Section 3.4.2. May also want to note forwarding as 4710 an email address portability issue. Note that, if changes are made 4711 in this area, they should be kept consistent with the description and 4712 discussion of the 251 and 551 in Section 4.2 and Section 3.5 as well 4713 as Section 3.4.1 to avoid introducing inconsistencies. In addition, 4714 there are some terminology issues about the use of the term "lists", 4715 identified in erratum 1820, that should be reviewed after any more 4716 substantive changes are made to the relevant sections. 4717 Ticket #12 and Ticket #34 (Ticket #34/ erratum 1820 resolved in -06 4718 and closed). 4720 G.7.6. Requirements for domain name and/or IP address in EHLO 4722 Text in Section 4.1.4; change made in -05. 4723 Ticket #19 (was ticket #47 -- done in rfc5321bis, more in A/S). 4725 G.7.7. Does the 'first digit only' and/or non-listed reply code text 4726 need clarification? (closed) 4728 Resolved. Text in Section 4.2 changed 2021-02-08 and CREF comment in 4729 Section 4.3.1 removed. 4731 Perhaps unresolved -- ongoing discussion on mailing list after IETF 4732 110. 4733 Ticket #13 (fixed and closed). 4735 G.7.8. Size limits (closed) 4737 Once a decision is made about line length rules for RFC 5322bis, 4738 review the size limit discussions in this document, particularly the 4739 CREF comment (Note in Draft) at the end of the introductory material 4740 to Section 4.5.3 to be sure this document says what we want it to 4741 say. (See the additional question about minimum quantities, etc., in 4742 Appendix G.7.19.) 4743 Ticket #14 (closed - no action) and maybe Ticket #38 (to A/S). 4745 G.7.9. Discussion of 'blind' copies and RCPT 4747 CREF comment in Section 7.2. May also need to discussion whether 4748 that terminology is politically incorrect and suggest a replacement. 4749 Ticket #15. 4751 G.7.10. Further clarifications needed to source routes? 4753 The current text largely deprecates the use of source routes but 4754 suggests that servers continue to support them. 4755 Ticket #17 (Closed 20220125). 4757 G.7.11. Should 1yz Be Revisited? (closed) 4759 RFC 5321 depreciated the "positive preliminary reply" response code 4760 category with first digit "1", so that the first digit of valid SMTP 4761 response codes must be 2, 3, 4, or 5. It has been suggested (see 4762 mail from Hector Santos with Subject "SMTP Reply code 1yz Positive 4763 Preliminary reply", March 5, 2020 12:56 -0500, on the SMTP list) that 4764 these codes should be reinstated to deal with some situations that 4765 became more plausible after 5321 was published. Do we need to take 4766 this up again? 4767 Ticket #18 (no, closed). 4769 G.7.12. Review Timeout Specifications 4771 RFC 5321 (and its predecessors going back to 821) specify minimum 4772 periods for client and server to wait before timing out. Are those 4773 intervals still appropriate in a world of faster processors and 4774 faster networks? Should they be updated and revised? Or should more 4775 qualifying language be added? 4776 Ticket #16. 4778 G.7.13. Possible SEND, SAML, SOML Loose End (closed) 4780 Per discussion (and Ticket #20), the text about SEND, SAML, and SOML 4781 has been removed from the main body of the document so that the only 4782 discussion of them now appears in Appendix F.6. Per the editor's 4783 note in that appendix, is any further discussion needed? 4784 Ticket #20 (closed) 4786 G.7.14. Abstract Update (closed) 4788 Does the Abstract need to be modified in the light of RFC 6409 or 4789 other changes? 4790 Ticket #52 (changes made; closed) 4792 G.7.15. Informative References to MIME and/or Message Submission 4793 (closed) 4795 Should RFC 2045 (MIME) and/or RFC 6409 (Message Submission) be 4796 referenced at the end of Section 1.2? 4797 Ticket #53 (more general reference to the A/S, closed). 4799 G.7.16. Mail Transaction Discussion 4801 Does the discussion of mail transactions need more work (see CREF in 4802 Section 3.3.)? 4804 G.7.17. Hop by hop Authentication and/or Encryption (closed) 4806 Should this document discuss hop-by-hop authentication or, for that 4807 matter, encryption? (See CREF in Section 2.) 4808 Propose "No, it shouldn't" (20211101 conversation with Todd, 4809 reaffirmed 20220121 plenary) 4810 Ticket #50 (work with in A/S. Closed). 4812 G.7.18. More Text About 554 Given 521, etc. 4814 Does reply code 554 need additional or different explanation in the 4815 light of the addition of the new 521 code and/or the new (in 5321bis 4816 Section 4.2.4.2? (See CREF in Section 4.2.3.) 4818 G.7.19. Minimum Lengths and Quantities 4820 Are the minimum lengths and quantities specified in Section 4.5.3 4821 still appropriate or do they need adjusting? (See CREF at the 4822 beginning of that section.) Also note potential interaction with the 4823 proposed LIMITS SMTP extension (draft-freed-smtp-limits) which may 4824 make this question OBE. 4826 G.8. Enhanced Reply Codes and DSNs 4828 Enhanced Mail System Status Codes (RFC 3463) [7] were added to SMTP 4829 before RFC 5321 was published and are now, together with a 4830 corresponding registry [43], widely deployed and in extensive use in 4831 the network. Similar, the structure and extensions options for 4832 Delivery Status Notifications [35] is implemented, deployed, and in 4833 wide use. Is it time to fold all or part of those mature 4834 specifications into the SMTP spec or at least to mention and 4835 normatively reference them? And, as an aside, do those specs need 4836 work or, if they are kept separate, is it time to move them to 4837 Internet Standard? 4839 At least one of the current references to RFC 3463 indicates that it 4840 SHOULD be used. That presumably makes the reference normative 4841 because one needs that specification to know what the present 4842 document requires. It has been moved in the -03 version of this 4843 draft, but, unless it is move to Internet Standard, it will require 4844 downref treatment. 4846 G.9. Revisiting Quoted Strings 4848 Recent discussions both in and out of the IETF have highlighted 4849 instances of non-compliance with the specification of a Local-part 4850 consisting of a Quoted-string, whether any content of QcontentSMTP 4851 that actually requires special treatment consists of qtextSMTP, 4852 quoted-pairSMTP, or both. Section 4.1.2 (of RFC 5321, repeated 4853 above) ends with a few paragraphs of warnings (essentially a partial 4854 applicability statement), the first of which cautions against 4855 cleverness with either Quoted-string or case sensitivity as a threat 4856 to interoperability. 4858 The Quoted-string portion of that discussion has apparently been 4859 widely not read or ignored. Do we need to do something else? If we 4860 do an Applicability Statement, would it be useful to either reference 4861 the discussion in this document from there or to move the discussion 4862 there and reference it (normatively?) from here? 4864 There has been a separate discussion of empty quoted strings in 4865 addresses, i.e., whether the production should be 4866 required to included at least one non-whitespace character. It is 4867 separate from this issue but would be further impacted or distorted 4868 from the considerations identified in this Section. 4870 Text modified in -07. 4871 Ticket #21. May also interact with Ticket #35. 4873 G.10. Internationalization 4875 RFC 5321 came long before work on internationalization of email 4876 addresses and headers (other than by use of encoded words in MINE) 4877 and specifically before the work of the EAI WG leading to the 4878 SMTPUTF8 specifications, specifically RFCs 6530ff. The second 4879 explanatory paragraph at the end of Section 4.1.2 ("Systems MUST NOT 4880 define mailboxes ...") is an extremely strong prohibition against the 4881 use of non-ASCII characters in SMTP commands and the requirements 4882 about message content in Section 2.3.1 an equally strong one for 4883 content. Would it be appropriate to add something like "in the 4884 absence of relevant extensions" there? Also, given [mis]behavior 4885 seen in the wild, does that paragraph (or an A/S) need an explicit 4886 caution about SMTP servers or clients assuming they can apply the 4887 popular web convention of using %NN sequences as a way to encode non- 4888 ASCII characters ( in RFC 3986) and assuming some later 4889 system will interpret it as they expect? Would it be appropriate to 4890 add an Internationalization Considerations section to the body of 4891 this document if only for the purpose of pointing people elsewhere? 4892 More broadly, while the EAI WG's extensions for non-ASCII headers and 4893 addresses are explicitly out of scope for the EMAILCORE WG (at least 4894 for 5321bis (and 5322bis), those documents make assumptions and 4895 interpretations of the core documents. Are there areas in which 4896 5321bis could and should be clarified to lay a more solid foundation 4897 for the EAI/SMTPUTF8 work and, if so, what are they? 4899 G.11. SMTP Clients, Servers, Senders, and Receivers 4901 RFC 821 used the terms "SMTP-sender" and "SMTP-receiver". In RFC 4902 2821 (and hence in 5321), we switched that to "client" and "server" 4903 (See the discussion in Section 1.2). In part because a relay is a 4904 server and then a client (in some recent practice, even interleaving 4905 the two functions by opening the connection to the next host in line 4906 and sending commands before the incoming transaction is complete), 4907 RFC 5321 continues to use the original terminology in some places. 4908 Should we revisit that usage, possibly even returning to consistent 4909 use of the original terminology? 4911 G.12. Extension Keywords Starting in 'X-' (closed) 4913 Section 2.2.2 contains a discussion of SMTP keywords starting in "X". 4914 Given general experience with such things and RFC 6648, is there any 4915 reason to not deprecate that practice entirely and remove that text? 4916 If we do so, should the former Section 4.1.5 be dropped or rewritten 4917 to make clear this is an obsolete practice? 4918 4.1.5 eliminated in rfc5321bis-06. 4919 Ticket #42 (resolved with -06 and closed). 4921 G.13. Deprecating HELO (closed) 4923 RFC 5321 (and 2821 before it) very carefully circle around the status 4924 of HELO, even recommending its use as a fallback when EHLO is sent 4925 and a "command not recognized" response is received. We are just a 4926 few months short of 20 years; is it time to deprecate the thing and 4927 clean out some or all of that text? And, given a recent (4Q2020) 4928 discussion on the EMAILCORE list, should EHLO be explicitly bound to 4929 SMTP over TCP with the older transports allowed only with HELO? 4930 While those questions may seem independent, separating them is fairly 4931 hard given the way the text is now constructed. 4933 Resolved 2021-01-19: No change 4934 Ticket #43 (closed). 4936 G.14. The FOR Clause in Trace Fields: Semantics, Security 4937 Considerations, and Other Issues 4939 The FOR clause in time-stamp ("Received:") fields is seriously under- 4940 defined. It is optional, the syntax is clear, but its semantics and 4941 use, while perhaps obvious from content and the application of common 4942 sense, have never been defined ("never" going back to 821). Do we 4943 want to better define it? Is there any chance that a definition 4944 would invalid existing, conforming and sensible, implementations? If 4945 we do want to define semantics, draft text and advice as to where it 4946 should go are invited. 4948 (Paragraph added 2021-08-18) 4949 In particular, recentdiscussions point strongly to the need for a 4950 statement to the effect that the value of the for clause must contain 4951 one of the addresses that caused the message to be routed to the 4952 recipient of this message copy (thanks Ned), that no mare than one 4953 address can appear, and that showing one address when there are 4954 multiple RCPT commands may be a security and/or privacy issue (thanks 4955 Ned and Viktor and see ). More detailed or specific 4957 guidance, including case analysis, are probably material for the A/s, 4958 but that is obviously up to the WG. 4960 Note the existing discussions in Section 7.2 and Section 7.6 as they 4961 may need adjustment, or at least cross-references, especially if FOR 4962 is more precisely defined. 4964 There is probably an error in Section 7.6. Its last sentence implies 4965 a possible interaction between messages with multiple recipients and 4966 the FOR clause of trace fields. However, because the syntax of the 4967 FOR clause only allows one Mailbox (or Path), it isn't clear if that 4968 statement is meaningful. Should it be revised to discuss other 4969 situations in which including FOR might not be desirable from a 4970 security or privacy standpoint? (See above -- this paragraph 4971 deliberately not changed in -04). 4972 Ticket #55 4974 G.15. Resistance to Attacks and Operational Necessity (closed) 4976 Section 7.8 is often cited as allowing an exception to the rules of 4977 the specification for reasons of operational necessity, not just 4978 attack resistance. I (JcK) believe the broader interpretation was 4979 intended by YAM (the section was new in RFC 5321). Recommendation: 4980 change the title to explicitly include "Local Operational 4981 Requirements" and add text to indicate that attack resistance is not 4982 the only possible source of such requirements. 4983 Ticket #48 (done, closed) 4985 G.16. Mandatory 8BITMIME 4987 There was extensive discussion on the mailing list in October 2021 4988 about messages with and without 8-bit (i.e., octets with the leading 4989 on) content and a tentative conclusion that support for 8BITMIME 4990 should be required. If that is the WG's conclusion, we need to 4991 figure out what to say and where to say it. 4993 Appendix H. RFC 5321 Errata Summary and Tentative Change Log 4995 [[RFC Editor: Please remove this section before publication.]] 4997 H.1. RFC 5321 Errata Summary 4999 This document addresses the following errata filed against RFC 5321 5000 since its publication in October 2008 [51]. As with the previous 5001 appendix, ticket numbers included below reference 5002 https://trac.ietf.org/trac/emailcore/report/1 . 5003 // [[Note in Draft: Unless marked "closed", items with comments below 5004 // have not yet been resolved as errata.]] 5006 1683 ABNF error. (closed) Section 4.4 5007 Ticket #23 (fixed, closed). 5009 4198 Description error. (closed) Section 4.2. 5010 RESOLVED 2020-12-14, ticket #24 (closed). 5012 2578 Syntax description error. (closed) Section 4.1.2 5013 Ticket #25 (fixed, closed) 5015 1543 Wrong code in description (closed) Section 3.8 5016 Ticket #26 (fixed, closed) 5018 4315 ABNF - IPv6 Section 4.1.3 (closed). 5019 // [5321bis]The IPv6 syntax has been adjusted since 5321 was 5020 // published (the erratum mentions RFC 5952, but RFC 6874 and 5021 draft- 5022 // carpenter-6man-rfc6874bis should also be considered). See the 5023 // rewritten form and the comment in the section cited in the 5024 // previous sentence, at least for the RFC 5952 issues. The 5025 editor 5026 // awaits instructions. See https://www.rfc-editor.org/errata/ 5027 // eid4315 5028 Ticket #27 (closed 2021-01-19). 5030 5414 ABNF for Quoted-string (closed) Section 4.1.2 5031 Ticket #22 (fixed, closed). 5033 1851 Location of text on unexpected close Section 4.1.1.5 (closed). 5034 Text moved per email 2020-12-31. 5035 Ticket #28 (fixed, closed). 5037 3447 Use of normative language (e.g., more "MUST"s), possible 5038 confusion in some sections Section 4.4. 5039 Ticket #7 5041 // [5321bis]As Barry notes in his verifier comments on the erratum 5042 // (see https://www.rfc-editor.org/errata/eid3447), the comments 5043 and 5044 // suggestions here raise a number of interesting (and difficult) 5045 // issues. One of the issues is that the core of RFCs 5321 (and 5046 // 2821) is text carried over from Jon Postel's RFC 821, a 5047 document 5048 // that was not only written in a different style than the IETF 5049 uses 5050 // today but that was written at a time when no one had dreamt of 5051 RFC 5052 // 2119 or even the IETF itself. It appears to me that trying to 5053 // patch that style might easily result in a document that is 5054 harder 5055 // to read as well as being error prone. If we want to get the 5056 // document entirely into contemporary style, we really should 5057 bite 5058 // the bullet and do a complete rewrite. To respond to a 5059 different 5060 // point in Barry's discussion, I think an explicit statement that 5061 // 5321/5322 and their predecessors differ in places and why would 5062 be 5063 // helpful. Text, and suggestions about where to put it, are 5064 // solicited. A list of differences might be a good idea too, but 5065 // getting it right might be more work than there is available 5066 energy 5067 // to do correctly. 5069 5711 Missing leading spaces in example Appendix D.3 (closed). 5071 // [5321bis]Well, this is interesting because the XML is correct 5072 and 5073 // the spaces are there, embedded in artwork. So either the 5074 XML2RFC 5075 // processor at the time took those leading spaces out or the RFC 5076 // Editor improved on the document and the change was not caught 5077 in 5078 // AUTH48, perhaps because rfcdiff ignores white space. We just 5079 need 5080 // to watch for future iterations. 5082 As of 2021-03-15, both the txt and html-ized versions of draft- 5083 ietf-emailcore-rfc5321bis-02 were showing identical output for 5084 both parts of the example, so the problem appears to be OBE at 5085 worst. 5086 Ticket #29 (closed 2021-03-16) 5088 4055 (closed) Erratum claims the the description of SPF and DKIM is 5089 wrong. It is not clear what 5321bis should really say about them, 5090 but the current text probably needs work (or dropping, which is 5091 what the proposed erratum suggests). 5092 Text changed; ticket should probably be closed after WG reviews 5093 -04. 5094 Ticket #30 (resolved and closed). 5096 // [5321bis]Note that rejected errata have _not_ been reviewed to see 5097 // if they contain anything useful that should be discussed again 5098 // with the possibility of rethinking and changing text. Volunteers 5099 // sought. 5101 H.2. Changes from RFC 5321 (published October 2008) to the initial 5102 (-00) version of this draft 5104 // This appendix will eventually need to be replaced by a real 5105 // section or standalone appendix describing changes between 5321 and 5106 // the final 5321bis. 5108 * Acknowledgments section (Section 9) trimmed back for new document. 5110 * Introductory paragraph to Appendix F extended to make it clear 5111 that these features were deprecated a long time ago and really 5112 should not be in use any more. 5114 * Adjusted some language to clarify that source routes really, 5115 really, should not be used or depended upon. 5117 * IPv6 address syntax replaced by a copy of the IPv6 URI syntax and 5118 a note added. 5120 * Production index added as a first step in tying all productions to 5121 their sources. As part of the effort to make the document more 5122 easily navigable, table of contents entries have been created for 5123 the individual command descriptions. 5125 * Clarified the relationship between the SMTP "letters, digits, and 5126 hyphens" and DNS "preferred name syntax" (Section 2.3.5). 5128 * Revised the reply code sections to add new 521 and 556 codes, 5129 clarify relationships, and be explicit about the requirement for 5130 clients to rely on first digits rather than the sequences in 5131 Section 4.3.2. 5133 * In conjunction with the above, explicitly obsolete RFCs 1846 and 5134 7504 5136 // That might not be right -- see email 2021-10-03). 5138 * Incorporated a correction reflecting Errata ID 2578. 5140 * Some small editorial changes made to eliminate redundant 5141 statements that were very close together. Other, equally small, 5142 editorial changes have been made to improve grammar or clarity. 5144 * A few questions, marked "[[5321bis Editor's Note:", or "[[Note in 5145 Draft" have been added for the group to resolve. Other questions, 5146 especially those in the errata summary, are simply included in 5147 narrative comments in CREFs. 5149 * Checked and rationalized "response" (to a command) and "reply 5150 code" terminology. One can talk about a "999 response" but only a 5151 "999 reply code". There is no such thing as a "response code". 5153 * Added note about length limit on mailbox names ("email 5154 addresses"). 5156 * Added an "errata summary" subsection to this change log/ 5157 comparison to 5321 in this Appendix. The entire Appendix will, of 5158 course, disappear at the time of RFC publication unless someone 5159 wants to make a strong case for retaining it. 5161 * Rationalized CREFs to 2821, 5321, 5321bis etc.; added note to 5162 readers below the Abstract. 5164 * Temporarily added a "Note on Reading This Working Draft" after the 5165 Abstract. 5167 H.3. Changes Among Versions of Rfc5321bis 5168 H.3.1. Changes from draft-klensin-rfc5321bis-00 (posted 2012-12-02) to 5169 -01 5171 Substantively, these two versions differ only by suppression of the 5172 CREF and other discussion associated with the evolution from RFC 2821 5173 to RFC 5321. That change includes an update to the document's Note 5174 to Readers, the date, the file name, and the addition of this change 5175 log subsection. 5177 H.3.2. Changes from draft-klensin-rfc5321bis-01 (20191203) to -02 5179 * Minor clarifications to improve text, e.g., addition of NOOP to 5180 the list of non-mail transaction examples in Section 4.1.4. 5182 * Added topics exposed in the ietf-smtp list and the IETF list 5183 "dogfood" discussion during December 2019 and an index listing of 5184 substantive issues identified only in CREFs in the prior draft as 5185 a new Appendix G.. 5187 H.3.3. Changes from draft-klensin-rfc5321bis-02 (2019-12-27) to -03 5189 * Added more text to Appendix G.7.1 to specifically call out the 5190 session-opening policy issues surrounding these codes. 5192 * Added discussion of "1yz" reinstatement in Appendix G.7.11. 5194 * Added discussion of timeouts in Appendix G.7.12. 5196 * Added subsection on Enhanced Status Codes and DSNs to the 5197 outstanding issues list Appendix G.8. 5199 * Replaced reference to RFC 1652 (8BITMIME) with the Internet 5200 Standard version, RFC 6152. 5202 * With help from cketti, clarified the ABNF productions whose 5203 terminals appear in other documents. 5205 * Added discussions of Quoted-string, Internationalization, and 5206 client-server versus sender-receiver terminology to Appendix G. 5208 * Added note to the Abstract. 5210 H.3.4. Changes from draft-klensin-rfc5321bis-03 (2020-07-02) to draft- 5211 ietf-emailcore-rfc5321bis-00 5213 * Added a paragraph about non-null quoted strings to Appendix G.9. 5215 * Added an explicit pointer to email insecurity and TLS to 5216 Appendix G.6. Inspired by Ben Kaduk's comment on the WG Charter, 5217 2020-09-09. 5219 * Converted document from individual to emailcore WG effort. 5221 H.3.5. Changes from draft-ietf-emailcore-rfc5321bis-00 (2020-10-06) to 5222 -01 5224 * Editorial: Corrected "blackslash" to "backslash" 5226 * Rewrote the introduction to Appendix G slightly to reflect the 5227 creation of the EMAILCORE WG. 5229 * Applied fixes for repeated use of EHLO. See Appendix G.2. 5231 * Added two new questions, one about "X" extensions (Appendix G.12) 5232 and one about the status of HELO (Appendix G.13). 5234 * Removed mention of SEND, SAML, SOML from the main body of the text 5235 (Ticket #20). 5237 * Added a warning about side effects to Appendix G.7.5. 5239 * Added ticket numbers to descriptions of issues and changes, 5240 adjusted some text so relationships would be more clear, and added 5241 subsections to the Appendix G and H lists to pick up on tickets 5242 that were not easily identified in those sections of with the 5243 text. 5245 * Made several additions to the Index, including one to deal with 5246 SEND et al., as above. 5248 H.3.6. Changes from draft-ietf-emailcore-rfc5321bis-01 (2020-12-25) to 5249 -02 5251 * Corrected discussion mailing list to point to emailcore@ietf.org 5252 in the introductory note. 5254 * Added new subsection(s) to Appendix G to reflect newly discovered 5255 issues. 5257 * Changed "as discussed in" references in Section 4.5.5 per ticket 5258 #45. 5260 * Corrected a misleading use of the term "mailbox" in Section 3.3. 5262 * Changed descriptions of use of first digit in replies per ticket 5263 #13. See Appendix G.7.7. 5265 * Moved paragraph per ticket #28, erratum 1851. 5267 * Added more clarifying cross-references, clarified some CREFs, and 5268 cleaned out some of those that no longer seemed relevant. 5270 * Removed "updates 1123" is unnecessary and obsolete. 5272 * Updated several references. 5274 H.3.7. Changes from draft-ietf-emailcore-rfc5321bis-02 (2021-02-21) to 5275 -03 5277 * Editorial: Fixed some instances of constructions like "RCPT TO 5278 command". The name of the command is RCPT. Sloppy editing in 5279 2008. 5281 * Added text and cross-references to clarify the role of 452 and 552 5282 in "too many recipients" situations. 5284 * Added Appendix G.15 to discuss changes to better reflect 5285 "operational necessity" issue. 5287 * Added detail for erratum 5711, ticket #29. 5289 * Added new subsections of Appendix G.7 to keep some previously- 5290 unnoted CREF notes from getting lost. Also removed some CREFs 5291 that were notes on changes made before the WG was created or 5292 appeared to no longer have value and trimmed or rewrote some of 5293 the remaining ones. 5295 * More discussion of Ticket #13, See Appendix G.7.7. 5297 * Identified Ticket #41 as closed. See Appendix Appendix G.7.3; 5298 notes removed from Section 2.3.5. 5300 * "SHOULD" requirement for interpreting 552 "too many recipients" 5301 removed from Section 4.5.3.1.10, explanation added, and text 5302 cleaned up. Also removed the parenthetical historical notes on 5303 the return code definitions in Section 4.2. See Appendix G.5. 5304 (Ticket #5) 5306 * Modified Appendix G.8 to add a note about the normative status of 5307 RFC 3463 and moved that reference. 5309 * Several clarifications to initiation and termination of mail 5310 transactions in Section 4.1.4. 5312 * Several additional minor editorial improvements. 5314 * Note for drafts -03 and -04 only, modified somewhat for -05 but 5315 outdated from -06 forward: Some issues are still outstanding: 5316 Notes were posted to the list on 2021-07-09 about tickets #7 5317 (5322bis issue), #10 , #14 (closed), #20 (closed), #30 (closed), 5318 and #42 (closed). Even though some comments about them appeared 5319 in the subsequent day or so, there appears to have been 5320 insufficient time for discussions to stabilize sufficiently for 5321 changes to be included in this version of the I-D. 5323 H.3.8. Changes from draft-ietf-emailcore-rfc5321bis-03 (2021-07-10) to 5324 -04 5326 * Clarified that the "period" in . is really the ASCII 5327 one in Section 3.3. 5329 // Editor's note: change treated as Editorial without a ticket. 5330 If 5331 // there are objections, speak up. 5333 * Several other small editorial corrections. 5335 * Added several notes about the possible need to add text to reflect 5336 the presence of MSAs and to clarify whether MUAs send messages 5337 directly to MTAs or whether, in that case, the MUAs are just 5338 incorporating MSA functions. 5340 * Added new text to Appendix G.14 reflecting discussions of the 5341 Received...FOR issue. 5343 * Adjusted discussion of erratum 4315 (Ticket #27) to reflect more 5344 recent IPv6 syntax developments. 5346 * Adjusted discussion of the various "mail not accepted" codes, 5347 rewrote Section 4.2.4.2, annotated and inserted cross-references 5348 in relevant response code descriptions and (tentatively) 5349 identified this document as obsoleting RFC 7505. Editor's guess, 5350 reinforced by a brief conversation with John Levine (lead author 5351 of 7505), is that we should incorporate text as needed and 5352 obsolete it. The changes include replacing the reference to the 5353 "nullMX" I-D with RFC 7505, which I am appalled that neither I nor 5354 anyone else noticed earlier. Cf. Appendix G.7.1, Section 4.2.4.2, 5355 and Ticket #6. 5357 H.3.9. Changes from draft-ietf-emailcore-rfc5321bis-04 (2021-10-03) to 5358 -05 5360 * Took a first step toward rewriting and updating the introductory 5361 material. It is only a first step; suggestions welcome. 5363 * Minor editorial fixes. 5365 * Correct text about domain name checking in Section 4.1.4, probably 5366 fixing ticket #19. See CREF added there. 5368 * Added Appendix G.16 a placeholder for the 8BITMIME discussion and 5369 possible action. 5371 * Additional changes to the description and organization of trace 5372 field materials. Intended to resolve the 5321bis part of Ticket 5373 #7. 5375 * Remaining patch to SEND, etc., discussion in Appendix F.6 applied 5376 and CREF removed. 5378 * Removed discussion of "X-" and edited associated text. The fix 5379 may or may not be sufficient to resolve Ticket #42 (later closed). 5381 * Verified that the problems of getting four-level sections (e.g., 5382 "4.1.1.1" and other command-specific ones) into the table of 5383 contents and the index reflecting page numbers still exist and 5384 updated the introductory note. 5386 H.3.10. Changes from draft-ietf-emailcore-rfc5321bis-05 (2021-10-24) to 5387 -06 5389 * Finished making changes for "X-" and commands starting in "X". 5390 Changes made in -05 were incomplete. This should allow closing 5391 Ticket #42. 5393 * Removed spurious "for use in delivery notifications" from 3.6.2. 5394 Was just a pasting-type error. 5396 * Changed "In other words" to "In particular" in Section 2.3.5 per 5397 Ticket #10 and July 2021 mailing list discussion. Removed 5398 associated CREF. 5400 * Converted to xml2rfc v3 (thanks to John Levine for doing the hard 5401 parts) and then modified the introductory note accordingly. 5403 * Started reworking the Abstract -- see revised CREF there. 5405 * Rewrote Section 2.3.3 slightly to note the existence of submission 5406 servers and removed the CREF. 5408 * Updated Appendix G.7.17 and slightly modified CREF note in 5409 Section 2 -- proposed to not get 5321bis involved with this 5410 (Ticket #50). 5412 * Rewrote parts of Section 3.4.2 to clarify text amd respond to 5413 Ticket #34. 5415 * Inserted suggested text info CREF at end of Section 1.2. Comments 5416 welcome. Soon. 5418 H.3.11. Changes from draft-ietf-emailcore-rfc5321bis-06 (2021-11-07) to 5419 -07 5421 * Reviewed closed tickets and discussion with co-chairs after IETF 5422 112 and updated text. Sections or items that are, according to 5423 the ticket list, completely closed have been identified by 5424 "(closed)" in or near their titles. 5426 * Changed the suggestion for references to other documents mentioned 5427 in G.7.14 and Section 1.2 to actual text. Cleaned things up and, 5428 per note from Alexey 2021-11-17, have marked Ticket #53 as closed. 5430 * New text added and old text replaced about quotes in 5431 Section 4.1.2, text rearranged and edited a bit per Appendix G.9, 5432 and CREF added about alternatives. Changes reflect mailing list 5433 comments through 5435 * Last sentence (about source routing) removed from Section 2.1. 5436 Also adjusted text in Section 3.3, Section 4.1.1.3 but work is 5437 still needed there (see new CREFs in that section) and 5438 Section 6.1. The former Appendix C and references to it have been 5439 removed, leaving a placeholder to avoid changing subsequent 5440 appendix numbering before IETF Last Call (and maybe its 5441 completion) No changes have yet been made to Appendix F.2 but it 5442 is likely to require some work in the next version of the 5443 document. This is entirely about Ticket #17, which should not be 5444 closed until that appendix is updated. 5446 H.3.12. Changes from draft-ietf-emailcore-rfc5321bis-07 (2021-12-04) to 5447 -08 5449 Other than the partial cleanup for "forwarding" and "aliasing" and 5450 miscellaneous editorial fixes and corrections (including cleaning out 5451 unused references), changes in this version reflect the conclusions 5452 of the EMAILCORE interim meeting held 2021-12-09. References to 5453 "slides" are to the deck at https://datatracker.ietf.org/doc/slides- 5454 interim-2021-emailcore-01-sessa-chairs-slides/ and the minutes at 5455 https://notes.ietf.org/notes-emailcore-interim-dec-2021 5457 * (Slides 9 through 12): Removed source route examples from 5458 Section 4.1.1.3 and added a new paragraph explaining what happened 5459 to them. For slides 11 and 12, see below for more general 5460 Appendix F.2 discussion. 5461 (Cf Appendix G.7.10 and Ticket #17.) 5463 * (Slides 13 through 14): Domain names, Section 2.3.5. Removed 5464 "resolvable". Changed "alias" to "host alias" (although, after 5465 looking at the actual text, the intent seems clear from the CNAME 5466 label comment and, of course, the term "host" has been 5467 controversial in DNS circles and the minutes are not clear on the 5468 desirability of this change). Inserted "MUST" for the FQDN. A 5469 cross-reference to the domain name discussion in this section has 5470 been added to Section 4.1.1.1 in an attempt to resolve that 5471 discussion. 5472 In going carefully through this material, it became obvious that 5473 the discussions in Section 2.3.5 and Section 5 were confusing and 5474 somewhat redundant. Those sections have been rewritten to clarify 5475 intent, hint that extensions may modify (or have modified) a few 5476 of the rules, improve cross-references, and remove redundant text. 5477 Domain name issues are still under discussion on the WG mailing 5478 list as of 2021-12-18 and it is possible that the above changes 5479 may have introduced new issues, so additional changes are 5480 possible. 5481 (Cf target="G-domain"/> and Tickets #9 and maybe #10.) 5483 * Aliasing and forwarding: 5484 Consolidated former sections 3.4 and 3.9 into a new Section 3.4, 5485 making them subsections. The new subsection probably still needs 5486 work and maybe an introductory paragraph, but even bringing the 5487 two subsections together may reduce some sources of confusion 5488 identified on the mailing list. Added cross-reference to security 5489 considerations from the new Section 3.4.1. 5491 All other issues discussed during the interim appear to be unresolved 5492 and were deferred to the mailing list. 5494 As what should be the third and final step in deprecation of source 5495 routes and removal of them from the main text, the appendix that 5496 discusses them (Appendix F.2) has been rewritten, adjusting language 5497 and incorporating some materials from the former Appendix C. 5499 H.3.13. Changes from draft-ietf-emailcore-rfc5321bis-08 (2021-12-31) to 5500 -09 5502 * Multiple small editorial changes. 5504 * Started tuning Appendix H.2 preparatory to an actual "Changes 5505 from" section. 5507 * Moved and rewrote a paragraph that seemed to be out of place from 5508 Section 4.4.1 to Section 4.1.1.3 per November discussion. See the 5509 note in the latter section for discussion. 5511 * Removed "for initial submission of messages" from Section 2.3.5 5512 and changed "may" to "MAY" in the last bullet point there, per 5513 Interim. Removed comment/ Editor's Note from that section: 5514 further instructions and evidence of consensus needed to do 5515 anything additional with it. 5516 Ticket #9 5518 * In Section 3.4.2, rewrote the first sentence to make it 5519 descriptive rather than normative. Also removed the last sentence 5520 of that paragraph. Both per the editor's understanding of the 5521 Interim's conclusions, but the latter was put in because of 5522 problems with people thinking changing the argument to the MAIL 5523 command also required changing "From:" in the headers, so this 5524 should be carefully reviewed on list. Comment removed from that 5525 section -- the dead horse has been kicked past recognition. 5526 Ticket #4. 5528 * In Appendix F.2, changed the requirement for server support to 5529 MAY, and prohibited client support, for source routing. Also made 5530 a small wording change. Per Interim. 5531 Ticket #17 5533 With this draft, comments in the running text ("//" at the beginning 5534 of lines) that seem to no longer be relevant either generally or 5535 after the discussions during the 2022-01-21 Interim are being 5536 removed. The "Notes on Reading..." at the beginning of the document 5537 (just below the Abstract) have been revised accordingly. Sections 5538 from which comments were removed this time include: 5540 * Abstract, comment introduced in -06 (No comments on it through -08 5541 are interpreted as consent; 5543 * Section 2 (any discussion needed will be in A/S); 5545 * Section 2.3.10 (discussion seems to have ended); 5547 * Section 4.1.1.3 (no further discussion during Interim, so assume 5548 comment is no longer needed); 5550 * Section 4.1.2 (no further discussion since -08 appeared or during 5551 Interim, assumed to not require further work); 5553 * Section 4.5.3.1 (further discussion will be in A/S); 5555 * Section 4.2.2 (this comment obsolete since revision -04 of this 5556 document). 5558 * Cross-checked ticket notes and annotations in this document 5559 against the ticket system. Consistent for closed tickets as of 5560 2022-01-31. 5562 Index 5564 A C S 5566 A 5568 Argument Syntax 5569 ALPHA Section 4.1.2, Paragraph 2, Item 1 5570 Additional-Registered-Clauses Section 4.4.1 5571 Addtl-Link Section 4.4.1 5572 Addtl-Protocol Section 4.4.1 5573 Argument Section 4.1.2 5574 Atom Section 4.1.2 5575 By-domain Section 4.4.1 5576 CFWS Section 4.1.2, Paragraph 2, Item 2 5577 CRLF Section 4.1.2, Paragraph 2, Item 1 5578 DIGIT Section 4.1.2, Paragraph 2, Item 1 5579 Domain Section 4.1.2 5580 Dot-string Section 4.1.2 5581 Extended-Domain Section 4.4.1 5582 FWS Section 4.1.2, Paragraph 2, Item 2 5583 For Section 4.4.1 5584 Forward-Path Section 4.1.2 5585 From-domain Section 4.4.1 5586 General-address-literal Section 4.1.3 5587 Greeting Section 4.2 5588 HEXDIG Section 4.1.2, Paragraph 2, Item 1 5589 ID Section 4.4.1 5590 IPv4-address-literal Section 4.1.3 5591 IPv6-addr Section 4.1.3 5592 IPv6-address-literal Section 4.1.3 5593 Keyword Section 4.1.2 5594 Ldh-str Section 4.1.2 5595 Let-dig Section 4.1.2 5596 Link Section 4.4.1 5597 Local-part Section 4.1.2 5598 Mail-parameters Section 4.1.2 5599 Mailbox Section 4.1.2 5600 Opt-info Section 4.4.1 5601 Path Section 4.1.2 5602 Protocol Section 4.4.1 5603 QcontentSMTP Section 4.1.2 5604 Quoted-string Section 4.1.2 5605 Rcpt-parameters Section 4.1.2 5606 Reply-code Section 4.2 5607 Reply-line Section 4.2 5608 Return-path-line Section 4.4.1 5609 Reverse-Path Section 4.1.2 5610 SP Section 4.1.2, Paragraph 2, Item 1 5611 Snum Section 4.1.3 5612 Stamp Section 4.4.1 5613 Standardized-tag Section 4.1.3 5614 String Section 4.1.2 5615 TCP-info Section 4.4.1 5616 Time-stamp-line Section 4.4.1 5617 Via Section 4.4.1 5618 With Section 4.4.1 5619 address-literal Section 4.1.2 5620 atext Section 4.1.2, Paragraph 2, Item 2 5621 dcontent Section 4.1.3 5622 esmtp-keyword Section 4.1.2 5623 esmtp-param Section 4.1.2 5624 esmtp-value Section 4.1.2 5625 h16 Section 4.1.3 5626 ls32 Section 4.1.3 5627 qtextSMTP Section 4.1.2 5628 quoted-pairSMTP Section 4.1.2 5629 sub-domain Section 4.1.2 5630 textstring Section 4.2 5632 C 5634 Command Syntax 5635 data Section 4.1.1.4, Paragraph 8, Item 1 5636 ehlo Section 3.2, Paragraph 1; Section 4.1.1.1, Paragraph 1 5637 expn Section 4.1.1.7, Paragraph 4, Item 1 5638 helo Section 4.1.1.1, Paragraph 1 5639 help Section 4.1.1.8, Paragraph 5, Item 1 5640 mail Section 4.1.1.2 5641 noop Section 4.1.1.9, Paragraph 4, Item 1 5642 quit Section 4.1.1.10, Paragraph 5, Item 1 5643 rcpt Section 4.1.1.3, Paragraph 16 5644 rset Section 4.1.1.5, Paragraph 4, Item 1 5645 send, saml, soml Appendix G.7.13, Paragraph 1 5646 vrfy Section 4.1.1.6, Paragraph 4, Item 1 5648 S 5650 Source Routes Appendix F.2 5651 A-d-l Appendix F.2 5652 At-domain Appendix F.2 5653 Path Appendix F.2 5655 Author's Address 5657 John C. Klensin 5658 1770 Massachusetts Ave, Suite 322 5659 Cambridge, MA 02140 5660 United States of America 5662 Email: john-ietf@jck.com