idnits 2.17.00 (12 Aug 2021) /tmp/idnits52160/draft-ietf-emailcore-rfc5321bis-06.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 : ---------------------------------------------------------------------------- ** The abstract seems to contain references ([5321bis]), which it shouldn't. Please replace those with straight textual mentions of the documents in question. == 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: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (8 November 2021) is 193 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 3116 -- Looks like a reference, but probably isn't: 'JcK 20210904' on line 2649 == Unused Reference: '41' is defined on line 4178, but no explicit reference was found in the text == Unused Reference: '43' is defined on line 4187, but no explicit reference was found in the text == Unused Reference: '44' is defined on line 4191, but no explicit reference was found in the text -- 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 '54', was also mentioned in '50'. Summary: 2 errors (**), 0 flaws (~~), 5 warnings (==), 14 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 EMAILCORE J. Klensin 3 Internet-Draft 8 November 2021 4 Obsoletes: 5321, 1846, 7504, 7505 (if approved) 5 Intended status: Standards Track 6 Expires: 12 May 2022 8 Simple Mail Transfer Protocol 9 draft-ietf-emailcore-rfc5321bis-06 11 Abstract 13 This document is a specification of the basic protocol for Internet 14 electronic mail transport. It consolidates, updates, and clarifies 15 several previous documents, making all or parts of most of them 16 obsolete. It covers the SMTP extension mechanisms and best practices 17 for the contemporary Internet, but does not provide details about 18 particular extensions. The document also provides information about 19 use of SMTP for other than strict mail transport and delivery. This 20 document replaces RFC 5321, the earlier version with the same title. 22 // JcK 20211029 Note in Draft: Adjusted in version -06. Decided the 23 // details belong in either the Introduction or the A/S, not the 24 // Abstract. And it makes the Abstract a tad shorter, which is good. 26 Notes on Reading This Working Draft 28 This working draft is extensively annotated with information about 29 changes made over the decade since RFC 5321 appeared, especially when 30 those changes might be controversial or should get careful review. 31 Anything marked in CREF comments with "[5321bis]" is current. In 32 general, unless those are marked with "[[Note in Draft", in the 33 contents of an "Editor's note", or are in the "Errata Summary" 34 appendix (Appendix H.1, they are just notes on changes that have 35 already been made and where those changes originated. As one can 36 tell from the dates (when they are given), this document has been 37 periodically updated over a very long period of time. 39 As people review or try to use this document, it may be worth paying 40 special attention to the historical discussion in Section 1.2. 42 This evolving draft should be discussed on the emailcore@ietf.org 43 list. 45 Status of This Memo 47 This Internet-Draft is submitted in full conformance with the 48 provisions of BCP 78 and BCP 79. 50 Internet-Drafts are working documents of the Internet Engineering 51 Task Force (IETF). Note that other groups may also distribute 52 working documents as Internet-Drafts. The list of current Internet- 53 Drafts is at https://datatracker.ietf.org/drafts/current/. 55 Internet-Drafts are draft documents valid for a maximum of six months 56 and may be updated, replaced, or obsoleted by other documents at any 57 time. It is inappropriate to use Internet-Drafts as reference 58 material or to cite them other than as "work in progress." 60 This Internet-Draft will expire on 12 May 2022. 62 Copyright Notice 64 Copyright (c) 2021 IETF Trust and the persons identified as the 65 document authors. All rights reserved. 67 This document is subject to BCP 78 and the IETF Trust's Legal 68 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 69 license-info) in effect on the date of publication of this document. 70 Please review these documents carefully, as they describe your rights 71 and restrictions with respect to this document. Code Components 72 extracted from this document must include Revised BSD License text as 73 described in Section 4.e of the Trust Legal Provisions and are 74 provided without warranty as described in the Revised BSD License. 76 Table of Contents 78 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 7 79 1.1. Transport of Electronic Mail . . . . . . . . . . . . . . 7 80 1.2. History and Context for This Document . . . . . . . . . . 7 81 1.3. Document Conventions . . . . . . . . . . . . . . . . . . 9 82 2. The SMTP Model . . . . . . . . . . . . . . . . . . . . . . . 9 83 2.1. Basic Structure . . . . . . . . . . . . . . . . . . . . . 9 84 2.2. The Extension Model . . . . . . . . . . . . . . . . . . . 11 85 2.2.1. Background . . . . . . . . . . . . . . . . . . . . . 12 86 2.2.2. Definition and Registration of Extensions . . . . . . 13 87 2.2.3. Special Issues with Extensions . . . . . . . . . . . 13 88 2.3. SMTP Terminology . . . . . . . . . . . . . . . . . . . . 14 89 2.3.1. Mail Objects . . . . . . . . . . . . . . . . . . . . 14 90 2.3.2. Senders and Receivers . . . . . . . . . . . . . . . . 14 91 2.3.3. Mail Agents and Message Stores . . . . . . . . . . . 15 92 2.3.4. Host . . . . . . . . . . . . . . . . . . . . . . . . 15 93 2.3.5. Domain Names . . . . . . . . . . . . . . . . . . . . 15 94 2.3.6. Buffer and State Table . . . . . . . . . . . . . . . 16 95 2.3.7. Commands and Replies . . . . . . . . . . . . . . . . 16 96 2.3.8. Lines . . . . . . . . . . . . . . . . . . . . . . . . 17 97 2.3.9. Message Content and Mail Data . . . . . . . . . . . . 17 98 2.3.10. Originator, Delivery, Relay, and Gateway Systems . . 17 99 2.3.11. Mailbox and Address . . . . . . . . . . . . . . . . . 18 100 2.4. General Syntax Principles and Transaction Model . . . . . 18 101 3. The SMTP Procedures: An Overview . . . . . . . . . . . . . . 20 102 3.1. Session Initiation . . . . . . . . . . . . . . . . . . . 20 103 3.2. Client Initiation . . . . . . . . . . . . . . . . . . . . 21 104 3.3. Mail Transactions . . . . . . . . . . . . . . . . . . . . 21 105 3.4. Forwarding for Address Correction or Updating . . . . . . 24 106 3.5. Commands for Debugging Addresses . . . . . . . . . . . . 25 107 3.5.1. Overview . . . . . . . . . . . . . . . . . . . . . . 25 108 3.5.2. VRFY Normal Response . . . . . . . . . . . . . . . . 27 109 3.5.3. Meaning of VRFY or EXPN Success Response . . . . . . 28 110 3.5.4. Semantics and Applications of EXPN . . . . . . . . . 28 111 3.6. Relaying and Mail Routing . . . . . . . . . . . . . . . . 29 112 3.6.1. Source Routes and Relaying . . . . . . . . . . . . . 29 113 3.6.2. Mail eXchange Records and Relaying . . . . . . . . . 29 114 3.6.3. Message Submission Servers as Relays . . . . . . . . 30 115 3.7. Mail Gatewaying . . . . . . . . . . . . . . . . . . . . . 31 116 3.7.1. Header Fields in Gatewaying . . . . . . . . . . . . . 31 117 3.7.2. Received Lines in Gatewaying . . . . . . . . . . . . 31 118 3.7.3. Addresses in Gatewaying . . . . . . . . . . . . . . . 32 119 3.7.4. Other Header Fields in Gatewaying . . . . . . . . . . 32 120 3.7.5. Envelopes in Gatewaying . . . . . . . . . . . . . . . 32 121 3.8. Terminating Sessions and Connections . . . . . . . . . . 33 122 3.9. Aliases and Mailing Lists . . . . . . . . . . . . . . . . 34 123 3.9.1. Simple Aliases . . . . . . . . . . . . . . . . . . . 34 124 3.9.2. Mailing Lists . . . . . . . . . . . . . . . . . . . . 34 125 4. The SMTP Specifications . . . . . . . . . . . . . . . . . . . 35 126 4.1. SMTP Commands . . . . . . . . . . . . . . . . . . . . . . 35 127 4.1.1. Command Semantics and Syntax . . . . . . . . . . . . 35 128 4.1.1.1. Extended HELLO (EHLO) or HELLO (HELO) . . . . . . 36 129 4.1.1.2. MAIL (MAIL) . . . . . . . . . . . . . . . . . . . 37 130 4.1.1.3. RECIPIENT (RCPT) . . . . . . . . . . . . . . . . 38 131 4.1.1.4. DATA (DATA) . . . . . . . . . . . . . . . . . . . 39 132 4.1.1.5. RESET (RSET) . . . . . . . . . . . . . . . . . . 41 133 4.1.1.6. VERIFY (VRFY) . . . . . . . . . . . . . . . . . . 41 134 4.1.1.7. EXPAND (EXPN) . . . . . . . . . . . . . . . . . . 41 135 4.1.1.8. HELP (HELP) . . . . . . . . . . . . . . . . . . . 42 136 4.1.1.9. NOOP (NOOP) . . . . . . . . . . . . . . . . . . . 42 137 4.1.1.10. QUIT (QUIT) . . . . . . . . . . . . . . . . . . . 42 138 4.1.2. Command Argument Syntax . . . . . . . . . . . . . . . 43 139 4.1.3. Address Literals . . . . . . . . . . . . . . . . . . 45 140 4.1.4. Order of Commands . . . . . . . . . . . . . . . . . . 47 141 4.2. SMTP Replies . . . . . . . . . . . . . . . . . . . . . . 49 142 4.2.1. Reply Code Severities and Theory . . . . . . . . . . 50 143 4.2.2. Reply Codes by Function Groups . . . . . . . . . . . 53 144 4.2.3. Reply Codes in Numeric Order . . . . . . . . . . . . 54 145 4.2.4. Some specific code situations and relationships . . . 56 147 4.3. Sequencing of Commands and Replies . . . . . . . . . . . 57 148 4.3.1. Sequencing Overview . . . . . . . . . . . . . . . . . 58 149 4.3.2. Command-Reply Sequences . . . . . . . . . . . . . . . 58 150 4.4. Trace Information . . . . . . . . . . . . . . . . . . . . 61 151 4.4.1. Received Header Field . . . . . . . . . . . . . . . . 61 152 4.5. Additional Implementation Issues . . . . . . . . . . . . 65 153 4.5.1. Minimum Implementation . . . . . . . . . . . . . . . 65 154 4.5.2. Transparency . . . . . . . . . . . . . . . . . . . . 66 155 4.5.3. Sizes and Timeouts . . . . . . . . . . . . . . . . . 66 156 4.5.3.1. Size Limits and Minimums . . . . . . . . . . . . 66 157 4.5.3.1.1. Local-part . . . . . . . . . . . . . . . . . 67 158 4.5.3.1.2. Domain . . . . . . . . . . . . . . . . . . . 67 159 4.5.3.1.3. Path . . . . . . . . . . . . . . . . . . . . 67 160 4.5.3.1.4. Command Line . . . . . . . . . . . . . . . . 67 161 4.5.3.1.5. Reply Line . . . . . . . . . . . . . . . . . 67 162 4.5.3.1.6. Text Line . . . . . . . . . . . . . . . . . . 67 163 4.5.3.1.7. Message Content . . . . . . . . . . . . . . . 68 164 4.5.3.1.8. Recipient Buffer . . . . . . . . . . . . . . 68 165 4.5.3.1.9. Treatment When Limits Exceeded . . . . . . . 68 166 4.5.3.1.10. Too Many Recipients Code . . . . . . . . . . 69 167 4.5.3.2. Timeouts . . . . . . . . . . . . . . . . . . . . 69 168 4.5.3.2.1. Initial 220 Message: 5 Minutes . . . . . . . 70 169 4.5.3.2.2. MAIL Command: 5 Minutes . . . . . . . . . . . 70 170 4.5.3.2.3. RCPT Command: 5 Minutes . . . . . . . . . . . 70 171 4.5.3.2.4. DATA Initiation: 2 Minutes . . . . . . . . . 70 172 4.5.3.2.5. Data Block: 3 Minutes . . . . . . . . . . . . 70 173 4.5.3.2.6. DATA Termination: 10 Minutes. . . . . . . . . 70 174 4.5.3.2.7. Server Timeout: 5 Minutes. . . . . . . . . . 70 175 4.5.4. Retry Strategies . . . . . . . . . . . . . . . . . . 70 176 4.5.5. Messages with a Null Reverse-Path . . . . . . . . . . 73 177 5. Address Resolution and Mail Handling . . . . . . . . . . . . 73 178 5.1. Locating the Target Host . . . . . . . . . . . . . . . . 73 179 5.2. IPv6 and MX Records . . . . . . . . . . . . . . . . . . . 75 180 6. Problem Detection and Handling . . . . . . . . . . . . . . . 76 181 6.1. Reliable Delivery and Replies by Email . . . . . . . . . 76 182 6.2. Unwanted, Unsolicited, and "Attack" Messages . . . . . . 77 183 6.3. Loop Detection . . . . . . . . . . . . . . . . . . . . . 78 184 6.4. Compensating for Irregularities . . . . . . . . . . . . . 78 185 7. Security Considerations . . . . . . . . . . . . . . . . . . . 80 186 7.1. Mail Security and Spoofing . . . . . . . . . . . . . . . 80 187 7.2. "Blind" Copies . . . . . . . . . . . . . . . . . . . . . 81 188 7.3. VRFY, EXPN, and Security . . . . . . . . . . . . . . . . 81 189 7.4. Mail Rerouting Based on the 251 and 551 Response Codes . 82 190 7.5. Information Disclosure in Announcements . . . . . . . . . 82 191 7.6. Information Disclosure in Trace Fields . . . . . . . . . 83 192 7.7. Information Disclosure in Message Forwarding . . . . . . 83 193 7.8. Local Operational Requirement and Resistance to 194 Attacks . . . . . . . . . . . . . . . . . . . . . . . . . 83 196 7.9. Scope of Operation of SMTP Servers . . . . . . . . . . . 83 197 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 84 198 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 85 199 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 86 200 10.1. Normative References . . . . . . . . . . . . . . . . . . 86 201 10.2. Informative References . . . . . . . . . . . . . . . . . 87 202 Appendix A. TCP Transport Service . . . . . . . . . . . . . . . 91 203 Appendix B. Generating SMTP Commands from RFC 822 Header 204 Fields . . . . . . . . . . . . . . . . . . . . . . . . . 91 205 Appendix C. Source Routes . . . . . . . . . . . . . . . . . . . 93 206 Appendix D. Scenarios . . . . . . . . . . . . . . . . . . . . . 94 207 D.1. A Typical SMTP Transaction Scenario . . . . . . . . . . . 94 208 D.2. Aborted SMTP Transaction Scenario . . . . . . . . . . . . 95 209 D.3. Relayed Mail Scenario . . . . . . . . . . . . . . . . . . 95 210 D.4. Verifying and Sending Scenario . . . . . . . . . . . . . 97 211 Appendix E. Other Gateway Issues . . . . . . . . . . . . . . . . 98 212 Appendix F. Deprecated Features of RFC 821 . . . . . . . . . . . 98 213 F.1. TURN . . . . . . . . . . . . . . . . . . . . . . . . . . 99 214 F.2. Source Routing . . . . . . . . . . . . . . . . . . . . . 99 215 F.3. HELO . . . . . . . . . . . . . . . . . . . . . . . . . . 99 216 F.4. #-literals . . . . . . . . . . . . . . . . . . . . . . . 99 217 F.5. Dates and Years . . . . . . . . . . . . . . . . . . . . . 100 218 F.6. Sending versus Mailing . . . . . . . . . . . . . . . . . 100 219 Appendix G. Other Outstanding Issues . . . . . . . . . . . . . . 100 220 G.1. IP Address literals . . . . . . . . . . . . . . . . . . . 101 221 G.2. Repeated Use of EHLO . . . . . . . . . . . . . . . . . . 101 222 G.3. Meaning of "MTA" and Related Terminology . . . . . . . . 102 223 G.4. Originator, or Originating System, Authentication . . . . 102 224 G.5. Remove or deprecate the work-around from code 552 to 225 452 . . . . . . . . . . . . . . . . . . . . . . . . . . 102 226 G.6. Clarify where the protocol stands with respect to 227 submission and TLS issues . . . . . . . . . . . . . . . 102 228 G.7. Probably-substantive Discussion Topics Identified in Other 229 Ways . . . . . . . . . . . . . . . . . . . . . . . . . . 103 230 G.7.1. Issues with 521, 554, and 556 codes . . . . . . . . . 103 231 G.7.2. SMTP Model, terminology, and relationship to RFC 232 5598 . . . . . . . . . . . . . . . . . . . . . . . . 103 233 G.7.3. Resolvable FQDNs and private domain names . . . . . . 103 234 G.7.4. Possible clarification about mail transactions and 235 transaction state . . . . . . . . . . . . . . . . . . 103 236 G.7.5. Issues with mailing lists, aliases, and forwarding . 104 237 G.7.6. Requirements for domain name and/or IP address in 238 EHLO . . . . . . . . . . . . . . . . . . . . . . . . 104 239 G.7.7. Does the 'first digit only' and/or non-listed reply 240 code text need clarification? . . . . . . . . . . . . 104 241 G.7.8. Size limits . . . . . . . . . . . . . . . . . . . . . 104 242 G.7.9. Discussion of 'blind' copies and RCPT . . . . . . . . 104 243 G.7.10. Further clarifications needed to source routes? . . . 105 244 G.7.11. Should 1yz Be Revisited? . . . . . . . . . . . . . . 105 245 G.7.12. Review Timeout Specifications . . . . . . . . . . . . 105 246 G.7.13. Possible SEND, SAML, SOML Loose End . . . . . . . . . 105 247 G.7.14. Abstract Update . . . . . . . . . . . . . . . . . . . 105 248 G.7.15. Informative References to MIME and/or Message 249 Submission . . . . . . . . . . . . . . . . . . . . . 105 250 G.7.16. Mail Transaction Discussion . . . . . . . . . . . . . 106 251 G.7.17. Hop by hop Authentication and/or Encryption . . . . . 106 252 G.7.18. More Text About 554 Given 521, etc. . . . . . . . . . 106 253 G.7.19. Minimum Lengths and Quantities . . . . . . . . . . . 106 254 G.8. Enhanced Reply Codes and DSNs . . . . . . . . . . . . . . 106 255 G.9. Revisiting Quoted Strings . . . . . . . . . . . . . . . . 107 256 G.10. Internationalization . . . . . . . . . . . . . . . . . . 107 257 G.11. SMTP Clients, Servers, Senders, and Receivers . . . . . . 108 258 G.12. Extension Keywords Starting in 'X-' . . . . . . . . . . . 108 259 G.13. Deprecating HELO . . . . . . . . . . . . . . . . . . . . 108 260 G.14. The FOR Clause in Trace Fields: Semantics, Security 261 Considerations, and Other Issues . . . . . . . . . . . . 109 262 G.15. Resistance to Attacks and Operational Necessity . . . . . 109 263 G.16. Mandatory 8BITMIME . . . . . . . . . . . . . . . . . . . 110 264 Appendix H. RFC 5321 Errata Summary and Tentative Change Log . . 110 265 H.1. RFC 5321 Errata Summary . . . . . . . . . . . . . . . . . 110 266 H.2. Changes from RFC 5321 (published October 2008) to the 267 initial (-00) version of this draft . . . . . . . . . . . 112 268 H.3. Changes Among Versions of Rfc5321bis . . . . . . . . . . 113 269 H.3.1. Changes from draft-klensin-rfc5321bis-00 (posted 270 2012-12-02) to -01 . . . . . . . . . . . . . . . . . 113 271 H.3.2. Changes from draft-klensin-rfc5321bis-01 (20191203) to 272 -02 . . . . . . . . . . . . . . . . . . . . . . . . . 113 273 H.3.3. Changes from draft-klensin-rfc5321bis-02 (2019-12-27) 274 to -03 . . . . . . . . . . . . . . . . . . . . . . . 114 275 H.3.4. Changes from draft-klensin-rfc5321bis-03 (2020-07-02) 276 to draft-ietf-emailcore-rfc5321bis-00 . . . . . . . . 114 277 H.3.5. Changes from draft-ietf-emailcore-rfc5321bis-00 278 (2020-10-06) to -01 . . . . . . . . . . . . . . . . . 114 279 H.3.6. Changes from draft-ietf-emailcore-rfc5321bis-01 280 (2020-12-25) to -02 . . . . . . . . . . . . . . . . . 115 281 H.3.7. Changes from draft-ietf-emailcore-rfc5321bis-02 282 (2021-02-21) to -03 . . . . . . . . . . . . . . . . . 115 283 H.3.8. Changes from draft-ietf-emailcore-rfc5321bis-03 284 (2021-07-10) to -04 . . . . . . . . . . . . . . . . . 116 285 H.3.9. Changes from draft-ietf-emailcore-rfc5321bis-04 286 (2021-10-03) to -05 . . . . . . . . . . . . . . . . . 117 287 H.3.10. Changes from draft-ietf-emailcore-rfc5321bis-05 288 (2021-10-24) to -06 . . . . . . . . . . . . . . . . . 118 289 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 290 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 120 292 1. Introduction 294 1.1. Transport of Electronic Mail 296 The objective of the Simple Mail Transfer Protocol (SMTP) is to 297 transfer mail reliably and efficiently. 299 SMTP is independent of the particular transmission subsystem and 300 requires only a reliable ordered data stream channel. While this 301 document specifically discusses transport over TCP, other transports 302 are possible. Appendices to RFC 821 [3] describe some of them. 304 An important feature of SMTP is its capability to transport mail 305 across multiple networks, usually referred to as "SMTP mail relaying" 306 (see Section 3.6). A network consists of the mutually-TCP-accessible 307 hosts on the public Internet, the mutually-TCP-accessible hosts on a 308 firewall-isolated TCP/IP Intranet, or hosts in some other LAN or WAN 309 environment utilizing a non-TCP transport-level protocol. Using 310 SMTP, a process can transfer mail to another process on the same 311 network or to some other network via a relay or gateway process 312 accessible to both networks. 314 In this way, a mail message may pass through a number of intermediate 315 relay or gateway hosts on its path from sender to ultimate recipient. 316 The Mail eXchanger mechanisms of the domain name system (RFC 1035 317 [4], RFC 974 [16], and Section 5 of this document) are used to 318 identify the appropriate next-hop destination for a message being 319 transported. 321 1.2. History and Context for This Document 323 This document is a specification of the basic protocol for the 324 Internet electronic mail transport. It consolidates, updates and 325 clarifies, but does not add new or change existing functionality of 326 the following: 328 * the original SMTP (Simple Mail Transfer Protocol) specification of 329 RFC 821 [3], 331 * domain name system requirements and implications for mail 332 transport from RFC 1035 [4] and RFC 974 [16], 334 * the clarifications and applicability statements in RFC 1123 [5], 336 * the new error codes added by RFC 1846 [20] and later by RFC 7504 337 [48], obsoleting both of those documents, and 339 * material drawn from the SMTP Extension mechanisms in RFC 1869 340 [22]. 342 It also includes editorial and clarification changes that were made 343 to RFC 2821 [30] to bring that specification to Draft Standard and 344 similar changes to RFC 5321 [50] to bring the current document to 345 Internet Standard. 347 It may help the reader to understand that, to reduce the risk of 348 introducing errors, large parts of the document essentially merge the 349 earlier specifications listed in the bullet points above rather than 350 providing a completely rewritten, reorganized, and integrated 351 description of SMTP. An index is provided to assist in the quest for 352 information. 354 It obsoletes RFCs 5321 [50] (the earlier version of this 355 specification), 1846 [20] and incorporates the substance of 7504 356 [48]7504 (specification of reply codes), and 7505 [49] (the "Null MX" 357 specification). 359 // JcK: 202107219: does the text that follows need rewriting? See 360 // comment in Abstract. 361 Although SMTP was designed as a mail transport and delivery protocol, 362 this specification also contains information that is important to its 363 use as a "mail submission" protocol, as recommended for Post Office 364 Protocol (POP) (RFC 937 [14], RFC 1939 [23]) and IMAP (RFC 3501 365 [36]). In general, the separate mail submission protocol specified 366 in RFC 6409 [42] is now preferred to direct use of SMTP; more 367 discussion of that subject appears in that document. 369 Section 2.3 provides definitions of terms specific to this document. 370 Except when the historical terminology is necessary for clarity, this 371 document uses the current 'client' and 'server' terminology to 372 identify the sending and receiving SMTP processes, respectively. 374 A companion document, RFC 5322 [12], discusses message header 375 sections and bodies and specifies formats and structures for them. 377 // [rfc5321bis 20210317] Would this be an appropriate place to 378 // mention RFC 2045 (MIME) and/or RFC 6409 (Message Submission)? 379 // Suggestion (20211104): Other relevant documents and their 380 // relationships are discussed in a forthcoming Applicability 381 // Statement [51]. 383 1.3. Document Conventions 385 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 386 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 387 document are to be interpreted as described in RFC 2119 [1]. As each 388 of these terms was intentionally and carefully chosen to improve the 389 interoperability of email, each use of these terms is to be treated 390 as a conformance requirement. 392 Because this document has a long history and to avoid the risk of 393 various errors and of confusing readers and documents that point to 394 this one, most examples and the domain names they contain are 395 preserved from RFC 2821. Readers are cautioned that these are 396 illustrative examples that should not actually be used in either code 397 or configuration files. 399 2. The SMTP Model 401 // [5321bis] [[Editor's Note: There have been extensive and repeated 402 // discussions on the SMTP and IETF lists about whether this document 403 // should say something about hop-by-hop (MTA-to-MTA) SMTP 404 // authentication and, if so, what?? Note that end to end message 405 // authentication is almost certainly out of scope for SMTP.]] Cf. 406 // Appendix G.7.17 408 2.1. Basic Structure 410 The SMTP design can be pictured as: 412 +----------+ +----------+ 413 +------+ | | | | 414 | User |<-->| | SMTP | | 415 +------+ | Client- |Commands/Replies| Server- | 416 +------+ | SMTP |<-------------->| SMTP | +------+ 417 | File |<-->| | and Mail | |<-->| File | 418 |System| | | | | |System| 419 +------+ +----------+ +----------+ +------+ 420 SMTP client SMTP server 422 When an SMTP client has a message to transmit, it establishes a two- 423 way transmission channel to an SMTP server. The responsibility of an 424 SMTP client is to transfer mail messages to one or more SMTP servers, 425 or report its failure to do so. 427 The means by which a mail message is presented to an SMTP client, and 428 how that client determines the identifier(s) ("names") of the 429 domain(s) to which mail messages are to be transferred, are local 430 matters. They are not addressed by this document. In some cases, 431 the designated domain(s), or those determined by an SMTP client, will 432 identify the final destination(s) of the mail message. In other 433 cases, common with SMTP clients associated with implementations of 434 the POP (RFC 937 [14], RFC 1939 [23]) or IMAP (RFC 3501 [36]) 435 protocols, or when the SMTP client is inside an isolated transport 436 service environment, the domain determined will identify an 437 intermediate destination through which all mail messages are to be 438 relayed. SMTP clients that transfer all traffic regardless of the 439 target domains associated with the individual messages, or that do 440 not maintain queues for retrying message transmissions that initially 441 cannot be completed, may otherwise conform to this specification but 442 are not considered fully-capable. Fully-capable SMTP 443 implementations, including the relays used by these less capable 444 ones, and their destinations, are expected to support all of the 445 queuing, retrying, and alternate address functions discussed in this 446 specification. In many situations and configurations, the less- 447 capable clients discussed above SHOULD be using the message 448 submission protocol (RFC 6409 [42]) rather than SMTP. 450 The means by which an SMTP client, once it has determined a target 451 domain, determines the identity of an SMTP server to which a copy of 452 a message is to be transferred, and then performs that transfer, are 453 covered by this document. To effect a mail transfer to an SMTP 454 server, an SMTP client establishes a two-way transmission channel to 455 that SMTP server. An SMTP client determines the address of an 456 appropriate host running an SMTP server by resolving a destination 457 domain name to either an intermediate Mail eXchanger host or a final 458 target host. 460 An SMTP server may be either the ultimate destination or an 461 intermediate "relay" (that is, it may assume the role of an SMTP 462 client after receiving the message) or "gateway" (that is, it may 463 transport the message further using some protocol other than SMTP). 464 SMTP commands are generated by the SMTP client and sent to the SMTP 465 server. SMTP replies are sent from the SMTP server to the SMTP 466 client in response to the commands. 468 In other words, message transfer can occur in a single connection 469 between the original SMTP-sender and the final SMTP-recipient, or can 470 occur in a series of hops through intermediary systems. In either 471 case, once the server has issued a success response at the end of the 472 mail data, a formal handoff of responsibility for the message occurs: 473 the protocol requires that a server MUST accept responsibility for 474 either delivering the message or properly reporting the failure to do 475 so (see Sections 6.1, 6.2, and 7.8, below). 477 Once the transmission channel is established and initial handshaking 478 is completed, the SMTP client normally initiates a mail transaction. 479 Such a transaction consists of a series of commands to specify the 480 originator and destination of the mail and transmission of the 481 message content (including any lines in the header section or other 482 structure) itself. When the same message is sent to multiple 483 recipients, this protocol encourages the transmission of only one 484 copy of the data for all recipients at the same destination (or 485 intermediate relay) host. 487 The server responds to each command with a reply; replies may 488 indicate that the command was accepted, that additional commands are 489 expected, or that a temporary or permanent error condition exists. 490 Commands specifying the sender or recipients may include server- 491 permitted SMTP service extension requests, as discussed in 492 Section 2.2. The dialog is purposely lock-step, one-at-a-time, 493 although this can be modified by mutually agreed upon extension 494 requests such as command pipelining (RFC 2920 [31]). 496 Once a given mail message has been transmitted, the client may either 497 request that the connection be shut down or may initiate other mail 498 transactions. In addition, an SMTP client may use a connection to an 499 SMTP server for ancillary services such as verification of email 500 addresses or retrieval of mailing list subscriber addresses. 502 As suggested above, this protocol provides mechanisms for the 503 transmission of mail. Historically, this transmission normally 504 occurred directly from the sending user's host to the receiving 505 user's host when the two hosts are connected to the same transport 506 service. When they are not connected to the same transport service, 507 transmission occurs via one or more relay SMTP servers. A very 508 common case in the Internet today involves submission of the original 509 message to an intermediate, "message submission" server, which is 510 similar to a relay but has some additional properties; such servers 511 are discussed in Section 2.3.10 and at some length in RFC 6409 [42]. 512 An intermediate host that acts as either an SMTP relay or as a 513 gateway into some other transmission environment is usually selected 514 through the use of the domain name service (DNS) Mail eXchanger 515 mechanism. Explicit "source" routing (see Section 5 and Appendix C 516 and Appendix F.2) SHOULD NOT be used. 518 2.2. The Extension Model 519 2.2.1. Background 521 In an effort that started in 1990, approximately a decade after RFC 522 821 was completed, the protocol was modified with a "service 523 extensions" model that permits the client and server to agree to 524 utilize shared functionality beyond the original SMTP requirements. 525 The SMTP extension mechanism defines a means whereby an extended SMTP 526 client and server may recognize each other, and the server can inform 527 the client as to the service extensions that it supports. 529 Contemporary SMTP implementations MUST support the basic extension 530 mechanisms. For instance, servers MUST support the EHLO command even 531 if they do not implement any specific extensions and clients SHOULD 532 preferentially utilize EHLO rather than HELO. (However, for 533 compatibility with older conforming implementations, SMTP clients and 534 servers MUST support the original HELO mechanisms as a fallback.) 535 Unless the different characteristics of HELO must be identified for 536 interoperability purposes, this document discusses only EHLO. 538 SMTP is widely deployed and high-quality implementations have proven 539 to be very robust. However, the Internet community now considers 540 some services to be important that were not anticipated when the 541 protocol was first designed. If support for those services is to be 542 added, it must be done in a way that permits older implementations to 543 continue working acceptably. The extension framework consists of: 545 * The SMTP command EHLO, superseding the earlier HELO, 547 * a registry of SMTP service extensions, 549 * additional parameters to the SMTP MAIL and RCPT commands, and 551 * optional replacements for commands defined in this protocol, such 552 as for DATA in non-ASCII transmissions (RFC 3030 [33]). 554 SMTP's strength comes primarily from its simplicity. Experience with 555 many protocols has shown that protocols with few options tend towards 556 ubiquity, whereas protocols with many options tend towards obscurity. 558 Each and every extension, regardless of its benefits, must be 559 carefully scrutinized with respect to its implementation, deployment, 560 and interoperability costs. In many cases, the cost of extending the 561 SMTP service will likely outweigh the benefit. 563 2.2.2. Definition and Registration of Extensions 565 The IANA maintains a registry of SMTP service extensions [55]. A 566 corresponding EHLO keyword value is associated with each extension. 567 Each service extension registered with the IANA must be defined in a 568 formal Standards-Track or IESG-approved Experimental protocol 569 document. The definition must include: 571 * the textual name of the SMTP service extension; 573 * the EHLO keyword value associated with the extension; 575 * the syntax and possible values of parameters associated with the 576 EHLO keyword value; 578 * any additional SMTP verbs associated with the extension 579 (additional verbs will usually be, but are not required to be, the 580 same as the EHLO keyword value); 582 * any new parameters the extension associates with the MAIL or RCPT 583 verbs; 585 * a description of how support for the extension affects the 586 behavior of a server and client SMTP; and 588 * the increment by which the extension is increasing the maximum 589 length of the commands MAIL and/or RCPT, over that specified in 590 this Standard. 592 Any keyword value presented in the EHLO response MUST correspond to a 593 Standard, Standards-Track, or IESG-approved Experimental SMTP service 594 extension registered with IANA. A conforming server MUST NOT offer 595 keyword values that are not described in a registered extension. 597 2.2.3. Special Issues with Extensions 599 Extensions that change fairly basic properties of SMTP operation are 600 permitted. The text in other sections of this document must be 601 understood in that context. In particular, extensions can change the 602 minimum limits specified in Section 4.5.3, can change the ASCII 603 character set requirement as mentioned above, or can introduce some 604 optional modes of message handling. 606 In particular, if an extension implies that the delivery path 607 normally supports special features of that extension, and an 608 intermediate SMTP system finds a next hop that does not support the 609 required extension, it MAY choose, based on the specific extension 610 and circumstances, to requeue the message and try later and/or try an 611 alternate MX host. If this strategy is employed, the timeout to fall 612 back to an unextended format (if one is available) SHOULD be less 613 than the normal timeout for bouncing as undeliverable (e.g., if 614 normal timeout is three days, the requeue timeout before attempting 615 to transmit the mail without the extension might be one day). 617 2.3. SMTP Terminology 619 2.3.1. Mail Objects 621 SMTP transports a mail object. A mail object contains an envelope 622 and content. 624 The SMTP envelope is sent as a series of SMTP protocol units 625 (described in Section 3). It consists of an originator address (to 626 which error reports should be directed), one or more recipient 627 addresses, and optional protocol extension material. Historically, 628 variations on the reverse-path (originator) address specification 629 command (MAIL) could be used to specify alternate delivery modes, 630 such as immediate display; those variations have now been deprecated 631 (see Appendix F and Appendix F.6). 633 The SMTP content is sent in the SMTP DATA protocol unit and has two 634 parts: the header section and the body. If the content conforms to 635 other contemporary standards, the header section consists of a 636 collection of header fields, each consisting of a header name, a 637 colon, and data, structured as in the message format specification 638 (RFC 5322 [12]); the body, if structured, is defined according to 639 MIME (RFC 2045 [25]). The content is textual in nature, expressed 640 using the US-ASCII repertoire [2]. Although SMTP extensions (such as 641 "8BITMIME", RFC 6152 [47]) may relax this restriction for the content 642 body, the content header fields are always encoded using the US-ASCII 643 repertoire. Two MIME extensions (RFC 2047 [26] and RFC 2231 [29]) 644 define an algorithm for representing header values outside the US- 645 ASCII repertoire, while still encoding them using the US-ASCII 646 repertoire. 648 2.3.2. Senders and Receivers 650 In RFC 821, the two hosts participating in an SMTP transaction were 651 described as the "SMTP-sender" and "SMTP-receiver". This document 652 has been changed to reflect current industry terminology and hence 653 refers to them as the "SMTP client" (or sometimes just "the client") 654 and "SMTP server" (or just "the server"), respectively. Since a 655 given host may act both as server and client in a relay situation, 656 "receiver" and "sender" terminology is still used where needed for 657 clarity. 659 2.3.3. Mail Agents and Message Stores 661 Additional mail system terminology became common after RFC 821 was 662 published and, where convenient, is used in this specification. In 663 particular, SMTP servers and clients provide a mail transport service 664 and therefore act as "Mail Transfer Agents" (MTAs). "Mail User 665 Agents" (MUAs or UAs) are normally thought of as the sources and 666 targets of mail. At the source, an MUA might collect mail to be 667 transmitted from a user and hand it off to an MTA or, more commonly 668 in recent years, a specialized variation on an MTA called a 669 "Submission Server" (MSA) [42]. . At the other end of the process, 670 the final ("delivery") MTA would be thought of as handing the mail 671 off to an MUA (or at least transferring responsibility to it, e.g., 672 by depositing the message in a "message store"). However, while 673 these terms are used with at least the appearance of great precision 674 in other environments, the implied boundaries between MUAs and MTAs 675 often do not accurately match common, and conforming, practices with 676 Internet mail. Hence, the reader should be cautious about inferring 677 the strong relationships and responsibilities that might be implied 678 if these terms were used elsewhere 680 2.3.4. Host 682 For the purposes of this specification, a host is a computer system 683 attached to the Internet (or, in some cases, to a private TCP/IP 684 network) and supporting the SMTP protocol. Hosts are known by names 685 (see the next section); they SHOULD NOT be identified by numerical 686 addresses, i.e., by address literals as described in Section 4.1.2. 688 2.3.5. Domain Names 690 A domain name (or often just a "domain") consists of one or more 691 components, separated by dots if more than one appears. In the case 692 of a top-level domain used by itself in an email address, a single 693 string is used without any dots. This makes the requirement, 694 described in more detail below, that only fully-qualified domain 695 names appear in SMTP transactions on the public Internet, 696 particularly important where top-level domains are involved. These 697 components ("labels" in DNS terminology, RFC 1035 [4]) are restricted 698 for SMTP purposes to consist of a sequence of letters, digits, and 699 hyphens drawn from the ASCII character set [2] and conforming to what 700 RFC 1035 Section 2.3.1 calls the "preferred name syntax". Domain 701 names are used as names of hosts and of other entities in the domain 702 name hierarchy. For example, a domain may refer to an alias (label 703 of a CNAME RR) or the label of Mail eXchanger records to be used to 704 deliver mail instead of representing a host name. See RFC 1035 [4] 705 and Section 5 of this specification. 707 The domain name, as described in this document and in RFC 1035 [4], 708 is the entire, fully-qualified name (often referred to as an "FQDN"). 709 A domain name that is not in FQDN form is no more than a local alias. 710 Local aliases MUST NOT appear in any SMTP transaction. 712 Only resolvable, fully-qualified domain names (FQDNs) are permitted 713 when domain names are used in SMTP. In particular, names that can be 714 resolved to MX RRs or address (i.e., A or AAAA) RRs (as discussed in 715 Section 5) are permitted, as are CNAME RRs whose targets can be 716 resolved, in turn, to MX or address RRs. Local nicknames or 717 unqualified names MUST NOT be used. There are two exceptions to the 718 rule requiring FQDNs: 720 * The domain name given in the EHLO command MUST be either a primary 721 host name (a domain name that resolves to an address RR) or, if 722 the host has no name, an address literal, as described in 723 Section 4.1.3 and discussed further in the EHLO discussion of 724 Section 4.1.4. 726 * The reserved mailbox name "postmaster" may be used in a RCPT 727 command without domain qualification (see Section 4.1.1.3) and 728 MUST be accepted if so used. 730 2.3.6. Buffer and State Table 732 SMTP sessions are stateful, with both parties carefully maintaining a 733 common view of the current state. In this document, we model this 734 state by a virtual "buffer" and a "state table" on the server that 735 may be used by the client to, for example, "clear the buffer" or 736 "reset the state table", causing the information in the buffer to be 737 discarded and the state to be returned to some previous state. 739 2.3.7. Commands and Replies 741 SMTP commands and, unless altered by a service extension, message 742 data, are transmitted from the sender to the receiver via the 743 transmission channel in "lines". 745 An SMTP reply is an acknowledgment (positive or negative) sent in 746 "lines" from receiver to sender via the transmission channel in 747 response to a command. The general form of a reply is a numeric 748 completion code (indicating failure or success) usually followed by a 749 text string. The codes are for use by programs and the text is 750 usually intended for human users. RFC 3463 [7], specifies further 751 structuring of the reply strings, including the use of supplemental 752 and more specific completion codes (see also RFC 5248 [46]). 754 2.3.8. Lines 756 Lines consist of zero or more data characters terminated by the 757 sequence ASCII character "CR" (hex value 0D) followed immediately by 758 ASCII character "LF" (hex value 0A). This termination sequence is 759 denoted as in this document. Conforming implementations MUST 760 NOT recognize or generate any other character or character sequence 761 as a line terminator. Limits MAY be imposed on line lengths by 762 servers (see Section 4). 764 In addition, the appearance of "bare" "CR" or "LF" characters in text 765 (i.e., either without the other) has a long history of causing 766 problems in mail implementations and applications that use the mail 767 system as a tool. SMTP client implementations MUST NOT transmit 768 these characters except when they are intended as line terminators 769 and then MUST, as indicated above, transmit them only as a 770 sequence. 772 2.3.9. Message Content and Mail Data 774 The terms "message content" and "mail data" are used interchangeably 775 in this document to describe the material transmitted after the DATA 776 command is accepted and before the end of data indication is 777 transmitted. Message content includes the message header section and 778 the possibly structured message body. In the absence of extensions, 779 both are required to be ASCII (see Section 2.3.1). The MIME 780 specification (RFC 2045 [25]) provides the standard mechanisms for 781 structured message bodies. 783 2.3.10. Originator, Delivery, Relay, and Gateway Systems 785 This specification makes a distinction among four types of SMTP 786 systems, based on the role those systems play in transmitting 787 electronic mail. An "originating" system (sometimes called an SMTP 788 originator) introduces mail into the Internet or, more generally, 789 into a transport service environment. A "delivery" SMTP system is 790 one that receives mail from a transport service environment and 791 passes it to a mail user agent or deposits it in a message store that 792 a mail user agent is expected to subsequently access. A "relay" SMTP 793 system (usually referred to just as a "relay") receives mail from an 794 SMTP client and transmits it, without modification to the message 795 data other than adding trace information (see Section 4.4), to 796 another SMTP server for further relaying or for delivery. 798 A "gateway" SMTP system (usually referred to just as a "gateway") 799 receives mail from a client system in one transport environment and 800 transmits it to a server system in another transport environment. 801 Differences in protocols or message semantics between the transport 802 environments on either side of a gateway may require that the gateway 803 system perform transformations to the message that are not permitted 804 to SMTP relay systems. For the purposes of this specification, 805 firewalls that rewrite addresses should be considered as gateways, 806 even if SMTP is used on both sides of them (see RFC 2979 [32]). 807 // [5321bis] [[Note in draft/Placeholder: There has been a request to 808 // expand this section, possibly into a more extensive model of 809 // Internet mail. Comments from others solicited. In particular, 810 // does RFC 5598 make that suggestion OBE?]] 812 2.3.11. Mailbox and Address 814 As used in this specification, an "address" is a character string 815 that identifies a user to whom mail will be sent or a location into 816 which mail will be deposited. The term "mailbox" refers to that 817 depository. The two terms are typically used interchangeably unless 818 the distinction between the location in which mail is placed (the 819 mailbox) and a reference to it (the address) is important. An 820 address normally consists of user and domain specifications. The 821 standard mailbox naming convention is defined to be "local- 822 part@domain"; contemporary usage permits a much broader set of 823 applications than simple "user names". Consequently, and due to a 824 long history of problems when intermediate hosts have attempted to 825 optimize transport by modifying them, the local-part MUST be 826 interpreted and assigned semantics only by the host specified in the 827 domain part of the address. 829 2.4. General Syntax Principles and Transaction Model 831 SMTP commands and replies have a rigid syntax. All commands begin 832 with a command verb. All replies begin with a three digit numeric 833 code. In some commands and replies, arguments are required following 834 the verb or reply code. Some commands do not accept arguments (after 835 the verb), and some reply codes are followed, sometimes optionally, 836 by free form text. In both cases, where text appears, it is 837 separated from the verb or reply code by a space character. Complete 838 definitions of commands and replies appear in Section 4. 840 Verbs and argument values (e.g., "TO:" or "to:" in the RCPT command 841 and extension name keywords) are not case sensitive, with the sole 842 exception in this specification of a mailbox local-part (SMTP 843 Extensions may explicitly specify case-sensitive elements). That is, 844 a command verb, an argument value other than a mailbox local-part, 845 and free form text MAY be encoded in upper case, lower case, or any 846 mixture of upper and lower case with no impact on its meaning. The 847 local-part of a mailbox MUST BE treated as case sensitive. 848 Therefore, SMTP implementations MUST take care to preserve the case 849 of mailbox local-parts. In particular, for some hosts, the user 850 "smith" is different from the user "Smith". However, exploiting the 851 case sensitivity of mailbox local-parts impedes interoperability and 852 is discouraged. Mailbox domains follow normal DNS rules and are 853 hence not case sensitive. 855 A few SMTP servers, in violation of this specification (and RFC 821) 856 require that command verbs be encoded by clients in upper case. 857 Implementations MAY wish to employ this encoding to accommodate those 858 servers. 860 The argument clause consists of a variable-length character string 861 ending with the end of the line, i.e., with the character sequence 862 . The receiver will take no action until this sequence is 863 received. 865 The syntax for each command is shown with the discussion of that 866 command. Common elements and parameters are shown in Section 4.1.2. 868 Commands and replies are composed of characters from the ASCII 869 character set [2]. When the transport service provides an 8-bit byte 870 (octet) transmission channel, each 7-bit character is transmitted, 871 right justified, in an octet with the high-order bit cleared to zero. 872 More specifically, the unextended SMTP service provides 7-bit 873 transport only. An originating SMTP client that has not successfully 874 negotiated an appropriate extension with a particular server (see the 875 next paragraph) MUST NOT transmit messages with information in the 876 high-order bit of octets. If such messages are transmitted in 877 violation of this rule, receiving SMTP servers MAY clear the high- 878 order bit or reject the message as invalid. In general, a relay SMTP 879 SHOULD assume that the message content it has received is valid and, 880 assuming that the envelope permits doing so, relay it without 881 inspecting that content. Of course, if the content is mislabeled and 882 the data path cannot accept the actual content, this may result in 883 the ultimate delivery of a severely garbled message to the recipient. 884 Delivery SMTP systems MAY reject such messages, or return them as 885 undeliverable, rather than deliver them. In the absence of a server- 886 offered extension explicitly permitting it, a sending SMTP system is 887 not permitted to send envelope commands in any character set other 888 than US-ASCII. Receiving systems SHOULD reject such commands, 889 normally using "500 syntax error - invalid character" replies. 891 8-bit message content transmission MAY be requested of the server by 892 a client using extended SMTP facilities, notably the "8BITMIME" 893 extension, RFC 6152 [47]. 8BITMIME SHOULD be supported by SMTP 894 servers. However, it MUST NOT be construed as authorization to 895 transmit unrestricted 8-bit material, nor does 8BITMIME authorize 896 transmission of any envelope material in other than ASCII. 8BITMIME 897 MUST NOT be requested by senders for material with the high bit on 898 that is not in MIME format with an appropriate content-transfer 899 encoding; servers MAY reject such messages. 901 The metalinguistic notation used in this document corresponds to the 902 "Augmented BNF" used in other Internet mail system documents. The 903 reader who is not familiar with that syntax should consult the ABNF 904 specification in RFC 5234 [11]. Metalanguage terms used in running 905 text are surrounded by pointed brackets (e.g., ) for clarity. 906 The reader is cautioned that the grammar expressed in the 907 metalanguage is not comprehensive. There are many instances in which 908 provisions in the text constrain or otherwise modify the syntax or 909 semantics implied by the grammar. 911 3. The SMTP Procedures: An Overview 913 This section contains descriptions of the procedures used in SMTP: 914 session initiation, mail transaction, forwarding mail, verifying 915 mailbox names and expanding mailing lists, and opening and closing 916 exchanges. Comments on relaying, a note on mail domains, and a 917 discussion of changing roles are included at the end of this section. 918 Several complete scenarios are presented in Appendix D. 920 3.1. Session Initiation 922 An SMTP session is initiated when a client opens a connection to a 923 server and the server responds with an opening message. 925 SMTP server implementations MAY include identification of their 926 software and version information in the connection greeting reply 927 after the 220 code, a practice that permits more efficient isolation 928 and repair of any problems. Implementations MAY make provision for 929 SMTP servers to disable the software and version announcement where 930 it causes security concerns. While some systems also identify their 931 contact point for mail problems, this is not a substitute for 932 maintaining the required "postmaster" address (see Section 4). 934 The SMTP protocol allows a server to formally reject a mail session 935 while still allowing the initial connection as follows: a 521 936 response MAY be given in the initial connection opening message 937 instead of the 220. A server taking this approach MUST still wait 938 for the client to send a QUIT (see Section 4.1.1.10) before closing 939 the connection and SHOULD respond to any intervening commands with 940 "503 bad sequence of commands". Since an attempt to make an SMTP 941 connection to such a system is probably in error, a server returning 942 a 521 944 // (or 554?) 945 response on connection opening SHOULD provide enough information in 946 the reply text to facilitate debugging of the sending system. See 947 Section 4.2.4.2. 949 3.2. Client Initiation 951 Once the server has sent the greeting (welcoming) message and the 952 client has received it, the client normally sends the EHLO command to 953 the server, indicating the client's identity. In addition to opening 954 the session, use of EHLO indicates that the client is able to process 955 service extensions and requests that the server provide a list of the 956 extensions it supports. Older SMTP systems that are unable to 957 support service extensions, and contemporary clients that do not 958 require service extensions in the mail session being initiated, MAY 959 use HELO instead of EHLO. Servers MUST NOT return the extended EHLO- 960 style response to a HELO command. For a particular connection 961 attempt, if the server returns a "command not recognized" response to 962 EHLO, the client SHOULD be able to fall back and send HELO. 964 In the EHLO command, the host sending the command identifies itself; 965 the command may be interpreted as saying "Hello, I am " (and, 966 in the case of EHLO, "and I support service extension requests"). 968 3.3. Mail Transactions 970 There are three steps to SMTP mail transactions. The transaction 971 starts with a MAIL command that gives the sender identification. (In 972 general, the MAIL command may be sent only when no mail transaction 973 is in progress; see Section 4.1.4.) A series of one or more RCPT 974 commands follows, giving the receiver information. Then, a DATA 975 command initiates transfer of the mail data and is terminated by the 976 "end of mail" data indicator, which also confirms (and terminates) 977 the transaction. 979 Mail transactions are also terminated by the RSET command 980 (Section 4.1.1.5), the sending of an EHLO command (Section 3.2), or 981 the sending of a QUIT command (Section 3.8) which terminates both any 982 active mail transaction and the SMTP connection. 984 The first step in the procedure is the MAIL command. 986 MAIL FROM: [SP ] 988 This command tells the SMTP-receiver that a new mail transaction is 989 starting and to reset all its state tables and buffers, including any 990 recipients or mail data. The portion of the first or 991 only argument contains the source mailbox (between "<" and ">" 992 brackets), which can be used to report errors (see Section 4.2 for a 993 discussion of error reporting). If accepted, the SMTP server returns 994 a "250 OK" reply. If the mailbox specification is not acceptable for 995 some reason, the server MUST return a reply indicating whether the 996 failure is permanent (i.e., will occur again if the client tries to 997 send the same address again) or temporary (i.e., the address might be 998 accepted if the client tries again later). Despite the apparent 999 scope of this requirement, there are circumstances in which the 1000 acceptability of the reverse-path may not be determined until one or 1001 more forward-paths (in RCPT commands) can be examined. In those 1002 cases, the server MAY reasonably accept the reverse-path (with a 250 1003 reply) and then report problems after the forward-paths are received 1004 and examined. Normally, failures produce 550 or 553 replies. 1006 Historically, the was permitted to contain more than 1007 just a mailbox; however, contemporary systems SHOULD NOT use source 1008 routing (see Appendix C). 1010 The optional are associated with negotiated SMTP 1011 service extensions (see Section 2.2). 1013 The second step in the procedure is the RCPT command. This step of 1014 the procedure can be repeated any number of times. 1016 RCPT TO: [ SP ] 1018 The first or only argument to this command includes a forward-path 1019 (normally a mailbox local-part and domain, always surrounded by "<" 1020 and ">" brackets) identifying one recipient. If accepted, the SMTP 1021 server returns a "250 OK" reply and stores the forward-path. If the 1022 recipient is known not to be a deliverable address, the SMTP server 1023 returns a 550 reply, typically with a string such as "no such user - 1024 " and the mailbox name (other circumstances and reply codes are 1025 possible). 1027 The can contain more than just a mailbox. 1028 Historically, the was permitted to contain a source 1029 routing list of hosts and the destination mailbox; however, 1030 contemporary SMTP clients SHOULD NOT utilize source routes (see 1031 Appendix C). Servers MUST be prepared to encounter a list of source 1032 routes in the forward-path, but they SHOULD ignore the routes or MAY 1033 decline to support the relaying they imply. Similarly, servers MAY 1034 decline to accept mail that is destined for other hosts or systems. 1035 These restrictions make a server useless as a relay for clients that 1036 do not support full SMTP functionality. Consequently, restricted- 1037 capability clients MUST NOT assume that any SMTP server on the 1038 Internet can be used as their mail processing (relaying) site. If a 1039 RCPT command appears without a previous MAIL command, the server MUST 1040 return a 503 "Bad sequence of commands" response. The optional 1041 are associated with negotiated SMTP service 1042 extensions (see Section 2.2). 1043 // [5321bis]: this section would be improved by being more specific 1044 // about where mail transactions begin and end and then talking about 1045 // "transaction state" here, rather than specific prior commands. 1046 // --JcK 1048 Since it has been a common source of errors, it is worth noting that 1049 spaces are not permitted on either side of the colon following FROM 1050 in the MAIL command or TO in the RCPT command. The syntax is exactly 1051 as given above. 1053 The third step in the procedure is the DATA command (or some 1054 alternative specified in a service extension). 1056 DATA 1058 If accepted, the SMTP server returns a 354 Intermediate reply and 1059 considers all succeeding lines up to but not including the end of 1060 mail data indicator to be the message text. When the end of text is 1061 successfully received and stored, the SMTP-receiver sends a "250 OK" 1062 reply. 1064 Since the mail data is sent on the transmission channel, the end of 1065 mail data must be indicated so that the command and reply dialog can 1066 be resumed. SMTP indicates the end of the mail data by sending a 1067 line containing only a "." (period or full stop, hex 2E). A 1068 transparency procedure is used to prevent this from interfering with 1069 the user's text (see Section 4.5.2). 1071 The end of mail data indicator also confirms the mail transaction and 1072 tells the SMTP server to now process the stored recipients and mail 1073 data. If accepted, the SMTP server returns a "250 OK" reply. The 1074 DATA command can fail at only two points in the protocol exchange: 1076 If there was no MAIL, or no RCPT, command, or all such commands were 1077 rejected, the server MAY return a "command out of sequence" (503) or 1078 "no valid recipients" (554) reply in response to the DATA command. 1079 If one of those replies (or any other 5yz reply) is received, the 1080 client MUST NOT send the message data; more generally, message data 1081 MUST NOT be sent unless a 354 reply is received. 1083 If the verb is initially accepted and the 354 reply issued, the DATA 1084 command should fail only if the mail transaction was incomplete (for 1085 example, no recipients), if resources were unavailable (including, of 1086 course, the server unexpectedly becoming unavailable), or if the 1087 server determines that the message should be rejected for policy or 1088 other reasons. 1090 However, in practice, some servers do not perform recipient 1091 verification until after the message text is received. These servers 1092 SHOULD treat a failure for one or more recipients as a "subsequent 1093 failure" and return a mail message as discussed in Section 6 and, in 1094 particular, in Section 6.1. Using a "550 mailbox not found" (or 1095 equivalent) reply code after the data are accepted makes it difficult 1096 or impossible for the client to determine which recipients failed. 1098 When the RFC 822 format ([13], [12]) is being used, the mail data 1099 include the header fields such as those named Date, Subject, To, Cc, 1100 and From. Server SMTP systems SHOULD NOT reject messages based on 1101 perceived defects in the RFC 822 or MIME (RFC 2045 [25]) message 1102 header section or message body. In particular, they MUST NOT reject 1103 messages in which the numbers of Resent-header fields do not match or 1104 Resent-to appears without Resent-from and/or Resent-date. 1106 Mail transaction commands MUST be used in the order discussed above. 1108 3.4. Forwarding for Address Correction or Updating 1110 Forwarding support is most often required to consolidate and simplify 1111 addresses within, or relative to, some enterprise and less frequently 1112 to establish addresses to link a person's prior address with a 1113 current one. Silent forwarding of messages (without server 1114 notification to the sender), for security or non-disclosure purposes, 1115 is common in the contemporary Internet. 1117 In both the enterprise and the "new address" cases, information 1118 hiding (and sometimes security) considerations argue against exposure 1119 of the "final" address through the SMTP protocol as a side effect of 1120 the forwarding activity. This may be especially important when the 1121 final address may not even be reachable by the sender. Consequently, 1122 the "forwarding" mechanisms described in Section 3.2 of RFC 821, and 1123 especially the 251 (corrected destination) and 551 reply codes from 1124 RCPT must be evaluated carefully by implementers and, when they are 1125 available, by those configuring systems (see also Section 7.4). 1127 In particular: 1129 * Servers MAY forward messages when they are aware of an address 1130 change. When they do so, they MAY either provide address-updating 1131 information with a 251 code, or may forward "silently" and return 1132 a 250 code. However, if a 251 code is used, they MUST NOT assume 1133 that the client will actually update address information or even 1134 return that information to the user. 1136 Alternately, 1138 * Servers MAY reject messages or return them as non-deliverable when 1139 they cannot be delivered precisely as addressed. When they do so, 1140 they MAY either provide address-updating information with a 551 1141 code, or may reject the message as undeliverable with a 550 code 1142 and no address-specific information. However, if a 551 code is 1143 used, they MUST NOT assume that the client will actually update 1144 address information or even return that information to the user. 1146 SMTP server implementations that support the 251 and/or 551 reply 1147 codes SHOULD provide configuration mechanisms so that sites that 1148 conclude that they would undesirably disclose information can disable 1149 or restrict their use. 1151 3.5. Commands for Debugging Addresses 1153 3.5.1. Overview 1155 SMTP provides commands to verify a user name or obtain the content of 1156 a mailing list. This is done with the VRFY and EXPN commands, which 1157 have character string arguments. Implementations SHOULD support VRFY 1158 and EXPN (however, see Section 3.5.2 and Section 7.3). 1160 For the VRFY command, the string is a user name or a user name and 1161 domain (see below). If a normal (i.e., 250) response is returned, 1162 the response MAY include the full name of the user and MUST include 1163 the mailbox of the user. It MUST be in either of the following 1164 forms: 1166 User Name 1167 local-part@domain 1169 When a name that is the argument to VRFY could identify more than one 1170 mailbox, the server MAY either note the ambiguity or identify the 1171 alternatives. In other words, any of the following are legitimate 1172 responses to VRFY: 1174 553 User ambiguous 1176 or 1177 553- Ambiguous; Possibilities are 1178 553-Joe Smith 1179 553-Harry Smith 1180 553 Melvin Smith 1182 or 1184 553-Ambiguous; Possibilities 1185 553- 1186 553- 1187 553 1189 Under normal circumstances, a client receiving a 553 reply would be 1190 expected to expose the result to the user. Use of exactly the forms 1191 given, and the "user ambiguous" or "ambiguous" keywords, possibly 1192 supplemented by extended reply codes, such as those described in RFC 1193 3463 [7], will facilitate automated translation into other languages 1194 as needed. Of course, a client that was highly automated or that was 1195 operating in another language than English might choose to try to 1196 translate the response to return some other indication to the user 1197 than the literal text of the reply, or to take some automated action 1198 such as consulting a directory service for additional information 1199 before reporting to the user. 1201 For the EXPN command, the string identifies a mailing list, and the 1202 successful (i.e., 250) multiline response MAY include the full name 1203 of the users and MUST give the mailboxes on the mailing list. 1205 In some hosts, the distinction between a mailing list and an alias 1206 for a single mailbox is a bit fuzzy, since a common data structure 1207 may hold both types of entries, and it is possible to have mailing 1208 lists containing only one mailbox. If a request is made to apply 1209 VRFY to a mailing list, a positive response MAY be given if a message 1210 so addressed would be delivered to everyone on the list, otherwise an 1211 error SHOULD be reported (e.g., "550 That is a mailing list, not a 1212 user" or "252 Unable to verify members of mailing list"). If a 1213 request is made to expand a user name, the server MAY return a 1214 positive response consisting of a list containing one name, or an 1215 error MAY be reported (e.g., "550 That is a user name, not a mailing 1216 list"). 1218 In the case of a successful multiline reply (normal for EXPN), 1219 exactly one mailbox is to be specified on each line of the reply. 1220 The case of an ambiguous request is discussed above. 1222 "User name" is a fuzzy term and has been used deliberately. An 1223 implementation of the VRFY or EXPN commands MUST include at least 1224 recognition of local mailboxes as "user names". However, since 1225 current Internet practice often results in a single host handling 1226 mail for multiple domains, hosts, especially hosts that provide this 1227 functionality, SHOULD accept the "local-part@domain" form as a "user 1228 name"; hosts MAY also choose to recognize other strings as "user 1229 names". 1231 The case of expanding a mailbox list requires a multiline reply, such 1232 as: 1234 C: EXPN Example-People 1235 S: 250-Jon Postel 1236 S: 250-Fred Fonebone 1237 S: 250 Sam Q. Smith 1239 or 1241 C: EXPN Executive-Washroom-List 1242 S: 550 Access Denied to You. 1244 The character string arguments of the VRFY and EXPN commands cannot 1245 be further restricted due to the variety of implementations of the 1246 user name and mailbox list concepts. On some systems, it may be 1247 appropriate for the argument of the EXPN command to be a file name 1248 for a file containing a mailing list, but again there are a variety 1249 of file naming conventions in the Internet. Similarly, historical 1250 variations in what is returned by these commands are such that the 1251 response SHOULD be interpreted very carefully, if at all, and SHOULD 1252 generally only be used for diagnostic purposes. 1254 3.5.2. VRFY Normal Response 1256 When normal (2yz or 551) responses are returned from a VRFY or EXPN 1257 request, the reply MUST include the name using a "" construction, where "domain" is a fully-qualified 1259 domain name. In circumstances exceptional enough to justify 1260 violating the intent of this specification, free-form text MAY be 1261 returned. In order to facilitate parsing by both computers and 1262 people, addresses SHOULD appear in pointed brackets. When addresses, 1263 rather than free-form debugging information, are returned, EXPN and 1264 VRFY MUST return only valid domain addresses that are usable in SMTP 1265 RCPT commands. Consequently, if an address implies delivery to a 1266 program or other system, the mailbox name used to reach that target 1267 MUST be given. Paths (explicit source routes) MUST NOT be returned 1268 by VRFY or EXPN. 1270 Server implementations SHOULD support both VRFY and EXPN. For 1271 security reasons, implementations MAY provide local installations a 1272 way to disable either or both of these commands through configuration 1273 options or the equivalent (see Section 7.3). When these commands are 1274 supported, they are not required to work across relays when relaying 1275 is supported. Since they were both optional in RFC 821, but VRFY was 1276 made mandatory in RFC 1123 [5], if EXPN is supported, it MUST be 1277 listed as a service extension in an EHLO response. VRFY MAY be 1278 listed as a convenience but, since support for it is required, SMTP 1279 clients are not required to check for its presence on the extension 1280 list before using it. 1282 3.5.3. Meaning of VRFY or EXPN Success Response 1284 A server MUST NOT return a 250 code in response to a VRFY or EXPN 1285 command unless it has actually verified the address. In particular, 1286 a server MUST NOT return 250 if all it has done is to verify that the 1287 syntax given is valid. In that case, 502 (Command not implemented) 1288 or 500 (Syntax error, command unrecognized) SHOULD be returned. As 1289 stated elsewhere, implementation (in the sense of actually validating 1290 addresses and returning information) of VRFY and EXPN are strongly 1291 recommended. Hence, implementations that return 500 or 502 for VRFY 1292 are not in full compliance with this specification. 1294 There may be circumstances where an address appears to be valid but 1295 cannot reasonably be verified in real time, particularly when a 1296 server is acting as a mail exchanger for another server or domain. 1297 "Apparent validity", in this case, would normally involve at least 1298 syntax checking and might involve verification that any domains 1299 specified were ones to which the host expected to be able to relay 1300 mail. In these situations, reply code 252 SHOULD be returned. These 1301 cases parallel the discussion of RCPT verification in Section 2.1. 1302 Similarly, the discussion in Section 3.4 applies to the use of reply 1303 codes 251 and 551 with VRFY (and EXPN) to indicate addresses that are 1304 recognized but that would be forwarded or rejected were mail received 1305 for them. Implementations generally SHOULD be more aggressive about 1306 address verification in the case of VRFY than in the case of RCPT, 1307 even if it takes a little longer to do so. 1309 3.5.4. Semantics and Applications of EXPN 1311 EXPN is often very useful in debugging and understanding problems 1312 with mailing lists and multiple-target-address aliases. Some systems 1313 have attempted to use source expansion of mailing lists as a means of 1314 eliminating duplicates. The propagation of aliasing systems with 1315 mail on the Internet for hosts (typically with MX and CNAME DNS 1316 records), for mailboxes (various types of local host aliases), and in 1317 various proxying arrangements has made it nearly impossible for these 1318 strategies to work consistently, and mail systems SHOULD NOT attempt 1319 them. 1321 3.6. Relaying and Mail Routing 1323 3.6.1. Source Routes and Relaying 1325 In general, the availability of Mail eXchanger records in the domain 1326 name system (RFC 1035 [4], RFC 974 [16]) makes the use of explicit 1327 source routes in the Internet mail system unnecessary. Many 1328 historical problems with the interpretation of explicit source routes 1329 have made their use undesirable. SMTP clients SHOULD NOT generate 1330 explicit source routes except under unusual circumstances. SMTP 1331 servers MAY decline to act as mail relays or to accept addresses that 1332 specify source routes. When route information is encountered, SMTP 1333 servers MAY ignore the route information and simply send to the final 1334 destination specified as the last element in the route and SHOULD do 1335 so. There has been an invalid practice of using names that do not 1336 appear in the DNS as destination names, with the senders counting on 1337 the intermediate hosts specified in source routing to resolve any 1338 problems. If source routes are stripped, this practice will cause 1339 failures. This is one of several reasons why SMTP clients MUST NOT 1340 generate invalid source routes or depend on serial resolution of 1341 names in such routes. 1343 When source routes are not used, the process described in RFC 821 for 1344 constructing a reverse-path from the forward-path is not applicable 1345 and the reverse-path at the time of delivery will simply be the 1346 address that appeared in the MAIL command. 1348 3.6.2. Mail eXchange Records and Relaying 1350 A relay SMTP server is usually the target of a DNS MX record that 1351 designates it, rather than the final delivery system. The relay 1352 server may accept or reject the task of relaying the mail in the same 1353 way it accepts or rejects mail for a local user. If it accepts the 1354 task, it then becomes an SMTP client, establishes a transmission 1355 channel to the next SMTP server specified in the DNS (according to 1356 the rules in Section 5), and sends it the mail. If it declines to 1357 relay mail to a particular address for policy reasons, a 550 response 1358 SHOULD be returned. 1360 // Text below reflects proposed replacement of the paragraph ( edited 1361 // version of suggestion by D Crocker 2021-03-17 17:23 email). Cf. 1362 // Ticket #30: 1363 This specification does not deal with the verification of return 1364 paths. Server efforts to verify a return path and actions to be 1365 taken under various circumstances are outside the scope of this 1366 specification. 1368 3.6.3. Message Submission Servers as Relays 1370 Many mail-sending clients exist, especially in conjunction with 1371 facilities that receive mail via POP3 or IMAP, that have limited 1372 capability to support some of the requirements of this specification, 1373 such as the ability to queue messages for subsequent delivery 1374 attempts. For these clients, it is common practice to make private 1375 arrangements to send all messages to a single server for processing 1376 and subsequent distribution. SMTP, as specified here, is not ideally 1377 suited for this role. A standardized mail submission protocol has 1378 been developed that is gradually superseding practices based on SMTP 1379 (see RFC 6409 [42]). In any event, because these arrangements are 1380 private and fall outside the scope of this specification, they are 1381 not described here. 1383 It is important to note that MX records can point to SMTP servers 1384 that act as gateways into other environments, not just SMTP relays 1385 and final delivery systems; see Sections 3.7 and 5. 1387 If an SMTP server has accepted the task of relaying the mail and 1388 later finds that the destination is incorrect or that the mail cannot 1389 be delivered for some other reason, then it MUST construct an 1390 "undeliverable mail" notification message and send it to the 1391 originator of the undeliverable mail (as indicated by the reverse- 1392 path). Formats specified for non-delivery reports by other standards 1393 (see, for example, RFC 3461 [34] and RFC 3464 [35]) SHOULD be used if 1394 possible. 1396 This notification message must be from the SMTP server at the relay 1397 host or the host that first determines that delivery cannot be 1398 accomplished. Of course, SMTP servers MUST NOT send notification 1399 messages about problems transporting notification messages. One way 1400 to prevent loops in error reporting is to specify a null reverse-path 1401 in the MAIL command of a notification message. When such a message 1402 is transmitted, the reverse-path MUST be set to null (see 1403 Section 4.5.5 for additional discussion). A MAIL command with a null 1404 reverse-path appears as follows: 1406 MAIL FROM:<> 1408 As discussed in Section 6.4, a relay SMTP has no need to inspect or 1409 act upon the header section or body of the message data and MUST NOT 1410 do so except to add its own "Received:" header field (Section 4.4.1 1411 and possibly other trace header fields) and, optionally, to attempt 1412 to detect looping in the mail system (see Section 6.3). Of course, 1413 this prohibition also applies to any modifications of these header 1414 fields or text (see also Section 7.9). 1416 3.7. Mail Gatewaying 1418 While the relay function discussed above operates within the Internet 1419 SMTP transport service environment, MX records or various forms of 1420 explicit routing may require that an intermediate SMTP server perform 1421 a translation function between one transport service and another. As 1422 discussed in Section 2.3.10, when such a system is at the boundary 1423 between two transport service environments, we refer to it as a 1424 "gateway" or "gateway SMTP". 1426 Gatewaying mail between different mail environments, such as 1427 different mail formats and protocols, is complex and does not easily 1428 yield to standardization. However, some general requirements may be 1429 given for a gateway between the Internet and another mail 1430 environment. 1432 3.7.1. Header Fields in Gatewaying 1434 Header fields MAY be rewritten when necessary as messages are 1435 gatewayed across mail environment boundaries. This may involve 1436 inspecting the message body or interpreting the local-part of the 1437 destination address in spite of the prohibitions in Section 6.4. 1439 Other mail systems gatewayed to the Internet often use a subset of 1440 the RFC 822 header section or provide similar functionality with a 1441 different syntax, but some of these mail systems do not have an 1442 equivalent to the SMTP envelope. Therefore, when a message leaves 1443 the Internet environment, it may be necessary to fold the SMTP 1444 envelope information into the message header section. A possible 1445 solution would be to create new header fields to carry the envelope 1446 information (e.g., "X-SMTP-MAIL:" and "X-SMTP-RCPT:"); however, this 1447 would require changes in mail programs in foreign environments and 1448 might risk disclosure of private information (see Section 7.2). 1450 3.7.2. Received Lines in Gatewaying 1452 When forwarding a message into or out of the Internet environment, a 1453 gateway MUST prepend a Received: line, but it MUST NOT alter in any 1454 way a Received: line that is already in the header section. 1456 "Received:" header fields of messages originating from other 1457 environments may not conform exactly to this specification. However, 1458 the most important use of Received: lines is for debugging mail 1459 faults, and this debugging can be severely hampered by well-meaning 1460 gateways that try to "fix" a Received: line. As another consequence 1461 of trace header fields arising in non-SMTP environments, receiving 1462 systems MUST NOT reject mail based on the format of a trace header 1463 field and SHOULD be extremely robust in the light of unexpected 1464 information or formats in those header fields. 1466 The gateway SHOULD indicate the environment and protocol in the "via" 1467 clauses of Received header field(s) that it supplies. 1469 3.7.3. Addresses in Gatewaying 1471 From the Internet side, the gateway SHOULD accept all valid address 1472 formats in SMTP commands and in the RFC 822 header section, and all 1473 valid RFC 822 messages. Addresses and header fields generated by 1474 gateways MUST conform to applicable standards (including this one and 1475 RFC 5322 [12]). Gateways are, of course, subject to the same rules 1476 for handling source routes as those described for other SMTP systems 1477 in Section 3.3. 1479 3.7.4. Other Header Fields in Gatewaying 1481 The gateway MUST ensure that all header fields of a message that it 1482 forwards into the Internet mail environment meet the requirements for 1483 Internet mail. In particular, all addresses in "From:", "To:", 1484 "Cc:", etc., header fields MUST be transformed (if necessary) to 1485 satisfy the standard header syntax of RFC 5322 [12], MUST reference 1486 only fully-qualified domain names, and MUST be effective and useful 1487 for sending replies. The translation algorithm used to convert mail 1488 from the Internet protocols to another environment's protocol SHOULD 1489 ensure that error messages from the foreign mail environment are 1490 delivered to the reverse-path from the SMTP envelope, not to an 1491 address in the "From:", "Sender:", or similar header fields of the 1492 message. 1494 3.7.5. Envelopes in Gatewaying 1496 Similarly, when forwarding a message from another environment into 1497 the Internet, the gateway SHOULD set the envelope return path in 1498 accordance with an error message return address, if supplied by the 1499 foreign environment. If the foreign environment has no equivalent 1500 concept, the gateway must select and use a best approximation, with 1501 the message originator's address as the default of last resort. 1503 3.8. Terminating Sessions and Connections 1505 An SMTP connection is terminated when the client sends a QUIT 1506 command. The server responds with a positive reply code, after which 1507 it closes the connection. 1509 An SMTP server MUST NOT intentionally close the connection under 1510 normal operational circumstances (see Section 7.8) except: 1512 * After receiving a QUIT command and responding with a 221 reply. 1514 * After detecting the need to shut down the SMTP service and 1515 returning a 421 reply code. This reply code can be issued after 1516 the server receives any command or, if necessary, asynchronously 1517 from command receipt (on the assumption that the client will 1518 receive it after the next command is issued). 1520 * After a timeout, as specified in Section 4.5.3.2, occurs waiting 1521 for the client to send a command or data. 1523 In particular, a server that closes connections in response to 1524 commands that are not understood is in violation of this 1525 specification. Servers are expected to be tolerant of unknown 1526 commands, issuing a 500 reply and awaiting further instructions from 1527 the client. 1529 An SMTP server that is forcibly shut down via external means SHOULD 1530 attempt to send a line containing a 421 reply code to the SMTP client 1531 before exiting. The SMTP client will normally read the 421 reply 1532 code after sending its next command. 1534 SMTP clients that experience a connection close, reset, or other 1535 communications failure due to circumstances not under their control 1536 (in violation of the intent of this specification but sometimes 1537 unavoidable) SHOULD, to maintain the robustness of the mail system, 1538 treat the mail transaction as if a 421 response had been received and 1539 act accordingly. 1541 There are circumstances, contrary to the intent of this 1542 specification, in which an SMTP server may receive an indication that 1543 the underlying TCP connection has been closed or reset. To preserve 1544 the robustness of the mail system, SMTP servers SHOULD be prepared 1545 for this condition and SHOULD treat it as if a QUIT had been received 1546 before the connection disappeared. 1548 3.9. Aliases and Mailing Lists 1550 // [5321bis] If "alias and list models" are explained elsewhere, 1551 // cross reference. Also note that this section appears to prohibit 1552 // an exploder from adding List-* headers. That needs to be explicit 1553 // or finessed. 1554 An SMTP-capable host SHOULD support both the alias and the list 1555 models of address expansion for multiple delivery. When a message is 1556 delivered or forwarded to each address of an expanded list form, the 1557 return address in the envelope ("MAIL FROM:") MUST be changed to be 1558 the address of a person or other entity who administers the list. 1559 However, in this case, the message header section (RFC 5322 [12]) 1560 MUST be left unchanged; in particular, the "From" field of the header 1561 section is unaffected. 1563 An important mail facility is a mechanism for multi-destination 1564 delivery of a single message, by transforming (or "expanding" or 1565 "exploding") a pseudo-mailbox address into a list of destination 1566 mailbox addresses. When a message is sent to such a pseudo-mailbox 1567 (sometimes called an "exploder"), copies are forwarded or 1568 redistributed to each mailbox in the expanded list. Servers SHOULD 1569 simply utilize the addresses on the list; application of heuristics 1570 or other matching rules to eliminate some addresses, such as that of 1571 the originator, is strongly discouraged. We classify such a pseudo- 1572 mailbox as an "alias" or a "list", depending upon the expansion 1573 rules. 1575 3.9.1. Simple Aliases 1577 To expand an alias, the recipient mailer simply replaces the pseudo- 1578 mailbox address in the envelope with each of the expanded addresses 1579 in turn; the rest of the envelope and the message body are left 1580 unchanged. The message is then delivered or forwarded to each 1581 expanded address. 1583 3.9.2. Mailing Lists 1585 Processing of a mailing list may be said to operate by 1586 "redistribution" rather than by "forwarding" (as in the simple alias 1587 case in the subsection above). To expand a list, the recipient 1588 mailer replaces the pseudo-mailbox address in the envelope with each 1589 of the expanded addresses in turn. The return (backward-pointing) 1590 address in the envelope is changed so that all error messages 1591 generated by the final deliveries will be returned to a list 1592 administrator, not to the message originator, who generally has no 1593 control over the contents of the list and will typically find error 1594 messages annoying. Note that the key difference between handling 1595 simple aliases Section 3.9.1 and redistribution (this subsection) is 1596 the change to the backward-pointing address. When a system managing 1597 a list constrains its processing to the very limited set of 1598 modifications and actions described here, it is acting as part of an 1599 MTA; such list processing, like alias processing, can be treated as a 1600 continuation of email transit. 1602 Mailing list management systems do exist that perform additional, 1603 sometimes extensive, modifications to a message and its envelope. 1604 Such mailing lists need to be viewed as MUAs that accept a message 1605 delivery and then submit a new message for multiple recipients. 1607 4. The SMTP Specifications 1609 4.1. SMTP Commands 1611 4.1.1. Command Semantics and Syntax 1613 The SMTP commands define the mail transfer or the mail system 1614 function requested by the user. SMTP commands are character strings 1615 terminated by . The commands themselves are alphabetic 1616 characters terminated by if parameters follow and 1617 otherwise. (In the interest of improved interoperability, SMTP 1618 receivers SHOULD tolerate trailing white space before the terminating 1619 .) The syntax of the local part of a mailbox MUST conform to 1620 receiver site conventions and the syntax specified in Section 4.1.2. 1621 The SMTP commands are discussed below. The SMTP replies are 1622 discussed in Section 4.2. 1624 A mail transaction involves several data objects that are 1625 communicated as arguments to different commands. The reverse-path is 1626 the argument of the MAIL command, the forward-path is the argument of 1627 the RCPT command, and the mail data is the argument of the DATA 1628 command. These arguments or data objects must be transmitted and 1629 held, pending the confirmation communicated by the end of mail data 1630 indication that finalizes the transaction. The model for this is 1631 that distinct buffers are provided to hold the types of data objects; 1632 that is, there is a reverse-path buffer, a forward-path buffer, and a 1633 mail data buffer. Specific commands cause information to be appended 1634 to a specific buffer, or cause one or more buffers to be cleared. 1636 Several commands (RSET, DATA, QUIT) are specified as not permitting 1637 parameters. In the absence of specific extensions offered by the 1638 server and accepted by the client, clients MUST NOT send such 1639 parameters and servers SHOULD reject commands containing them as 1640 having invalid syntax. 1642 4.1.1.1. Extended HELLO (EHLO) or HELLO (HELO) 1644 These commands are used to identify the SMTP client to the SMTP 1645 server. The argument clause contains the fully-qualified domain name 1646 of the SMTP client, if one is available. In situations in which the 1647 SMTP client system does not have a meaningful domain name (e.g., when 1648 its address is dynamically allocated and no reverse mapping record is 1649 available), the client SHOULD send an address literal (see 1650 Section 4.1.3). 1652 RFC 2821, and some earlier informal practices, encouraged following 1653 the literal by information that would help to identify the client 1654 system. That convention was not widely supported, and many SMTP 1655 servers considered it an error. In the interest of interoperability, 1656 it is probably wise for servers to be prepared for this string to 1657 occur, but SMTP clients SHOULD NOT send it. 1659 The SMTP server identifies itself to the SMTP client in the 1660 connection greeting reply and in the response to this command. 1662 A client SMTP SHOULD start an SMTP session by issuing the EHLO 1663 command. If the SMTP server supports the SMTP service extensions, it 1664 will give a successful response, a failure response, or an error 1665 response. If the SMTP server, in violation of this specification, 1666 does not support any SMTP service extensions, it will generate an 1667 error response. Older client SMTP systems MAY, as discussed above, 1668 use HELO (as specified in RFC 821) instead of EHLO, and servers MUST 1669 support the HELO command and reply properly to it. In any event, a 1670 client MUST issue HELO or EHLO before starting a mail transaction. 1672 These commands, and a "250 OK" reply to one of them, confirm that 1673 both the SMTP client and the SMTP server are in the initial state, 1674 that is, there is no transaction in progress and all state tables and 1675 buffers are cleared. 1677 Syntax: 1679 ehlo = "EHLO" SP ( Domain / address-literal ) CRLF 1681 helo = "HELO" SP Domain CRLF 1683 Normally, the response to EHLO will be a multiline reply. Each line 1684 of the response contains a keyword and, optionally, one or more 1685 parameters. Following the normal syntax for multiline replies, these 1686 keywords follow the code (250) and a hyphen for all but the last 1687 line, and the code and a space for the last line. The syntax for a 1688 positive response, using the ABNF notation and terminal symbols of 1689 RFC 5234 [11], is: 1691 ehlo-ok-rsp = ( "250" SP Domain [ SP ehlo-greet ] CRLF ) 1692 / ( "250-" Domain [ SP ehlo-greet ] CRLF 1693 *( "250-" ehlo-line CRLF ) 1694 "250" SP ehlo-line CRLF ) 1696 ehlo-greet = 1*(%d0-9 / %d11-12 / %d14-127) 1697 ; string of any characters other than CR or LF 1699 ehlo-line = ehlo-keyword *( SP ehlo-param ) 1701 ehlo-keyword = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-") 1702 ; additional syntax of ehlo-params depends on 1703 ; ehlo-keyword 1705 ehlo-param = 1*(%d33-126) 1706 ; any CHAR excluding and all 1707 ; control characters (US-ASCII 0-31 and 127 1708 ; inclusive) 1710 Although EHLO keywords may be specified in upper, lower, or mixed 1711 case, they MUST always be recognized and processed in a case- 1712 insensitive manner. This is simply an extension of practices 1713 specified in RFC 821 and Section 2.4. 1715 The EHLO response MUST contain keywords (and associated parameters if 1716 required) for all commands not listed as "required" in Section 4.5.1. 1718 4.1.1.2. MAIL (MAIL) 1720 This command is used to initiate a mail transaction in which the mail 1721 data is delivered to an SMTP server that may, in turn, deliver it to 1722 one or more mailboxes or pass it on to another system (possibly using 1723 SMTP). The argument clause contains a reverse-path and may contain 1724 optional parameters. In general, the MAIL command may be sent only 1725 when no mail transaction is in progress, see Section 4.1.4. 1727 The reverse-path consists of the sender mailbox. Historically, that 1728 mailbox might optionally have been preceded by a list of hosts, but 1729 that behavior is now deprecated (see Appendix C). In some types of 1730 reporting messages for which a reply is likely to cause a mail loop 1731 (for example, mail delivery and non-delivery notifications), the 1732 reverse-path may be null (see Section 3.6). 1734 This command clears the reverse-path buffer, the forward-path buffer, 1735 and the mail data buffer, and it inserts the reverse-path information 1736 from its argument clause into the reverse-path buffer. 1738 If service extensions were negotiated, the MAIL command may also 1739 carry parameters associated with a particular service extension. 1741 Syntax: 1743 mail = "MAIL FROM:" Reverse-path 1744 [SP Mail-parameters] CRLF 1746 4.1.1.3. RECIPIENT (RCPT) 1748 This command is used to identify an individual recipient of the mail 1749 data; multiple recipients are specified by multiple uses of this 1750 command. The argument clause contains a forward-path and may contain 1751 optional parameters. 1753 The forward-path normally consists of the required destination 1754 mailbox. Sending systems SHOULD NOT generate the optional list of 1755 hosts known as a source route. Receiving systems MUST recognize 1756 source route syntax but SHOULD strip off the source route 1757 specification and utilize the domain name associated with the mailbox 1758 as if the source route had not been provided. 1760 Similarly, relay hosts SHOULD strip or ignore source routes, and 1761 names MUST NOT be copied into the reverse-path. When mail reaches 1762 its ultimate destination (the forward-path contains only a 1763 destination mailbox), the SMTP server inserts it into the destination 1764 mailbox in accordance with its host mail conventions. 1766 This command appends its forward-path argument to the forward-path 1767 buffer; it does not change the reverse-path buffer nor the mail data 1768 buffer. 1770 For example, mail received at relay host xyz.com with envelope 1771 commands 1773 MAIL FROM: 1774 RCPT TO:<@hosta.int,@jkl.org:userc@d.bar.org> 1776 will normally be sent directly on to host d.bar.org with envelope 1777 commands 1779 MAIL FROM: 1780 RCPT TO: 1782 As provided in Appendix C, xyz.com MAY also choose to relay the 1783 message to hosta.int, using the envelope commands 1784 MAIL FROM: 1785 RCPT TO:<@hosta.int,@jkl.org:userc@d.bar.org> 1787 or to jkl.org, using the envelope commands 1789 MAIL FROM: 1790 RCPT TO:<@jkl.org:userc@d.bar.org> 1792 Attempting to use relaying this way is now strongly discouraged. 1793 Since hosts are not required to relay mail at all, xyz.com MAY also 1794 reject the message entirely when the RCPT command is received, using 1795 a 550 code (since this is a "policy reason"). 1797 If service extensions were negotiated, the RCPT command may also 1798 carry parameters associated with a particular service extension 1799 offered by the server. The client MUST NOT transmit parameters other 1800 than those associated with a service extension offered by the server 1801 in its EHLO response. 1803 Syntax: 1805 rcpt = "RCPT TO:" ( "" / "" / 1806 Forward-path ) [SP Rcpt-parameters] CRLF 1808 Note that, in a departure from the usual rules for 1809 local-parts, the "Postmaster" string shown above is 1810 treated as case-insensitive. 1812 4.1.1.4. DATA (DATA) 1814 The receiver normally sends a 354 response to DATA, and then treats 1815 the lines (strings ending in sequences, as described in 1816 Section 2.3.7) following the command as mail data from the sender. 1817 This command causes the mail data to be appended to the mail data 1818 buffer. The mail data may contain any of the 128 ASCII character 1819 codes, although experience has indicated that use of control 1820 characters other than SP, HT, CR, and LF may cause problems and 1821 SHOULD be avoided when possible. 1823 The mail data are terminated by a line containing only a period, that 1824 is, the character sequence ".", where the first is 1825 actually the terminator of the previous line (see Section 4.5.2). 1826 This is the end of mail data indication. The first of this 1827 terminating sequence is also the that ends the final line of 1828 the data (message text) or, if there was no mail data, ends the DATA 1829 command itself (the "no mail data" case does not conform to this 1830 specification since it would require that neither the trace header 1831 fields required by this specification nor the message header section 1832 required by RFC 5322 [12] be transmitted). An extra MUST NOT 1833 be added, as that would cause an empty line to be added to the 1834 message. The only exception to this rule would arise if the message 1835 body were passed to the originating SMTP-sender with a final "line" 1836 that did not end in ; in that case, the originating SMTP system 1837 MUST either reject the message as invalid or add in order to 1838 have the receiving SMTP server recognize the "end of data" condition. 1840 The custom of accepting lines ending only in , as a concession to 1841 non-conforming behavior on the part of some UNIX systems, has proven 1842 to cause more interoperability problems than it solves, and SMTP 1843 server systems MUST NOT do this, even in the name of improved 1844 robustness. In particular, the sequence "." (bare line 1845 feeds, without carriage returns) MUST NOT be treated as equivalent to 1846 . as the end of mail data indication. 1848 Receipt of the end of mail data indication requires the server to 1849 process the stored mail transaction information. This processing 1850 consumes the information in the reverse-path buffer, the forward-path 1851 buffer, and the mail data buffer, and on the completion of this 1852 command these buffers are cleared. If the processing is successful, 1853 the receiver MUST send an OK reply. If the processing fails, the 1854 receiver MUST send a failure reply. The SMTP model does not allow 1855 for partial failures at this point: either the message is accepted by 1856 the server for delivery and a positive response is returned or it is 1857 not accepted and a failure reply is returned. In sending a positive 1858 "250 OK" completion reply to the end of data indication, the receiver 1859 takes full responsibility for the message (see Section 6.1). Errors 1860 that are diagnosed subsequently MUST be reported in a mail message, 1861 as discussed in Section 4.4. 1863 When the SMTP server accepts a message either for relaying or for 1864 final delivery, it inserts a trace record (also referred to 1865 interchangeably as a "time stamp line" or "Received" line) at the top 1866 of the mail data. This trace record indicates the identity of the 1867 host that sent the message, the identity of the host that received 1868 the message (and is inserting this time stamp), and the date and time 1869 the message was received. Relayed messages will have multiple time 1870 stamp lines. Details for formation of these lines, including their 1871 syntax, is specified in Section 4.4. 1873 Additional discussion about the operation of the DATA command appears 1874 in Section 3.3. 1876 Syntax: 1878 data = "DATA" CRLF 1880 4.1.1.5. RESET (RSET) 1882 This command specifies that the current mail transaction will be 1883 aborted. Any stored sender, recipients, and mail data MUST be 1884 discarded, and all buffers and state tables cleared. The receiver 1885 MUST send a "250 OK" reply to a RSET command with no arguments. A 1886 reset command may be issued by the client at any time. It is 1887 effectively equivalent to a NOOP (i.e., it has no effect) if issued 1888 immediately after EHLO, before EHLO is issued in the session, after 1889 an end of data indicator has been sent and acknowledged, or 1890 immediately before a QUIT. An SMTP server MUST NOT close the 1891 connection as the result of receiving a RSET; that action is reserved 1892 for QUIT (see Section 4.1.1.10). 1894 Since EHLO implies some additional processing and response by the 1895 server, RSET will normally be more efficient than reissuing that 1896 command, even though the formal semantics are the same. 1898 Syntax: 1900 rset = "RSET" CRLF 1902 4.1.1.6. VERIFY (VRFY) 1904 This command asks the receiver to confirm that the argument 1905 identifies a user or mailbox. If it is a user name, information is 1906 returned as specified in Section 3.5. 1908 This command has no effect on the reverse-path buffer, the forward- 1909 path buffer, or the mail data buffer. 1911 Syntax: 1913 vrfy = "VRFY" SP String CRLF 1915 4.1.1.7. EXPAND (EXPN) 1917 This command asks the receiver to confirm that the argument 1918 identifies a mailing list, and if so, to return the membership of 1919 that list. If the command is successful, a reply is returned 1920 containing information as described in Section 3.5. This reply will 1921 have multiple lines except in the trivial case of a one-member list. 1923 This command has no effect on the reverse-path buffer, the forward- 1924 path buffer, or the mail data buffer, and it may be issued at any 1925 time. 1927 Syntax: 1929 expn = "EXPN" SP String CRLF 1931 4.1.1.8. HELP (HELP) 1933 This command causes the server to send helpful information to the 1934 client. The command MAY take an argument (e.g., any command name) 1935 and return more specific information as a response. 1937 This command has no effect on the reverse-path buffer, the forward- 1938 path buffer, or the mail data buffer, and it may be issued at any 1939 time. 1941 SMTP servers SHOULD support HELP without arguments and MAY support it 1942 with arguments. 1944 Syntax: 1946 help = "HELP" [ SP String ] CRLF 1948 4.1.1.9. NOOP (NOOP) 1950 This command does not affect any parameters or previously entered 1951 commands. It specifies no action other than that the receiver send a 1952 "250 OK" reply. 1954 This command has no effect on the reverse-path buffer, the forward- 1955 path buffer, or the mail data buffer, and it may be issued at any 1956 time. If a parameter string is specified, servers SHOULD ignore it. 1958 Syntax: 1960 noop = "NOOP" [ SP String ] CRLF 1962 4.1.1.10. QUIT (QUIT) 1964 This command specifies that the receiver MUST send a "221 OK" reply, 1965 and then close the transmission channel. 1967 The receiver MUST NOT intentionally close the transmission channel 1968 until it receives and replies to a QUIT command (even if there was an 1969 error). The sender MUST NOT intentionally close the transmission 1970 channel until it sends a QUIT command, and it SHOULD wait until it 1971 receives the reply (even if there was an error response to a previous 1972 command). If the connection is closed prematurely due to violations 1973 of the above or system or network failure, the server MUST cancel any 1974 pending transaction, but not undo any previously completed 1975 transaction, and generally MUST act as if the command or transaction 1976 in progress had received a temporary error (i.e., a 4yz response). 1978 The QUIT command may be issued at any time. Any current uncompleted 1979 mail transaction will be aborted. 1981 Syntax: 1983 quit = "QUIT" CRLF 1985 4.1.1.11. Mail-Parameter and Rcpt-Parameter Error Responses 1987 If the server SMTP does not recognize or cannot implement one or more 1988 of the parameters associated with a particular MAIL or RCPT command, 1989 it will return code 555. 1991 If, for some reason, the server is temporarily unable to accommodate 1992 one or more of the parameters associated with a MAIL or RCPT command, 1993 and if the definition of the specific parameter does not mandate the 1994 use of another code, it should return code 455. 1996 Errors specific to particular parameters and their values will be 1997 specified in the parameter's defining RFC. 1999 4.1.2. Command Argument Syntax 2001 The syntax of the argument clauses of the above commands (using the 2002 syntax specified in RFC 5234 [11] where applicable) is given below. 2003 Some of the productions given below are used only in conjunction with 2004 source routes as described in Appendix C. Some terminals not defined 2005 in this document, but are defined elsewhere, specifically: 2007 * In the "core" syntax in Appendix B of RFC 5234 [11]: ALPHA , CRLF 2008 , DIGIT , HEXDIG , and SP 2010 * In the message format syntax in RFC 5322 [12]: atext , CFWS , and 2011 FWS . 2013 Reverse-path = Path / "<>" 2015 Forward-path = Path 2017 Path = "<" [ A-d-l ":" ] Mailbox ">" 2019 A-d-l = At-domain *( "," At-domain ) 2020 ; Note that this form, the so-called "source 2021 ; route", MUST BE accepted, SHOULD NOT be 2022 ; generated, and SHOULD be ignored. 2024 At-domain = "@" Domain 2025 Mail-parameters = esmtp-param *(SP esmtp-param) 2027 Rcpt-parameters = esmtp-param *(SP esmtp-param) 2029 esmtp-param = esmtp-keyword ["=" esmtp-value] 2031 esmtp-keyword = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-") 2033 esmtp-value = 1*(%d33-60 / %d62-126) 2034 ; any CHAR excluding "=", SP, and control 2035 ; characters. If this string is an email address, 2036 ; i.e., a Mailbox, then the "xtext" syntax [34] 2037 ; SHOULD be used. 2039 Keyword = Ldh-str 2041 Argument = Atom 2043 Domain = sub-domain *("." sub-domain) 2045 sub-domain = Let-dig [Ldh-str] 2047 Let-dig = ALPHA / DIGIT 2049 Ldh-str = *( ALPHA / DIGIT / "-" ) Let-dig 2051 address-literal = "[" ( IPv4-address-literal / 2052 IPv6-address-literal / 2053 General-address-literal ) "]" 2054 ; See Section 4.1.3 2056 Mailbox = Local-part "@" ( Domain / address-literal ) 2058 Local-part = Dot-string / Quoted-string 2059 ; MAY be case-sensitive 2061 Dot-string = Atom *("." Atom) 2063 Atom = 1*atext 2065 Quoted-string = DQUOTE 1*QcontentSMTP DQUOTE 2067 QcontentSMTP = qtextSMTP / quoted-pairSMTP 2069 quoted-pairSMTP = %d92 %d32-126 2070 ; i.e., backslash followed by any ASCII 2071 ; graphic (including itself) or SPace 2073 qtextSMTP = %d32-33 / %d35-91 / %d93-126 2074 ; i.e., within a quoted string, any 2075 ; ASCII graphic or space is permitted 2076 ; without backslash-quoting except 2077 ; double-quote and the backslash itself. 2079 String = Atom / Quoted-string 2081 While the above definition for Local-part is relatively permissive, 2082 for maximum interoperability, a host that expects to receive mail 2083 SHOULD avoid defining mailboxes where the Local-part requires (or 2084 uses) the Quoted-string form or where the Local-part is case- 2085 sensitive. For any purposes that require generating or comparing 2086 Local-parts (e.g., to specific mailbox names), all quoted forms MUST 2087 be treated as equivalent, and the sending system SHOULD transmit the 2088 form that uses the minimum quoting possible. 2090 Systems MUST NOT define mailboxes in such a way as to require the use 2091 in SMTP of non-ASCII characters (octets with the high order bit set 2092 to one) or ASCII "control characters" (decimal value 0-31 and 127). 2093 These characters MUST NOT be used in MAIL or RCPT commands or other 2094 commands that require mailbox names. 2096 Note that the backslash, "\", is a quote character, which is used to 2097 indicate that the next character is to be used literally (instead of 2098 its normal interpretation). For example, "Joe\,Smith" indicates a 2099 single nine-character user name string with the comma being the 2100 fourth character of that string. 2102 To promote interoperability and consistent with long-standing 2103 guidance about conservative use of the DNS in naming and applications 2104 (e.g., see Section 2.3.1 of the base DNS document, RFC 1035 [4]), 2105 characters outside the set of alphabetic characters, digits, and 2106 hyphen MUST NOT appear in domain name labels for SMTP clients or 2107 servers. In particular, the underscore character is not permitted. 2108 SMTP servers that receive a command in which invalid character codes 2109 have been employed, and for which there are no other reasons for 2110 rejection, MUST reject that command with a 501 response (this rule, 2111 like others, could be overridden by appropriate SMTP extensions). 2113 4.1.3. Address Literals 2115 Sometimes a host is not known to the domain name system and 2116 communication (and, in particular, communication to report and repair 2117 the error) is blocked. To bypass this barrier, a special literal 2118 form of the address is allowed as an alternative to a domain name. 2119 For IPv4 addresses, this form uses four small decimal integers 2120 separated by dots and enclosed by brackets such as [123.255.37.2], 2121 which indicates an (IPv4) Internet Address in sequence-of-octets 2122 form. For IPv6 and other forms of addressing that might eventually 2123 be standardized, the form consists of a standardized "tag" that 2124 identifies the address syntax, a colon, and the address itself, in a 2125 format specified as part of the relevant standards (i.e., RFC 4291 2126 [10] for IPv6). 2128 // [5321bis] Proposed erratum 4315 (2015-03-27) suggests yet another 2129 // modification to the IPv6 address literal syntax, based on part on 2130 // RFC 5952. We should consider whether those, or other, 2131 // modifications are appropriate and/or whether, given both the 2132 // issues of spam/malware and servers supporting multiple domains, it 2133 // it time to deprecate mailboxes containing address literals 2134 // entirely (EHLO fields may be a different issue). If we are going 2135 // to allow IPv6 address literals, it may be time to incorporate 2136 // something by reference rather than including specific syntax here 2137 // (RFC 5952 is 14 pages long and does not contain any ABNF). 2139 Specifically: 2141 IPv4-address-literal = Snum 3("." Snum) 2143 IPv6-address-literal = "IPv6:" IPv6-addr 2145 General-address-literal = Standardized-tag ":" 1*dcontent 2147 Standardized-tag = Ldh-str 2148 ; Standardized-tag MUST be specified in a 2149 ; Standards-Track RFC and registered with IANA 2151 dcontent = %d33-90 / ; Printable US-ASCII 2152 %d94-126 ; excl. "[", "\", "]" 2154 Snum = 1*3DIGIT 2155 ; representing a decimal integer 2156 ; value in the range 0 through 255 2158 IPv6-addr = 6( h16 ":" ) ls32 2159 / "::" 5( h16 ":" ) ls32 2160 / [ h16 ] "::" 4( h16 ":" ) ls32 2161 / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32 2162 / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32 2163 / [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32 2164 / [ *4( h16 ":" ) h16 ] "::" ls32 2165 / [ *5( h16 ":" ) h16 ] "::" h16 2166 / [ *6( h16 ":" ) h16 ] "::" 2167 ; This definition is consistent with the one for 2168 ; URIs [40]. 2170 ls32 = ( h16 ":" h16 ) / IPv4address 2171 ; least-significant 32 bits of address 2173 h16 = 1*4HEXDIG 2174 ; 16 bits of address represented in hexadecimal 2176 4.1.4. Order of Commands 2178 There are restrictions on the order in which these commands may be 2179 used. 2181 A session that will contain mail transactions MUST first be 2182 initialized by the use of the EHLO command. An SMTP server SHOULD 2183 accept commands for non-mail transactions (e.g., VRFY, EXPN, or NOOP) 2184 without this initialization. 2186 An EHLO command MAY be issued by a client later in the session. If 2187 it is issued after the session begins and the EHLO command is 2188 acceptable to the SMTP server, the SMTP server MUST clear all buffers 2189 and reset the state exactly as if a RSET command had been issued 2190 (specifically, it terminates any mail transaction that was in 2191 progress, see Section 3.3). In other words, the sequence of RSET 2192 followed immediately by EHLO is redundant, but not harmful other than 2193 in the performance cost of executing unnecessary commands. However 2194 the response to an additional EHLO command MAY be different from that 2195 from prior ones; the client MUST rely only on the responses from the 2196 most recent EHLO command. 2198 If the EHLO command is not acceptable to the SMTP server, 501, 500, 2199 502, or 550 failure replies MUST be returned as appropriate. The 2200 SMTP server MUST stay in the same state after transmitting these 2201 replies that it was in before the EHLO was received. 2203 The SMTP client MUST, if possible, ensure that the domain parameter 2204 to the EHLO command is a primary host name as specified for this 2205 command in Section 2.3.5. If this is not possible (e.g., when the 2206 client's address is dynamically assigned and the client does not have 2207 an obvious name), an address literal SHOULD be substituted for the 2208 domain name. 2210 An SMTP server MAY verify that the domain name argument in the EHLO 2211 command has an address record matching the IP address of the client. 2213 // JcK 20211022: Note that Alessandro's email of 2021-10-13 proposes 2214 // adding "See [A/S] for further discussion." after that sentence. 2215 // Noting that phrasing could get us in trouble if the A/S takes a 2216 // long time to complete, can the WG please make a decision? 2217 // JcK 20211022: Additional question: should we be clear that this 2218 // refers to a forward lookup of the domain name, not a reverse 2219 // lookup of the address? 2221 The NOOP, HELP, EXPN, VRFY, and RSET commands can be used at any time 2222 during a session, or without previously initializing a session. SMTP 2223 servers SHOULD process these normally (that is, not return a 503 2224 code) even if no EHLO command has yet been received; clients SHOULD 2225 open a session with EHLO before sending these commands. 2227 If these rules are followed, the example in RFC 821 that shows "550 2228 access denied to you" in response to an EXPN command is incorrect 2229 unless an EHLO command precedes the EXPN or the denial of access is 2230 based on the client's IP address or other authentication or 2231 authorization-determining mechanisms. 2233 A mail transaction begins with a MAIL command and then consists of 2234 one or more RCPT commands, and a DATA command, in that order. A mail 2235 transaction may be aborted by the RSET, a new EHLO, or the QUIT 2236 command. 2238 SMTP extensions (see Section 2.2) may create additional commands that 2239 initiate, abort, or end the transaction.More generally, any new 2240 command MUST clearly document any effect it has on the transaction 2241 state. 2243 There may be zero or more transactions in a session. MAIL MUST NOT 2244 be sent if a mail transaction is already open, i.e., it should be 2245 sent only if no mail transaction had been started in the session, or 2246 if the previous one successfully concluded with a successful DATA 2247 command, or if the previous one was aborted, e.g., with a RSET or new 2248 EHLO. 2249 // [5321bis] See comment about changing this convoluted discussion to 2250 // talk about 'mail transaction' above. --Jck (and see Ticket #11 2251 // correspondence with Alexey 2021-07-06) 2253 If the transaction beginning command argument is not acceptable, a 2254 501 failure reply MUST be returned and the SMTP server MUST stay in 2255 the same state. If the commands in a transaction are out of order to 2256 the degree that they cannot be processed by the server, a 503 failure 2257 reply MUST be returned and the SMTP server MUST stay in the same 2258 state. 2260 The last command in a session MUST be the QUIT command. The QUIT 2261 command SHOULD be used by the client SMTP to request connection 2262 closure, even when no session opening command was sent and accepted. 2264 4.2. SMTP Replies 2266 Replies to SMTP commands serve to ensure the synchronization of 2267 requests and actions in the process of mail transfer and to guarantee 2268 that the SMTP client always knows the state of the SMTP server. 2269 Every command MUST generate exactly one reply. 2271 The details of the command-reply sequence are described in 2272 Section 4.3. 2274 An SMTP reply consists of a three digit number (transmitted as three 2275 numeric characters) followed by some text unless specified otherwise 2276 in this document. The number is for use by automata to determine 2277 what state to enter next; the text is for the human user. The three 2278 digits contain enough encoded information that the SMTP client need 2279 not examine the text and may either discard it or pass it on to the 2280 user, as appropriate. Exceptions are as noted elsewhere in this 2281 document. In particular, the 220, 221, 251, 421, and 551 reply codes 2282 are associated with message text that must be parsed and interpreted 2283 by machines. In the general case, the text may be receiver dependent 2284 and context dependent, so there are likely to be varying texts for 2285 each reply code. A discussion of the theory of reply codes is given 2286 in Section 4.2.1. Formally, a reply is defined to be the sequence: a 2287 three-digit code, , one line of text, and , or a multiline 2288 reply (as defined in the same section). Since, in violation of this 2289 specification, the text is sometimes not sent, clients that do not 2290 receive it SHOULD be prepared to process the code alone (with or 2291 without a trailing space character). Only the EHLO, EXPN, and HELP 2292 commands are expected to result in multiline replies in normal 2293 circumstances; however, multiline replies are allowed for any 2294 command. 2296 In ABNF, server responses are: 2298 Greeting = ( "220 " (Domain / address-literal) 2299 [ SP textstring ] CRLF ) / 2300 ( "220-" (Domain / address-literal) 2301 [ SP textstring ] CRLF 2302 *( "220-" [ textstring ] CRLF ) 2303 "220" [ SP textstring ] CRLF ) 2305 textstring = 1*(%d09 / %d32-126) ; HT, SP, Printable US-ASCII 2307 Reply-line = *( Reply-code "-" [ textstring ] CRLF ) 2308 Reply-code [ SP textstring ] CRLF 2310 Reply-code = %x32-35 %x30-35 %x30-39 2311 where "Greeting" appears only in the 220 response that announces that 2312 the server is opening its part of the connection. (Other possible 2313 server responses upon connection follow the syntax of Reply-line.) 2315 An SMTP server SHOULD send only the reply codes listed in this 2316 document or additions to the list as discussed below. An SMTP server 2317 SHOULD use the text shown in the examples whenever appropriate. 2319 An SMTP client MUST determine its actions only by the reply code, not 2320 by the text (except for the "change of address" 251 and 551 and, if 2321 necessary, 220, 221, and 421 replies); in the general case, any text, 2322 including no text at all (although senders SHOULD NOT send bare 2323 codes), MUST be acceptable. The space (blank) following the reply 2324 code is considered part of the text. A Sender-SMTP MUST first test 2325 the whole 3 digit reply code it receives, as well as any accompanying 2326 supplemental codes or information (see RFC 3463 [7] and RFC 5248 2327 [46]). If the full reply code is not recognized, and the additional 2328 information is not recognized or missing, the Sender-SMTP MUST use 2329 the first digit (severity indication) of a reply code it receives. 2331 The list of codes that appears below MUST NOT be construed as 2332 permanent. While the addition of new codes should be a rare and 2333 significant activity, with supplemental information in the textual 2334 part of the response (including enhanced status codes [7] and the 2335 successors to that specification) being preferred, new codes may be 2336 added as the result of new Standards or Standards-Track 2337 specifications. Consequently, a sender-SMTP MUST be prepared to 2338 handle codes not specified in this document and MUST do so by 2339 interpreting the first digit only. 2341 In the absence of extensions negotiated with the client, SMTP servers 2342 MUST NOT send reply codes whose first digits are other than 2, 3, 4, 2343 or 5. Clients that receive such out-of-range codes SHOULD normally 2344 treat them as fatal errors and terminate the mail transaction. 2346 4.2.1. Reply Code Severities and Theory 2348 The three digits of the reply each have a special significance. The 2349 first digit denotes whether the response is good, bad, or incomplete. 2350 An unsophisticated SMTP client, or one that receives an unexpected 2351 code, will be able to determine its next action (proceed as planned, 2352 redo, retrench, etc.) by examining this first digit. An SMTP client 2353 that wants to know approximately what kind of error occurred (e.g., 2354 mail system error, command syntax error) may examine the second 2355 digit. The third digit and any supplemental information that may be 2356 present is reserved for the finest gradation of information. 2358 There are four values for the first digit of the reply code: 2360 2yz Positive Completion reply 2361 The requested action has been successfully completed. A new 2362 request may be initiated. 2364 3yz Positive Intermediate reply 2365 The command has been accepted, but the requested action is being 2366 held in abeyance, pending receipt of further information. The 2367 SMTP client should send another command specifying this 2368 information. This reply is used in command sequence groups (i.e., 2369 in DATA). 2371 4yz Transient Negative Completion reply 2372 The command was not accepted, and the requested action did not 2373 occur. However, the error condition is temporary, and the action 2374 may be requested again. The sender should return to the beginning 2375 of the command sequence (if any). It is difficult to assign a 2376 meaning to "transient" when two different sites (receiver- and 2377 sender-SMTP agents) must agree on the interpretation. Each reply 2378 in this category might have a different time value, but the SMTP 2379 client SHOULD try again. A rule of thumb to determine whether a 2380 reply fits into the 4yz or the 5yz category (see below) is that 2381 replies are 4yz if they can be successful if repeated without any 2382 change in command form or in properties of the sender or receiver 2383 (that is, the command is repeated identically and the receiver 2384 does not put up a new implementation). 2386 5yz Permanent Negative Completion reply 2387 The command was not accepted and the requested action did not 2388 occur. The SMTP client SHOULD NOT repeat the exact request (in 2389 the same sequence). Even some "permanent" error conditions can be 2390 corrected, so the human user may want to direct the SMTP client to 2391 reinitiate the command sequence by direct action at some point in 2392 the future (e.g., after the spelling has been changed, or the user 2393 has altered the account status). 2395 It is worth noting that the file transfer protocol (FTP) [15] uses a 2396 very similar code architecture and that the SMTP codes are based on 2397 the FTP model. However, SMTP uses a one-command, one-response model 2398 (while FTP is asynchronous) and FTP's 1yz codes are not part of the 2399 SMTP model. 2401 The second digit encodes responses in specific categories: 2403 x0z Syntax: These replies refer to syntax errors, syntactically 2404 correct commands that do not fit any functional category, and 2405 unimplemented or superfluous commands. 2407 x1z Information: These are replies to requests for information, such 2408 as status or help. 2410 x2z Connections: These are replies referring to the transmission 2411 channel. 2413 x3z Unspecified. 2415 x4z Unspecified. 2417 x5z Mail system: These replies indicate the status of the receiver 2418 mail system vis-a-vis the requested transfer or other mail system 2419 action. 2421 The third digit gives a finer gradation of meaning in each category 2422 specified by the second digit. The list of replies illustrates this. 2423 Each reply text is recommended rather than mandatory, and may even 2424 change according to the command with which it is associated. On the 2425 other hand, the reply codes must strictly follow the specifications 2426 in this section. Receiver implementations should not invent new 2427 codes for slightly different situations from the ones described here, 2428 but rather adapt codes already defined. 2430 For example, a command such as NOOP, whose successful execution does 2431 not offer the SMTP client any new information, will return a 250 2432 reply. The reply is 502 when the command requests an unimplemented 2433 non-site-specific action. A refinement of that is the 504 reply for 2434 a command that is implemented, but that requests an unimplemented 2435 parameter. 2437 The reply text may be longer than a single line; in these cases the 2438 complete text must be marked so the SMTP client knows when it can 2439 stop reading the reply. This requires a special format to indicate a 2440 multiple line reply. 2442 The format for multiline replies requires that every line, except the 2443 last, begin with the reply code, followed immediately by a hyphen, 2444 "-" (also known as minus), followed by text. The last line will 2445 begin with the reply code, followed immediately by , optionally 2446 some text, and . As noted above, servers SHOULD send the 2447 if subsequent text is not sent, but clients MUST be prepared for it 2448 to be omitted. 2450 For example: 2452 250-First line 2453 250-Second line 2454 250-234 Text beginning with numbers 2455 250 The last line 2457 In a multiline reply, the reply code on each of the lines MUST be the 2458 same. It is reasonable for the client to rely on this, so it can 2459 make processing decisions based on the code in any line, assuming 2460 that all others will be the same. In a few cases, there is important 2461 data for the client in the reply "text". The client will be able to 2462 identify these cases from the current context. 2464 4.2.2. Reply Codes by Function Groups 2466 500 Syntax error, command unrecognized (This may include errors such 2467 as command line too long) 2469 501 Syntax error in parameters or arguments 2471 502 Command not implemented (see Section 4.2.4.1) 2473 503 Bad sequence of commands 2475 504 Command parameter not implemented 2477 211 System status, or system help reply 2479 214 Help message (Information on how to use the receiver or the 2480 meaning of a particular non-standard command; this reply is useful 2481 only to the human user) 2483 220 Service ready 2485 221 Service closing transmission channel 2487 421 Service not available, closing transmission channel 2488 (This may be a reply to any command if the service knows it must 2489 shut down) 2491 521 No mail service here. 2493 556 No mail service at this domain. 2495 250 Requested mail action okay, completed 2497 251 User not local; will forward to (See Section 3.4) 2499 252 Cannot VRFY user, but will accept message and attempt delivery 2500 (See Section 3.5.3) 2502 455 Server unable to accommodate parameters 2504 555 MAIL FROM/RCPT TO parameters not recognized or not implemented 2505 450 Requested mail action not taken: mailbox unavailable (e.g., 2506 mailbox busy or temporarily blocked for policy reasons) 2508 550 Requested action not taken: mailbox unavailable (e.g., mailbox 2509 not found, no access, or command rejected for policy reasons) 2511 451 Requested action aborted: error in processing 2513 551 User not local; please try (See Section 3.4) 2515 452 Requested action not taken: insufficient system storage 2516 (preferred code for "too many recipients", see Section 4.5.3.1.10) 2518 552 Requested mail action aborted: exceeded storage allocation. 2520 553 Requested action not taken: mailbox name not allowed (e.g., 2521 mailbox syntax incorrect) 2523 354 Start mail input; end with . 2525 554 Transaction failed (Or, historically in the case of a 2526 connection-opening response, "No SMTP service here". 521 is now 2527 preferred for that function at connection-opening if the server 2528 never accepts mail.) 2530 // [5321bis] [[Note in Draft: Revise above statement in the light 2531 of 2532 // new 521 code?? -- revised with rfc5321bis-04]] 2534 4.2.3. Reply Codes in Numeric Order 2536 211 System status, or system help reply 2538 214 Help message (Information on how to use the receiver or the 2539 meaning of a particular non-standard command; this reply is useful 2540 only to the human user) 2542 220 Service ready 2544 221 Service closing transmission channel 2546 250 Requested mail action okay, completed 2548 251 User not local; will forward to (See Section 3.4) 2550 252 Cannot VRFY user, but will accept message and attempt delivery 2551 (See Section 3.5.3) 2553 354 Start mail input; end with . 2555 421 Service not available, closing transmission channel 2556 (This may be a reply to any command if the service knows it must 2557 shut down) 2559 450 Requested mail action not taken: mailbox unavailable (e.g., 2560 mailbox busy or temporarily blocked for policy reasons) 2562 451 Requested action aborted: local error in processing 2564 452 Requested action not taken: insufficient system storage (also 2565 preferred code for "too many recipients", see Section 4.5.3.1.10) 2567 455 Server unable to accommodate parameters 2569 500 Syntax error, command unrecognized (This may include errors such 2570 as command line too long) 2572 501 Syntax error in parameters or arguments 2574 502 Command not implemented (see Section 4.2.4.1) 2576 503 Bad sequence of commands 2578 504 Command parameter not implemented 2580 521 No mail service (See Section 4.2.4.2.) 2582 550 Requested action not taken: mailbox unavailable (e.g., mailbox 2583 not found, no access, or command rejected for policy reasons) 2585 551 User not local; please try (See Section 3.4) 2587 552 Requested mail action aborted: exceeded storage allocation. 2589 553 Requested action not taken: mailbox name not allowed (e.g., 2590 mailbox syntax incorrect) 2592 554 Transaction failed (Or, in the case of a connection-opening 2593 response, "No SMTP service here" although 521 is now preferred for 2594 the latter. See Section 4.2.4.2.) 2596 555 MAIL FROM/RCPT TO parameters not recognized or not implemented 2598 556 No mail service at this domain. (See Section 4.2.4.2.) 2600 4.2.4. Some specific code situations and relationships 2602 4.2.4.1. Reply Code 502 2604 Questions have been raised as to when reply code 502 (Command not 2605 implemented) SHOULD be returned in preference to other codes. 502 2606 SHOULD be used when the command is actually recognized by the SMTP 2607 server, but not implemented. If the command is not recognized, code 2608 500 SHOULD be returned. Extended SMTP systems MUST NOT list 2609 capabilities in response to EHLO for which they will return 502 (or 2610 500) replies. 2612 4.2.4.2. "No mail accepted" situations and the 521, 554, and 556 codes 2614 Codes 521, 554, and 556 are all used to report different types of "no 2615 mail accepted" situations. They differ as follows. 521 is an 2616 indication from a system answering on the SMTP port that it does not 2617 support SMTP service (a so-called "dummy server" as discussed in RFC 2618 7504 [48] and elsewhere). Obviously, it requires that system exist 2619 and that a connection can be made successfully to it. Because a 2620 system that does not accept any mail cannot meaningfully accept a 2621 RCPT command, any commands (other than QUIT) issued after an SMTP 2622 server has issued a 521 reply are client (sender) errors. 2624 When a domain does not intend to accept mail and wishes to publish 2625 that fact rather than being subjected to connection attempts, the 2626 best way to accomplish that is to use the "Null MX" convention. This 2627 is done by advertising a single MX RR (see Section 3.3.9 of (RFC 1035 2628 [4]) with an RDATA section consisting of preference number 0 and a 2629 zero-length label, written in master files as ".", as the exchange 2630 domain, to denote that there exists no mail exchanger for that 2631 domain. Reply code 556 is then used by a message submission or 2632 intermediate SMTP system (see Section 1.1) to report that it cannot 2633 forward the message further because it knows from the DNS entry that 2634 the recipient domain does not accept mail. If, despite publishing 2635 the DNS entry, the server domain chooses to respond on the SMTP port, 2636 it SHOULD respond with the 556 code as well. The details of the Null 2637 MX convention were first defined in RFC 7505 [49]; see that document 2638 for additional discussion of the rationale for that convention. 2640 Reply code 554 would normally be used in response to a RCPT command 2641 (or extension command with similar intent) when the SMTP system 2642 identifies a domain that it can (or has) determined never accepts 2643 mail. Other codes, including 554 and the temporary 450, are used for 2644 more transient situations and situations in which an SMTP server 2645 cannot or will not deliver to (or accept mail for) a particular 2646 system or mailbox for policy reasons rather than ones directly 2647 related to SMTP processing. 2649 // [JcK 20210904]: do we want/need to discuss temporary server 2650 // outages? And is the discussion above sufficient to obsolete RFC 2651 // 7505 or do we need either more text or some pretense to claim to 2652 // update it. 2654 4.2.4.3. Reply Codes after DATA and the Subsequent . 2656 When an SMTP server returns a positive completion status (2yz code) 2657 after the DATA command is completed with ., it accepts 2658 responsibility for: 2660 * delivering the message (if the recipient mailbox exists), or 2662 * if attempts to deliver the message fail due to transient 2663 conditions, retrying delivery some reasonable number of times at 2664 intervals as specified in Section 4.5.4. 2666 * if attempts to deliver the message fail due to permanent 2667 conditions, or if repeated attempts to deliver the message fail 2668 due to transient conditions, returning appropriate notification to 2669 the sender of the original message (using the address in the SMTP 2670 MAIL command). 2672 When an SMTP server returns a temporary error status (4yz) code after 2673 the DATA command is completed with ., it MUST NOT make a 2674 subsequent attempt to deliver that message. The SMTP client retains 2675 responsibility for the delivery of that message and may either return 2676 it to the user or requeue it for a subsequent attempt (see 2677 Section 4.5.4.1). 2679 The user who originated the message SHOULD be able to interpret the 2680 return of a transient failure status (by mail message or otherwise) 2681 as a non-delivery indication, just as a permanent failure would be 2682 interpreted. If the client SMTP successfully handles these 2683 conditions, the user will not receive such a reply. 2685 When an SMTP server returns a permanent error status (5yz) code after 2686 the DATA command is completed with ., it MUST NOT make 2687 any subsequent attempt to deliver the message. As with temporary 2688 error status codes, the SMTP client retains responsibility for the 2689 message, but SHOULD NOT again attempt delivery to the same server 2690 without user review of the message and response and appropriate 2691 intervention. 2693 4.3. Sequencing of Commands and Replies 2694 4.3.1. Sequencing Overview 2696 The communication between the sender and receiver is an alternating 2697 dialogue, controlled by the sender. As such, the sender issues a 2698 command and the receiver responds with a reply. Unless other 2699 arrangements are negotiated through service extensions, the sender 2700 MUST wait for this response before sending further commands. One 2701 important reply is the connection greeting. Normally, a receiver 2702 will send a 220 "Service ready" reply when the connection is 2703 completed. The sender SHOULD wait for this greeting message before 2704 sending any commands. 2706 Note: all the greeting-type replies have the official name (the 2707 fully-qualified primary domain name) of the server host as the first 2708 word following the reply code. Sometimes the host will have no 2709 meaningful name. See Section 4.1.3 for a discussion of alternatives 2710 in these situations. 2712 For example, 2714 220 ISIF.USC.EDU Service ready 2716 or 2718 220 mail.example.com SuperSMTP v 6.1.2 Service ready 2720 or 2722 220 [10.0.0.1] Clueless host service ready 2724 The table below lists alternative success and failure replies for 2725 each command. These SHOULD be strictly adhered to. A receiver MAY 2726 substitute text in the replies, but the meanings and actions implied 2727 by the code numbers and by the specific command reply sequence MUST 2728 be preserved. However, in order to provide robustness as SMTP is 2729 extended and evolves, the discussion in Section 4.2.1 still applies: 2730 all SMTP clients MUST be prepared to accept any code that conforms to 2731 the discussion in that section and MUST be prepared to interpret it 2732 on the basis of its first digit only. 2734 4.3.2. Command-Reply Sequences 2736 Each command is listed with its usual possible replies. The prefixes 2737 used before the possible replies are "I" for intermediate, "S" for 2738 success, and "E" for error. Since some servers may generate other 2739 replies under special circumstances, and to allow for future 2740 extension, SMTP clients SHOULD, when possible, interpret only the 2741 first digit of the reply and MUST be prepared to deal with 2742 unrecognized reply codes by interpreting the first digit only. 2743 Unless extended using the mechanisms described in Section 2.2, SMTP 2744 servers MUST NOT transmit reply codes to an SMTP client that are 2745 other than three digits or that do not start in a digit between 2 and 2746 5 inclusive. 2748 These sequencing rules and, in principle, the codes themselves, can 2749 be extended or modified by SMTP extensions offered by the server and 2750 accepted (requested) by the client. However, if the target is more 2751 precise granularity in the codes, rather than codes for completely 2752 new purposes, the system described in RFC 3463 [7] SHOULD be used in 2753 preference to the invention of new codes. 2755 In addition to the codes listed below, any SMTP command can return 2756 any of the following codes if the corresponding unusual circumstances 2757 are encountered: 2759 500 For the "command line too long" case or if the command name was 2760 not recognized. Note that producing a "command not recognized" 2761 error in response to the required subset of these commands is a 2762 violation of this specification. Similarly, producing a "command 2763 too long" message for a command line shorter than 512 characters 2764 would violate the provisions of Section 4.5.3.1.4. 2766 501 Syntax error in command or arguments. In order to provide for 2767 future extensions, commands that are specified in this document as 2768 not accepting arguments (DATA, RSET, QUIT) SHOULD return a 501 2769 message if arguments are supplied in the absence of EHLO- 2770 advertised extensions. 2772 421 Service shutting down and closing transmission channel 2774 Specific sequences are: 2776 CONNECTION ESTABLISHMENT 2778 - S: 220 2779 E: 521, 554, 556 2781 EHLO or HELO 2783 - S: 250 2784 E: 504 (a conforming implementation could return this code only 2785 in fairly obscure cases), 550, 502 (permitted only with an old- 2786 style server that does not support EHLO) 2788 MAIL 2789 - S: 250 2790 E: 552, 451, 452, 550, 553, 503, 455, 555 2792 RCPT 2794 - S: 250, 251 (but see Section 3.4 for discussion of 251 and 551) 2795 E: 550, 551, 552 (obsolete for "too many recipients; see 2796 Section 4.5.3.1.10, 553, 450, 451, 452, 503, 455, 555 2798 DATA 2800 - I: 354 -> data -> S: 250 2802 o E: 552, 554, 451, 452 2804 o E: 450, 550 (rejections for policy reasons) 2806 - E: 503, 554 2808 RSET 2810 - S: 250 2812 VRFY 2814 - S: 250, 251, 252 2815 E: 550, 551, 553, 502, 504 2817 EXPN 2819 - S: 250, 252 2820 E: 550, 500, 502, 504 2822 HELP 2824 - S: 211, 214 2825 E: 502, 504 2827 NOOP 2829 - S: 250 2831 QUIT 2833 - S: 221 2835 4.4. Trace Information 2837 Trace information is used to provide an audit trail of message 2838 handling. In addition, it indicates a route back to the sender of 2839 the message. 2841 4.4.1. Received Header Field 2843 When an SMTP server receives a message for delivery or further 2844 processing, it MUST insert trace (often referred to as "time stamp" 2845 or "Received" information) at the beginning of the message content, 2846 as discussed in Section 4.1.1.4. 2848 This line MUST be structured as follows: 2850 * The FROM clause, which MUST be supplied in an SMTP environment, 2851 SHOULD contain both (1) the name of the source host as presented 2852 in the EHLO command and (2) an address literal containing the IP 2853 address of the source, determined from the TCP connection. 2855 * The ID clause MAY contain an "@" as suggested in RFC 822, but this 2856 is not required. 2858 * If the FOR clause appears, it MUST contain exactly one 2859 entry, even when multiple RCPT commands have been given. Multiple 2860 s raise some security issues and have been deprecated, see 2861 Section 7.2. 2863 An Internet mail program MUST NOT change or delete a Received: line 2864 that was previously added to the message header section. SMTP 2865 servers MUST prepend Received lines to messages; they MUST NOT change 2866 the order of existing lines or insert Received lines in any other 2867 location. 2869 As the Internet grows, comparability of Received header fields is 2870 important for detecting problems, especially slow relays. SMTP 2871 servers that create Received header fields SHOULD use explicit 2872 offsets in the dates (e.g., -0800), rather than time zone names of 2873 any type. Local time (with an offset) SHOULD be used rather than UT 2874 when feasible. This formulation allows slightly more information 2875 about local circumstances to be specified. If UT is needed, the 2876 receiver need merely do some simple arithmetic to convert the values. 2877 Use of UT loses information about the time zone-location of the 2878 server. If it is desired to supply a time zone name, it SHOULD be 2879 included in a comment. 2881 When the delivery SMTP server makes the "final delivery" of a 2882 message, it inserts a return-path line at the beginning of the mail 2883 data. This use of return-path is required; mail systems MUST support 2884 it. The return-path line preserves the information in the from the MAIL command. Here, final delivery means the message 2886 has left the SMTP environment. Normally, this would mean it had been 2887 delivered to the destination user or an associated mail drop, but in 2888 some cases it may be further processed and transmitted by another 2889 mail system. 2891 It is possible for the mailbox in the return path to be different 2892 from the actual sender's mailbox, for example, if error responses are 2893 to be delivered to a special error handling mailbox rather than to 2894 the message sender. When mailing lists are involved, this 2895 arrangement is common and useful as a means of directing errors to 2896 the list maintainer rather than the message originator. 2898 The text above implies that the final mail data will begin with a 2899 return path line, followed by one or more time stamp lines. These 2900 lines will be followed by the rest of the mail data: first the 2901 balance of the mail header section and then the body (RFC 5322 [12]). 2903 It is sometimes difficult for an SMTP server to determine whether or 2904 not it is making final delivery since forwarding or other operations 2905 may occur after the message is accepted for delivery. Consequently, 2906 any further (forwarding, gateway, or relay) systems MAY remove the 2907 return path and rebuild the MAIL command as needed to ensure that 2908 exactly one such line appears in a delivered message. 2910 A message-originating SMTP system SHOULD NOT send a message that 2911 already contains a Return-path header field. SMTP servers performing 2912 a relay function MUST NOT inspect the message data, and especially 2913 not to the extent needed to determine if Return-path header fields 2914 are present. SMTP servers making final delivery MAY remove Return- 2915 path header fields before adding their own. 2917 The primary purpose of the Return-path is to designate the address to 2918 which messages indicating non-delivery or other mail system failures 2919 are to be sent. For this to be unambiguous, exactly one return path 2920 SHOULD be present when the message is delivered. Systems using RFC 2921 822 syntax with non-SMTP transports SHOULD designate an unambiguous 2922 address, associated with the transport envelope, to which error 2923 reports (e.g., non-delivery messages) should be sent. 2925 Historical note: Text in RFC 822 that appears to contraindicate the 2926 use of the Return-path header field (or the envelope reverse-path 2927 address from the MAIL command) if the destination for error messages 2928 is not applicable on the Internet. The reverse-path address (as 2929 copied into the Return-path) MUST be used as the target of any mail 2930 containing delivery error messages. 2932 In particular: 2934 * a gateway from SMTP -> elsewhere SHOULD insert a return-path 2935 header field, unless it is known that the "elsewhere" transport 2936 also uses Internet domain addresses and maintains the envelope 2937 sender address separately. 2939 * a gateway from elsewhere -> SMTP SHOULD delete any return-path 2940 header field present in the message, and either copy that 2941 information to the SMTP envelope or combine it with information 2942 present in the envelope of the other transport system to construct 2943 the reverse-path argument to the MAIL command in the SMTP 2944 envelope. 2946 The server must give special treatment to cases in which the 2947 processing following the end of mail data indication is only 2948 partially successful. This could happen if, after accepting several 2949 recipients and the mail data, the SMTP server finds that the mail 2950 data could be successfully delivered to some, but not all, of the 2951 recipients. In such cases, the response to the DATA command MUST be 2952 an OK reply. However, the SMTP server MUST compose and send an 2953 "undeliverable mail" notification message to the originator of the 2954 message. 2956 // [JcK/Alexey 20211104] The following paragraph does not seem to 2957 // belong in this section. Where should it be moved? 2958 A single notification listing all of the failed recipients or 2959 separate notification messages MUST be sent for each failed 2960 recipient. For economy of processing by the sender, the former 2961 SHOULD be used when possible. All notification messages about 2962 undeliverable mail MUST be sent using the MAIL command and MUST use a 2963 null return path as discussed in Section 3.6. 2965 The time stamp line and the return path line are formally defined as 2966 follows (the definitions for "FWS" and "CFWS" appear in RFC 5322 2967 [12]): 2969 Return-path-line = "Return-Path:" FWS Reverse-path 2971 Time-stamp-line = "Received:" FWS Stamp 2972 Stamp = From-domain By-domain Opt-info [CFWS] ";" 2973 FWS date-time 2974 ; where "date-time" is as defined in RFC 5322 [12] 2975 ; but the "obs-" forms, especially two-digit 2976 ; years, are prohibited in SMTP and MUST NOT be used. 2978 From-domain = "FROM" FWS Extended-Domain 2980 By-domain = CFWS "BY" FWS Extended-Domain 2982 Extended-Domain = Domain / 2983 ( Domain FWS "(" TCP-info ")" ) / 2984 ( address-literal FWS "(" TCP-info ")" ) 2986 TCP-info = address-literal / ( Domain FWS address-literal ) 2987 ; Information derived by server from TCP connection 2988 ; not client EHLO. 2990 Opt-info = [Via] [With] [ID] [For] 2991 [Additional-Registered-Clauses] 2993 Via = CFWS "VIA" FWS Link 2995 With = CFWS "WITH" FWS Protocol 2997 ID = CFWS "ID" FWS ( Atom / msg-id ) 2998 ; msg-id is defined in RFC 5322 [12] 3000 For = CFWS "FOR" FWS ( Path / Mailbox ) 3002 Additional-Registered-Clauses = 1* (CFWS Atom FWS String) 3004 // [5321bis] 5321 errata #1683, 20090215, 3005 ; Additional standard clauses may be added in this 3006 ; location by future standards and registration with 3007 ; IANA. SMTP servers SHOULD NOT use unregistered 3008 ; names. See Section 8. 3010 Link = "TCP" / Addtl-Link 3012 Addtl-Link = Atom 3013 ; Additional standard names for links are 3014 ; registered with the Internet Assigned Numbers 3015 ; Authority (IANA). "Via" is primarily of value 3016 ; with non-Internet transports. SMTP servers 3017 ; SHOULD NOT use unregistered names. 3019 Protocol = "ESMTP" / "SMTP" / Attdl-Protocol 3020 Addtl-Protocol = Atom 3021 ; Additional standard names for protocols are 3022 ; registered with the Internet Assigned Numbers 3023 ; Authority (IANA) in the "mail parameters" 3024 ; registry [8]. SMTP servers SHOULD NOT 3025 ; use unregistered names. 3027 4.5. Additional Implementation Issues 3029 4.5.1. Minimum Implementation 3031 In order to make SMTP workable, the following minimum implementation 3032 MUST be provided by all receivers. The following commands MUST be 3033 supported to conform to this specification: 3035 EHLO 3036 HELO 3037 MAIL 3038 RCPT 3039 DATA 3040 RSET 3041 NOOP 3042 QUIT 3043 VRFY 3045 Any system that includes an SMTP server supporting mail relaying or 3046 delivery MUST support the reserved mailbox "postmaster" as a case- 3047 insensitive local name. This postmaster address is not strictly 3048 necessary if the server always returns 554 on connection opening (as 3049 described in Section 3.1). The requirement to accept mail for 3050 postmaster implies that RCPT commands that specify a mailbox for 3051 postmaster at any of the domains for which the SMTP server provides 3052 mail service, as well as the special case of "RCPT TO:" 3053 (with no domain specification), MUST be supported. 3055 SMTP systems are expected to make every reasonable effort to accept 3056 mail directed to Postmaster from any other system on the Internet. 3057 In extreme cases -- such as to contain a denial of service attack or 3058 other breach of security -- an SMTP server may block mail directed to 3059 Postmaster. However, such arrangements SHOULD be narrowly tailored 3060 so as to avoid blocking messages that are not part of such attacks. 3062 4.5.2. Transparency 3064 Without some provision for data transparency, the character sequence 3065 "." ends the mail text and cannot be sent by the user. 3066 In general, users are not aware of such "forbidden" sequences. To 3067 allow all user composed text to be transmitted transparently, the 3068 following procedures are used: 3070 * Before sending a line of mail text, the SMTP client checks the 3071 first character of the line. If it is a period, one additional 3072 period is inserted at the beginning of the line. 3074 * When a line of mail text is received by the SMTP server, it checks 3075 the line. If the line is composed of a single period, it is 3076 treated as the end of mail indicator. If the first character is a 3077 period and there are other characters on the line, the first 3078 character is deleted. 3080 The mail data may contain any of the 128 ASCII characters. All 3081 characters are to be delivered to the recipient's mailbox, including 3082 spaces, vertical and horizontal tabs, and other control characters. 3083 If the transmission channel provides an 8-bit byte (octet) data 3084 stream, the 7-bit ASCII codes are transmitted, right justified, in 3085 the octets, with the high-order bits cleared to zero. See 3086 Section 3.6 for special treatment of these conditions in SMTP systems 3087 serving a relay function. 3089 In some systems, it may be necessary to transform the data as it is 3090 received and stored. This may be necessary for hosts that use a 3091 different character set than ASCII as their local character set, that 3092 store data in records rather than strings, or which use special 3093 character sequences as delimiters inside mailboxes. If such 3094 transformations are necessary, they MUST be reversible, especially if 3095 they are applied to mail being relayed. 3097 4.5.3. Sizes and Timeouts 3099 4.5.3.1. Size Limits and Minimums 3101 There are several objects that have required minimum/maximum sizes. 3102 Every implementation MUST be able to receive objects of at least 3103 these sizes. Objects larger than these sizes SHOULD be avoided when 3104 possible. However, some Internet mail constructs such as encoded 3105 X.400 addresses (RFC 2156 [27]) will often require larger objects. 3106 Clients MAY attempt to transmit these, but MUST be prepared for a 3107 server to reject them if they cannot be handled by it. To the 3108 maximum extent possible, implementation techniques that impose no 3109 limits on the length of these objects should be used. 3111 Extensions to SMTP may involve the use of characters that occupy more 3112 than a single octet each. This section therefore specifies lengths 3113 in octets where absolute lengths, rather than character counts, are 3114 intended. 3116 // [5321bis] [[Note in Draft: Klensin 20191126: Given the controversy 3117 // on the SMTP mailing list between 20191123 and now about maximum 3118 // lengths, is the above adequate or is further tuning of the limit 3119 // text below needed? 3121 4.5.3.1.1. Local-part 3123 The maximum total length of a user name or other local-part is 64 3124 octets. 3126 4.5.3.1.2. Domain 3128 The maximum total length of a domain name or number is 255 octets. 3130 4.5.3.1.3. Path 3132 The maximum total length of a reverse-path or forward-path is 256 3133 octets (including the punctuation and element separators). 3135 4.5.3.1.4. Command Line 3137 The maximum total length of a command line including the command word 3138 and the is 512 octets. SMTP extensions may be used to 3139 increase this limit. 3141 4.5.3.1.5. Reply Line 3143 The maximum total length of a reply line including the reply code and 3144 the is 512 octets. More information may be conveyed through 3145 multiple-line replies. 3147 4.5.3.1.6. Text Line 3149 The maximum total length of a text line including the is 1000 3150 octets (not counting the leading dot duplicated for transparency). 3151 This number may be increased by the use of SMTP Service Extensions. 3153 4.5.3.1.7. Message Content 3155 The maximum total length of a message content (including any message 3156 header section as well as the message body) MUST BE at least 64K 3157 octets. Since the introduction of Internet Standards for multimedia 3158 mail (RFC 2045 [25]), message lengths on the Internet have grown 3159 dramatically, and message size restrictions should be avoided if at 3160 all possible. SMTP server systems that must impose restrictions 3161 SHOULD implement the "SIZE" service extension of RFC 1870 [6], and 3162 SMTP client systems that will send large messages SHOULD utilize it 3163 when possible. 3165 4.5.3.1.8. Recipient Buffer 3167 The minimum total number of recipients that MUST be buffered is 100 3168 recipients. Rejection of messages (for excessive recipients) with 3169 fewer than 100 RCPT commands is a violation of this specification. 3170 The general principle that relaying SMTP server MUST NOT, and 3171 delivery SMTP servers SHOULD NOT, perform validation tests on message 3172 header fields suggests that messages SHOULD NOT be rejected based on 3173 the total number of recipients shown in header fields. A server that 3174 imposes a limit on the number of recipients MUST behave in an orderly 3175 fashion, such as rejecting additional addresses over its limit rather 3176 than silently discarding addresses previously accepted. A client 3177 that needs to deliver a message containing over 100 RCPT commands 3178 SHOULD be prepared to transmit in 100-recipient "chunks" if the 3179 server declines to accept more than 100 recipients in a single 3180 message. 3182 4.5.3.1.9. Treatment When Limits Exceeded 3184 Errors due to exceeding these limits may be reported by using the 3185 reply codes. Some examples of reply codes are: 3187 500 Line too long. 3189 or 3191 501 Path too long 3193 or 3195 452 Too many recipients (see below) 3197 or 3199 552 Too much mail data (historically also used for too many 3200 recipients (see below). 3202 4.5.3.1.10. Too Many Recipients Code 3204 RFC 821 [3] incorrectly listed the error where an SMTP server 3205 exhausts its implementation limit on the number of RCPT commands 3206 ("too many recipients") as having reply code 552. The correct reply 3207 code for this condition is 452. At the time RFC 5321 was written, 3208 the use of response code 552 by servers was sufficiently common that 3209 client implementation were advised to simply treat it as if 452 had 3210 been sent. That advice is no longer necessary or useful. 3212 When a conforming SMTP server encounters this condition, it has at 3213 least 100 successful RCPT commands in its recipient buffer. If the 3214 server is able to accept the message, then at least these 100 3215 addresses will be removed from the SMTP client's queue. When the 3216 client attempts retransmission of those addresses that received 452 3217 responses, at least 100 of these will be able to fit in the SMTP 3218 server's recipient buffer. Each retransmission attempt that is able 3219 to deliver anything will be able to dispose of at least 100 of these 3220 recipients. 3222 If an SMTP server has an implementation limit on the number of RCPT 3223 commands and this limit is exhausted, it MUST use a response code of 3224 452. If the server has a configured site-policy limitation on the 3225 number of RCPT commands, it MAY instead use a 5yz response code. In 3226 particular, if the intent is to prohibit messages with more than a 3227 site-specified number of recipients, rather than merely limit the 3228 number of recipients in a given mail transaction, it would be 3229 reasonable to return a 503 response to any DATA command received 3230 subsequent to the 452 code or to simply return the 503 after DATA 3231 without returning any previous negative response. 3233 4.5.3.2. Timeouts 3235 An SMTP client MUST provide a timeout mechanism. It MUST use per- 3236 command timeouts rather than somehow trying to time the entire mail 3237 transaction. Timeouts SHOULD be easily reconfigurable, preferably 3238 without recompiling the SMTP code. To implement this, a timer is set 3239 for each SMTP command and for each buffer of the data transfer. The 3240 latter means that the overall timeout is inherently proportional to 3241 the size of the message. 3243 Based on extensive experience with busy mail-relay hosts, the minimum 3244 per-command timeout values SHOULD be as follows: 3246 4.5.3.2.1. Initial 220 Message: 5 Minutes 3248 An SMTP client process needs to distinguish between a failed TCP 3249 connection and a delay in receiving the initial 220 greeting message. 3250 Many SMTP servers accept a TCP connection but delay delivery of the 3251 220 message until their system load permits more mail to be 3252 processed. 3254 4.5.3.2.2. MAIL Command: 5 Minutes 3256 4.5.3.2.3. RCPT Command: 5 Minutes 3258 A longer timeout is required if processing of mailing lists and 3259 aliases is not deferred until after the message was accepted. 3261 4.5.3.2.4. DATA Initiation: 2 Minutes 3263 This is while awaiting the "354 Start Input" reply to a DATA command. 3265 4.5.3.2.5. Data Block: 3 Minutes 3267 This is while awaiting the completion of each TCP SEND call 3268 transmitting a chunk of data. 3270 4.5.3.2.6. DATA Termination: 10 Minutes. 3272 This is while awaiting the "250 OK" reply. When the receiver gets 3273 the final period terminating the message data, it typically performs 3274 processing to deliver the message to a user mailbox. A spurious 3275 timeout at this point would be very wasteful and would typically 3276 result in delivery of multiple copies of the message, since it has 3277 been successfully sent and the server has accepted responsibility for 3278 delivery. See Section 6.1 for additional discussion. 3280 4.5.3.2.7. Server Timeout: 5 Minutes. 3282 An SMTP server SHOULD have a timeout of at least 5 minutes while it 3283 is awaiting the next command from the sender. 3285 4.5.4. Retry Strategies 3287 The common structure of a host SMTP implementation includes user 3288 mailboxes, one or more areas for queuing messages in transit, and one 3289 or more daemon processes for sending and receiving mail. The exact 3290 structure will vary depending on the needs of the users on the host 3291 and the number and size of mailing lists supported by the host. We 3292 describe several optimizations that have proved helpful, particularly 3293 for mailers supporting high traffic levels. 3295 Any queuing strategy MUST include timeouts on all activities on a 3296 per-command basis. A queuing strategy MUST NOT send error messages 3297 in response to error messages under any circumstances. 3299 4.5.4.1. Sending Strategy 3301 The general model for an SMTP client is one or more processes that 3302 periodically attempt to transmit outgoing mail. In a typical system, 3303 the program that composes a message has some method for requesting 3304 immediate attention for a new piece of outgoing mail, while mail that 3305 cannot be transmitted immediately MUST be queued and periodically 3306 retried by the sender. A mail queue entry will include not only the 3307 message itself but also the envelope information. 3309 The sender MUST delay retrying a particular destination after one 3310 attempt has failed. In general, the retry interval SHOULD be at 3311 least 30 minutes; however, more sophisticated and variable strategies 3312 will be beneficial when the SMTP client can determine the reason for 3313 non-delivery. 3315 Retries continue until the message is transmitted or the sender gives 3316 up; the give-up time generally needs to be at least 4-5 days. It MAY 3317 be appropriate to set a shorter maximum number of retries for non- 3318 delivery notifications and equivalent error messages than for 3319 standard messages. The parameters to the retry algorithm MUST be 3320 configurable. 3322 A client SHOULD keep a list of hosts it cannot reach and 3323 corresponding connection timeouts, rather than just retrying queued 3324 mail items. 3326 Experience suggests that failures are typically transient (the target 3327 system or its connection has crashed), favoring a policy of two 3328 connection attempts in the first hour the message is in the queue, 3329 and then backing off to one every two or three hours. 3331 The SMTP client can shorten the queuing delay in cooperation with the 3332 SMTP server. For example, if mail is received from a particular 3333 address, it is likely that mail queued for that host can now be sent. 3334 Application of this principle may, in many cases, eliminate the 3335 requirement for an explicit "send queues now" function such as ETRN, 3336 RFC 1985 [24]. 3338 The strategy may be further modified as a result of multiple 3339 addresses per host (see below) to optimize delivery time versus 3340 resource usage. 3342 An SMTP client may have a large queue of messages for each 3343 unavailable destination host. If all of these messages were retried 3344 in every retry cycle, there would be excessive Internet overhead and 3345 the sending system would be blocked for a long period. Note that an 3346 SMTP client can generally determine that a delivery attempt has 3347 failed only after a timeout of several minutes, and even a one-minute 3348 timeout per connection will result in a very large delay if retries 3349 are repeated for dozens, or even hundreds, of queued messages to the 3350 same host. 3352 At the same time, SMTP clients SHOULD use great care in caching 3353 negative responses from servers. In an extreme case, if EHLO is 3354 issued multiple times during the same SMTP connection, different 3355 answers may be returned by the server. More significantly, 5yz 3356 responses to the MAIL command MUST NOT be cached. 3358 When a mail message is to be delivered to multiple recipients, and 3359 the SMTP server to which a copy of the message is to be sent is the 3360 same for multiple recipients, then only one copy of the message 3361 SHOULD be transmitted. That is, the SMTP client SHOULD use the 3362 command sequence: MAIL, RCPT, RCPT, ..., RCPT, DATA instead of the 3363 sequence: MAIL, RCPT, DATA, ..., MAIL, RCPT, DATA. However, if there 3364 are very many addresses, a limit on the number of RCPT commands per 3365 MAIL command MAY be imposed. This efficiency feature SHOULD be 3366 implemented. 3368 Similarly, to achieve timely delivery, the SMTP client MAY support 3369 multiple concurrent outgoing mail transactions. However, some limit 3370 may be appropriate to protect the host from devoting all its 3371 resources to mail. 3373 4.5.4.2. Receiving Strategy 3375 The SMTP server SHOULD attempt to keep a pending listen on the SMTP 3376 port (specified by IANA as port 25) at all times. This requires the 3377 support of multiple incoming TCP connections for SMTP. Some limit 3378 MAY be imposed, but servers that cannot handle more than one SMTP 3379 transaction at a time are not in conformance with the intent of this 3380 specification. 3382 As discussed above, when the SMTP server receives mail from a 3383 particular host address, it could activate its own SMTP queuing 3384 mechanisms to retry any mail pending for that host address. 3386 4.5.5. Messages with a Null Reverse-Path 3388 There are several types of notification messages that are required by 3389 existing and proposed Standards to be sent with a null reverse-path, 3390 namely non-delivery notifications as discussed in Section 3.6.2 and 3391 Section 3.6.3, other kinds of Delivery Status Notifications (DSNs, 3392 RFC 3461 [34]), and Message Disposition Notifications (MDNs, RFC 8098 3393 [37]). All of these kinds of messages are notifications about a 3394 previous message, and they are sent to the reverse-path of the 3395 previous mail message. (If the delivery of such a notification 3396 message fails, that usually indicates a problem with the mail system 3397 of the host to which the notification message is addressed. For this 3398 reason, at some hosts the MTA is set up to forward such failed 3399 notification messages to someone who is able to fix problems with the 3400 mail system, e.g., via the postmaster alias.) 3402 All other types of messages (i.e., any message which is not required 3403 by a Standards-Track RFC to have a null reverse-path) SHOULD be sent 3404 with a valid, non-null reverse-path. 3406 Implementers of automated email processors should be careful to make 3407 sure that the various kinds of messages with a null reverse-path are 3408 handled correctly. In particular, such systems SHOULD NOT reply to 3409 messages with a null reverse-path, and they SHOULD NOT add a non-null 3410 reverse-path, or change a null reverse-path to a non-null one, to 3411 such messages when forwarding. 3413 5. Address Resolution and Mail Handling 3415 5.1. Locating the Target Host 3417 Once an SMTP client lexically identifies a domain to which mail will 3418 be delivered for processing (as described in Sections 2.3.5 and 3.6), 3419 a DNS lookup MUST be performed to resolve the domain name (RFC 1035 3420 [4]). The names are expected to be fully-qualified domain names 3421 (FQDNs): mechanisms for inferring FQDNs from partial names or local 3422 aliases are outside of this specification. Due to a history of 3423 problems, SMTP servers used for initial submission of messages SHOULD 3424 NOT make such inferences (Message Submission Servers [42] have 3425 somewhat more flexibility) and intermediate (relay) SMTP servers MUST 3426 NOT make them. 3428 The lookup first attempts to locate an MX record associated with the 3429 name. If a CNAME record is found, the resulting name is processed as 3430 if it were the initial name. If a non-existent domain error is 3431 returned, this situation MUST be reported as an error. If a 3432 temporary error is returned, the message MUST be queued and retried 3433 later (see Section 4.5.4.1). If an empty list of MXs is returned, 3434 the address is treated as if it was associated with an implicit MX 3435 RR, with a preference of 0, pointing to that host. If MX records are 3436 present, but none of them are usable, or the implicit MX is unusable, 3437 this situation MUST be reported as an error. 3439 If one or more MX RRs are found for a given name, SMTP systems MUST 3440 NOT utilize any address RRs associated with that name unless they are 3441 located using the MX RRs; the "implicit MX" rule above applies only 3442 if there are no MX records present. If MX records are present, but 3443 none of them are usable, this situation MUST be reported as an error. 3445 When a domain name associated with an MX RR is looked up and the 3446 associated data field obtained, the data field of that response MUST 3447 contain a domain name that conforms to the specifications of 3448 Section 2.3.5. 3449 [[5321bis Editor's Note: Depending on how the "null MX" discussion 3450 unfolds, some additional text may be in order here (20140718)]] 3451 That domain name, when queried, MUST return at least one address 3452 record (e.g., A or AAAA RR) that gives the IP address of the SMTP 3453 server to which the message should be directed. Any other response, 3454 specifically including a value that will return a CNAME record when 3455 queried, lies outside the scope of this Standard. The prohibition on 3456 labels in the data that resolve to CNAMEs is discussed in more detail 3457 in RFC 2181, Section 10.3 [28]. 3459 When the lookup succeeds, the mapping can result in a list of 3460 alternative delivery addresses rather than a single address, because 3461 of multiple MX records, multihoming, or both. To provide reliable 3462 mail transmission, the SMTP client MUST be able to try (and retry) 3463 each of the relevant addresses in this list in order, until a 3464 delivery attempt succeeds. However, there MAY also be a configurable 3465 limit on the number of alternate addresses that can be tried. In any 3466 case, the SMTP client SHOULD try at least two addresses. 3468 Two types of information are used to rank the host addresses: 3469 multiple MX records, and multihomed hosts. 3471 MX records contain a preference indication that MUST be used in 3472 sorting if more than one such record appears (see below). Lower 3473 numbers are more preferred than higher ones. If there are multiple 3474 destinations with the same preference and there is no clear reason to 3475 favor one (e.g., by recognition of an easily reached address), then 3476 the sender-SMTP MUST randomize them to spread the load across 3477 multiple mail exchangers for a specific organization. 3479 The destination host (perhaps taken from the preferred MX record) may 3480 be multihomed, in which case the domain name resolver will return a 3481 list of alternative IP addresses. It is the responsibility of the 3482 domain name resolver interface to have ordered this list by 3483 decreasing preference if necessary, and the SMTP sender MUST try them 3484 in the order presented. 3486 Although the capability to try multiple alternative addresses is 3487 required, specific installations may want to limit or disable the use 3488 of alternative addresses. The question of whether a sender should 3489 attempt retries using the different addresses of a multihomed host 3490 has been controversial. The main argument for using the multiple 3491 addresses is that it maximizes the probability of timely delivery, 3492 and indeed sometimes the probability of any delivery; the counter- 3493 argument is that it may result in unnecessary resource use. Note 3494 that resource use is also strongly determined by the sending strategy 3495 discussed in Section 4.5.4.1. 3497 If an SMTP server receives a message with a destination for which it 3498 is a designated Mail eXchanger, it MAY relay the message (potentially 3499 after having rewritten the MAIL FROM and/or RCPT TO addresses), make 3500 final delivery of the message, or hand it off using some mechanism 3501 outside the SMTP-provided transport environment. Of course, neither 3502 of the latter require that the list of MX records be examined 3503 further. 3505 If it determines that it should relay the message without rewriting 3506 the address, it MUST sort the MX records to determine candidates for 3507 delivery. The records are first ordered by preference, with the 3508 lowest-numbered records being most preferred. The relay host MUST 3509 then inspect the list for any of the names or addresses by which it 3510 might be known in mail transactions. If a matching record is found, 3511 all records at that preference level and higher-numbered ones MUST be 3512 discarded from consideration. If there are no records left at that 3513 point, it is an error condition, and the message MUST be returned as 3514 undeliverable. If records do remain, they SHOULD be tried, best 3515 preference first, as described above. 3517 5.2. IPv6 and MX Records 3519 In the contemporary Internet, SMTP clients and servers may be hosted 3520 on IPv4 systems, IPv6 systems, or dual-stack systems that are 3521 compatible with either version of the Internet Protocol. The host 3522 domains to which MX records point may, consequently, contain "A RR"s 3523 (IPv4), "AAAA RR"s (IPv6), or any combination of them. While RFC 3524 3974 [39] discusses some operational experience in mixed 3525 environments, it was not comprehensive enough to justify 3526 standardization, and some of its recommendations appear to be 3527 inconsistent with this specification. The appropriate actions to be 3528 taken either will depend on local circumstances, such as performance 3529 of the relevant networks and any conversions that might be necessary, 3530 or will be obvious (e.g., an IPv6-only client need not attempt to 3531 look up A RRs or attempt to reach IPv4-only servers). Designers of 3532 SMTP implementations that might run in IPv6 or dual-stack 3533 environments should study the procedures above, especially the 3534 comments about multihomed hosts, and, preferably, provide mechanisms 3535 to facilitate operational tuning and mail interoperability between 3536 IPv4 and IPv6 systems while considering local circumstances. 3538 6. Problem Detection and Handling 3540 6.1. Reliable Delivery and Replies by Email 3542 When the receiver-SMTP accepts a piece of mail (by sending a "250 OK" 3543 message in response to DATA), it is accepting responsibility for 3544 delivering or relaying the message. It must take this responsibility 3545 seriously. It MUST NOT lose the message for frivolous reasons, such 3546 as because the host later crashes or because of a predictable 3547 resource shortage. Some reasons that are not considered frivolous 3548 are discussed in the next subsection and in Section 7.8. 3550 If there is a delivery failure after acceptance of a message, the 3551 receiver-SMTP MUST formulate and mail a notification message. This 3552 notification MUST be sent using a null ("<>") reverse-path in the 3553 envelope. The recipient of this notification MUST be the address 3554 from the envelope return path (or the Return-Path: line). However, 3555 if this address is null ("<>"), the receiver-SMTP MUST NOT send a 3556 notification. Obviously, nothing in this section can or should 3557 prohibit local decisions (i.e., as part of the same system 3558 environment as the receiver-SMTP) to log or otherwise transmit 3559 information about null address events locally if that is desired. If 3560 the address is an explicit source route, it MUST be stripped down to 3561 its final hop. 3563 For example, suppose that an error notification must be sent for a 3564 message that arrived with: 3566 MAIL FROM:<@a,@b:user@d> 3568 The notification message MUST be sent using: 3570 RCPT TO: 3572 Some delivery failures after the message is accepted by SMTP will be 3573 unavoidable. For example, it may be impossible for the receiving 3574 SMTP server to validate all the delivery addresses in RCPT command(s) 3575 due to a "soft" domain system error, because the target is a mailing 3576 list (see earlier discussion of RCPT), or because the server is 3577 acting as a relay and has no immediate access to the delivering 3578 system. 3580 To avoid receiving duplicate messages as the result of timeouts, a 3581 receiver-SMTP MUST seek to minimize the time required to respond to 3582 the final . end of data indicator. See RFC 1047 [17] for 3583 a discussion of this problem. 3585 6.2. Unwanted, Unsolicited, and "Attack" Messages 3587 Utility and predictability of the Internet mail system requires that 3588 messages that can be delivered should be delivered, regardless of any 3589 syntax or other faults associated with those messages and regardless 3590 of their content. If they cannot be delivered, and cannot be 3591 rejected by the SMTP server during the SMTP transaction, they should 3592 be "bounced" (returned with non-delivery notification messages) as 3593 described above. In today's world, in which many SMTP server 3594 operators have discovered that the quantity of undesirable bulk email 3595 vastly exceeds the quantity of desired mail and in which accepting a 3596 message may trigger additional undesirable traffic by providing 3597 verification of the address, those principles may not be practical. 3599 As discussed in Section 7.8 and Section 7.9 below, dropping mail 3600 without notification of the sender is permitted in practice. 3601 However, it is extremely dangerous and violates a long tradition and 3602 community expectations that mail is either delivered or returned. If 3603 silent message-dropping is misused, it could easily undermine 3604 confidence in the reliability of the Internet's mail systems. So 3605 silent dropping of messages should be considered only in those cases 3606 where there is very high confidence that the messages are seriously 3607 fraudulent or otherwise inappropriate. 3609 To stretch the principle of delivery if possible even further, it may 3610 be a rational policy to not deliver mail that has an invalid return 3611 address, although the history of the network is that users are 3612 typically better served by delivering any message that can be 3613 delivered. Reliably determining that a return address is invalid can 3614 be a difficult and time-consuming process, especially if the putative 3615 sending system is not directly accessible or does not fully and 3616 accurately support VRFY and, even if a "drop messages with invalid 3617 return addresses" policy is adopted, it SHOULD be applied only when 3618 there is near-certainty that the return addresses are, in fact, 3619 invalid. 3621 Conversely, if a message is rejected because it is found to contain 3622 hostile content (a decision that is outside the scope of an SMTP 3623 server as defined in this document), rejection ("bounce") messages 3624 SHOULD NOT be sent unless the receiving site is confident that those 3625 messages will be usefully delivered. The preference and default in 3626 these cases is to avoid sending non-delivery messages when the 3627 incoming message is determined to contain hostile content. 3629 6.3. Loop Detection 3631 Simple counting of the number of "Received:" header fields in a 3632 message has proven to be an effective, although rarely optimal, 3633 method of detecting loops in mail systems. SMTP servers using this 3634 technique SHOULD use a large rejection threshold, normally at least 3635 100 Received entries. Whatever mechanisms are used, servers MUST 3636 contain provisions for detecting and stopping trivial loops. 3638 6.4. Compensating for Irregularities 3640 Unfortunately, variations, creative interpretations, and outright 3641 violations of Internet mail protocols do occur; some would suggest 3642 that they occur quite frequently. The debate as to whether a well- 3643 behaved SMTP receiver or relay should reject a malformed message, 3644 attempt to pass it on unchanged, or attempt to repair it to increase 3645 the odds of successful delivery (or subsequent reply) began almost 3646 with the dawn of structured network mail and shows no signs of 3647 abating. Advocates of rejection claim that attempted repairs are 3648 rarely completely adequate and that rejection of bad messages is the 3649 only way to get the offending software repaired. Advocates of 3650 "repair" or "deliver no matter what" argue that users prefer that 3651 mail go through it if at all possible and that there are significant 3652 market pressures in that direction. In practice, these market 3653 pressures may be more important to particular vendors than strict 3654 conformance to the standards, regardless of the preference of the 3655 actual developers. 3657 The problems associated with ill-formed messages were exacerbated by 3658 the introduction of the split-UA mail reading protocols (Post Office 3659 Protocol (POP) version 2 [14], Post Office Protocol (POP) version 3 3660 [23], IMAP version 2 [19], and PCMAIL [18]). These protocols 3661 encouraged the use of SMTP as a posting (message submission) 3662 protocol, and SMTP servers as relay systems for these client hosts 3663 (which are often only intermittently connected to the Internet). 3664 Historically, many of those client machines lacked some of the 3665 mechanisms and information assumed by SMTP (and indeed, by the mail 3666 format protocol, RFC 822 [13]). Some could not keep adequate track 3667 of time; others had no concept of time zones; still others could not 3668 identify their own names or addresses; and, of course, none could 3669 satisfy the assumptions that underlay RFC 822's conception of 3670 authenticated addresses. 3672 In response to these weak SMTP clients, many SMTP systems now 3673 complete messages that are delivered to them in incomplete or 3674 incorrect form. This strategy is generally considered appropriate 3675 when the server can identify or authenticate the client, and there 3676 are prior agreements between them. By contrast, there is at best 3677 great concern about fixes applied by a relay or delivery SMTP server 3678 that has little or no knowledge of the user or client machine. Many 3679 of these issues are addressed by using a separate protocol, such as 3680 that defined in RFC 6409 [42], for message submission, rather than 3681 using originating SMTP servers for that purpose. 3683 The following changes to a message being processed MAY be applied 3684 when necessary by an originating SMTP server, or one used as the 3685 target of SMTP as an initial posting (message submission) protocol: 3687 * Addition of a message-id field when none appears 3689 * Addition of a date, time, or time zone when none appears 3691 * Correction of addresses to proper FQDN format 3693 The less information the server has about the client, the less likely 3694 these changes are to be correct and the more caution and conservatism 3695 should be applied when considering whether or not to perform fixes 3696 and how. These changes MUST NOT be applied by an SMTP server that 3697 provides an intermediate relay function. 3699 In all cases, properly operating clients supplying correct 3700 information are preferred to corrections by the SMTP server. In all 3701 cases, documentation SHOULD be provided in trace header fields and/or 3702 header field comments for actions performed by the servers. 3704 7. Security Considerations 3706 7.1. Mail Security and Spoofing 3708 SMTP mail is inherently insecure in that it is feasible for even 3709 fairly casual users to negotiate directly with receiving and relaying 3710 SMTP servers and create messages that will trick a naive recipient 3711 into believing that they came from somewhere else. Constructing such 3712 a message so that the "spoofed" behavior cannot be detected by an 3713 expert is somewhat more difficult, but not sufficiently so as to be a 3714 deterrent to someone who is determined and knowledgeable. 3715 Consequently, as knowledge of Internet mail increases, so does the 3716 knowledge that SMTP mail inherently cannot be authenticated, or 3717 integrity checks provided, at the transport level. Real mail 3718 security lies only in end-to-end methods involving the message 3719 bodies, such as those that use digital signatures (see RFC 1847 [21] 3720 and, e.g., Pretty Good Privacy (PGP) in RFC 4880 [45] or Secure/ 3721 Multipurpose Internet Mail Extensions (S/MIME) in RFC 8551 [38]). 3723 Various protocol extensions and configuration options that provide 3724 authentication at the transport level (e.g., from an SMTP client to 3725 an SMTP server) improve somewhat on the traditional situation 3726 described above. However, in general, they only authenticate one 3727 server to another rather than a chain of relays and servers, much 3728 less authenticating users or user machines. Consequently, unless 3729 they are accompanied by careful handoffs of responsibility in a 3730 carefully designed trust environment, they remain inherently weaker 3731 than end-to-end mechanisms that use digitally signed messages rather 3732 than depending on the integrity of the transport system. 3734 Efforts to make it more difficult for users to set envelope return 3735 path and header "From" fields to point to valid addresses other than 3736 their own are largely misguided: they frustrate legitimate 3737 applications in which mail is sent by one user on behalf of another, 3738 in which error (or normal) replies should be directed to a special 3739 address, or in which a single message is sent to multiple recipients 3740 on different hosts. (Systems that provide convenient ways for users 3741 to alter these header fields on a per-message basis should attempt to 3742 establish a primary and permanent mailbox address for the user so 3743 that Sender header fields within the message data can be generated 3744 sensibly.) 3746 This specification does not further address the authentication issues 3747 associated with SMTP other than to advocate that useful functionality 3748 not be disabled in the hope of providing some small margin of 3749 protection against a user who is trying to fake mail. 3751 7.2. "Blind" Copies 3753 Addresses that do not appear in the message header section may appear 3754 in the RCPT commands to an SMTP server for a number of reasons. The 3755 two most common involve the use of a mailing address as a "list 3756 exploder" (a single address that resolves into multiple addresses) 3757 and the appearance of "blind copies". Especially when more than one 3758 RCPT command is present, and in order to avoid defeating some of the 3759 purpose of these mechanisms, SMTP clients and servers SHOULD NOT copy 3760 the full set of RCPT command arguments into the header section, 3761 either as part of trace header fields or as informational or private- 3762 extension header fields. 3763 // [rfc5321bis] [[Note in draft - Suggestion from 20070124 that got 3764 // lost: delete "especially" and "the full set of" -- copying the 3765 // first one can be as harmful as copying all of them, at least 3766 // without verifying that the addresses do appear in the headers. 3767 // See G.7.9 and ticket #15.Since this rule is often violated in 3768 practice, and cannot be enforced, sending SMTP systems that are aware 3769 of "bcc" use MAY find it helpful to send each blind copy as a 3770 separate message transaction containing only a single RCPT command. 3772 There is no inherent relationship between either "reverse" (from the 3773 MAIL command) or "forward" (RCPT) addresses in the SMTP transaction 3774 ("envelope") and the addresses in the header section. Receiving 3775 systems SHOULD NOT attempt to deduce such relationships and use them 3776 to alter the header section of the message for delivery. The popular 3777 "Apparently-to" header field is a violation of this principle as well 3778 as a common source of unintended information disclosure and SHOULD 3779 NOT be used. 3781 7.3. VRFY, EXPN, and Security 3783 As discussed in Section 3.5, individual sites may want to disable 3784 either or both of VRFY or EXPN for security reasons (see below). As 3785 a corollary to the above, implementations that permit this MUST NOT 3786 appear to have verified addresses that are not, in fact, verified. 3787 If a site disables these commands for security reasons, the SMTP 3788 server MUST return a 252 response, rather than a code that could be 3789 confused with successful or unsuccessful verification. 3791 Returning a 250 reply code with the address listed in the VRFY 3792 command after having checked it only for syntax violates this rule. 3793 Of course, an implementation that "supports" VRFY by always returning 3794 550 whether or not the address is valid is equally not in 3795 conformance. 3797 On the public Internet, the contents of mailing lists have become 3798 popular as an address information source for so-called "spammers." 3799 The use of EXPN to "harvest" addresses has increased as list 3800 administrators have installed protections against inappropriate uses 3801 of the lists themselves. However, VRFY and EXPN are still useful for 3802 authenticated users and within an administrative domain. For 3803 example, VRFY and EXPN are useful for performing internal audits of 3804 how email gets routed to check and to make sure no one is 3805 automatically forwarding sensitive mail outside the organization. 3806 Sites implementing SMTP authentication may choose to make VRFY and 3807 EXPN available only to authenticated requestors. Implementations 3808 SHOULD still provide support for EXPN, but sites SHOULD carefully 3809 evaluate the tradeoffs. 3811 Whether disabling VRFY provides any real marginal security depends on 3812 a series of other conditions. In many cases, RCPT commands can be 3813 used to obtain the same information about address validity. On the 3814 other hand, especially in situations where determination of address 3815 validity for RCPT commands is deferred until after the DATA command 3816 is received, RCPT may return no information at all, while VRFY is 3817 expected to make a serious attempt to determine validity before 3818 generating a response code (see discussion above). 3820 7.4. Mail Rerouting Based on the 251 and 551 Response Codes 3822 Before a client uses the 251 or 551 reply codes from a RCPT command 3823 to automatically update its future behavior (e.g., updating the 3824 user's address book), it should be certain of the server's 3825 authenticity. If it does not, it may be subject to a man in the 3826 middle attack. 3828 7.5. Information Disclosure in Announcements 3830 There has been an ongoing debate about the tradeoffs between the 3831 debugging advantages of announcing server type and version (and, 3832 sometimes, even server domain name) in the greeting response or in 3833 response to the HELP command and the disadvantages of exposing 3834 information that might be useful in a potential hostile attack. The 3835 utility of the debugging information is beyond doubt. Those who 3836 argue for making it available point out that it is far better to 3837 actually secure an SMTP server rather than hope that trying to 3838 conceal known vulnerabilities by hiding the server's precise identity 3839 will provide more protection. Sites are encouraged to evaluate the 3840 tradeoff with that issue in mind; implementations SHOULD minimally 3841 provide for making type and version information available in some way 3842 to other network hosts. 3844 7.6. Information Disclosure in Trace Fields 3846 In some circumstances, such as when mail originates from within a LAN 3847 whose hosts are not directly on the public Internet, trace (e.g., 3848 "Received") header fields produced in conformance with this 3849 specification may disclose host names and similar information that 3850 would not normally be available. This ordinarily does not pose a 3851 problem, but sites with special concerns about name disclosure should 3852 be aware of it. Also, the optional FOR clause should be supplied 3853 with caution or not at all when multiple recipients are involved lest 3854 it inadvertently disclose the identities of "blind copy" recipients 3855 to others. 3857 7.7. Information Disclosure in Message Forwarding 3859 As discussed in Section 3.4, use of the 251 or 551 reply codes to 3860 identify the replacement address associated with a mailbox may 3861 inadvertently disclose sensitive information. Sites that are 3862 concerned about those issues should ensure that they select and 3863 configure servers appropriately. 3865 7.8. Local Operational Requirement and Resistance to Attacks 3867 In recent years, there has been an increase of attacks on SMTP 3868 servers, either in conjunction with attempts to discover addresses 3869 for sending unsolicited messages or simply to make the servers 3870 inaccessible to others (i.e., as an application-level denial of 3871 service attack). There may also be important local circumstances 3872 that justify departures from some of the limits specified in this 3873 documents especially ones involving maximums or minimums. While the 3874 means of doing so are beyond the scope of this Standard, rational 3875 operational behavior requires that servers be permitted to detect 3876 such attacks and take action to defend themselves. For example, if a 3877 server determines that a large number of RCPT commands are being 3878 sent, most or all with invalid addresses, as part of such an attack, 3879 it would be reasonable for the server to close the connection after 3880 generating an appropriate number of 5yz (normally 550) replies. 3882 7.9. Scope of Operation of SMTP Servers 3884 It is a well-established principle that an SMTP server may refuse to 3885 accept mail for any operational or technical reason that makes sense 3886 to the site providing the server. However, cooperation among sites 3887 and installations makes the Internet possible. If sites take 3888 excessive advantage of the right to reject traffic, the ubiquity of 3889 email availability (one of the strengths of the Internet) will be 3890 threatened; considerable care should be taken and balance maintained 3891 if a site decides to be selective about the traffic it will accept 3892 and process. 3894 In recent years, use of the relay function through arbitrary sites 3895 has been used as part of hostile efforts to hide the actual origins 3896 of mail. Some sites have decided to limit the use of the relay 3897 function to known or identifiable sources, and implementations SHOULD 3898 provide the capability to perform this type of filtering. When mail 3899 is rejected for these or other policy reasons, a 550 code SHOULD be 3900 used in response to EHLO (or HELO), MAIL, or RCPT as appropriate. 3902 8. IANA Considerations 3904 IANA maintains three registries in support of this specification, all 3905 of which were created for RFC 2821 or earlier. This document expands 3906 the third one as specified below. The registry references listed are 3907 as of the time of publication; IANA does not guarantee the locations 3908 associated with the URLs. The registries are as follows: 3910 * The first, "Simple Mail Transfer Protocol (SMTP) Service 3911 Extensions" [52], consists of SMTP service extensions with the 3912 associated keywords, and, as needed, parameters and verbs. 3913 Entries may be made only for service extensions (and associated 3914 keywords, parameters, or verbs) that are defined in Standards- 3915 Track or Experimental RFCs specifically approved by the IESG for 3916 this purpose. 3918 * The second registry, "Address Literal Tags" [53], consists of 3919 "tags" that identify forms of domain literals other than those for 3920 IPv4 addresses (specified in RFC 821 and in this document). The 3921 initial entry in that registry is for IPv6 addresses (specified in 3922 this document). Additional literal types require standardization 3923 before being used; none are anticipated at this time. 3925 * The third, "Mail Transmission Types" [52], established by RFC 821 3926 and renewed by this specification, is a registry of link and 3927 protocol identifiers to be used with the "via" and "with" 3928 subclauses of the time stamp ("Received:" header field) described 3929 in Section 4.4. Link and protocol identifiers in addition to 3930 those specified in this document may be registered only by 3931 standardization or by way of an RFC-documented, IESG-approved, 3932 Experimental protocol extension. This name space is for 3933 identification and not limited in size: the IESG is encouraged to 3934 approve on the basis of clear documentation and a distinct method 3935 rather than preferences about the properties of the method itself. 3936 An additional subsection has been added to the "VIA link types" 3937 and "WITH protocol types" subsections of this registry to contain 3938 registrations of "Additional-registered-clauses" as described 3939 above. The registry will contain clause names, a description, a 3940 summary of the syntax of the associated String, and a reference. 3941 As new clauses are defined, they may, in principle, specify 3942 creation of their own registries if the Strings consist of 3943 reserved terms or keywords rather than less restricted strings. 3944 As with link and protocol identifiers, additional clauses may be 3945 registered only by standardization or by way of an RFC-documented, 3946 IESG-approved, Experimental protocol extension. The additional 3947 clause name space is for identification and is not limited in 3948 size: the IESG is encouraged to approve on the basis of clear 3949 documentation, actual use or strong signs that the clause will be 3950 used, and a distinct requirement rather than preferences about the 3951 properties of the clause itself. 3953 In addition, if additional trace header fields (i.e., in addition to 3954 Return-path and Received) are ever created, those trace fields MUST 3955 be added to the IANA registry established by BCP 90 (RFC 3864) [9] 3956 for use with RFC 5322 [12]. 3958 9. Acknowledgments 3960 Many people contributed to the development of RFCs 2821 and 5321. 3961 Those documents should be consulted for those acknowledgments. 3963 Neither this document nor RFCs 2821 or 5321 would have been possible 3964 without the many contribution and insights of the late Jon Postel. 3965 Those contributions of course include the original specification of 3966 SMTP in RFC 821. A considerable quantity of text from RFC 821 still 3967 appears in this document as do several of Jon's original examples 3968 that have been updated only as needed to reflect other changes in the 3969 specification. 3971 The following filed errata against RFC 5321 that were not rejected at 3972 the time of submission: Jasen Betts, Adrien de Croy Guillaume Fortin- 3973 Debigare Roberto Javier Godoy, David Romerstein, Dominic Sayers, 3974 Rodrigo Speller, Alessandro Vesely, and Brett Watson. Some of those 3975 individuals made additional suggestions after the EMAILCORE WG was 3976 initiated. In addition to the above, several of whom continued to 3977 make other suggestions, specific suggestions that led to corrections 3978 and improvements in early versions of the current specification were 3979 received from Dave Crocker, Ned Freed, Arnt Gulbrandsen, Tony Hansen, 3980 Barry Leiba, Ivar Lumi, Pete Resnick, Hector Santos, Paul Smith and 3981 others. 3983 chetti contributed an analysis that clarified the ABNF productions 3984 that implicitly reference other documents. 3986 The EMAILCORE Working Group was chartered in September 2020 with 3987 Alexey Melnikov and Seth Blank as co-chairs. Todd Herr replaced Seth 3988 Blank early in 2021. Without their leadership and technical 3989 contributions, this document would never have been completed. 3991 10. References 3993 10.1. Normative References 3995 [1] Bradner, S., "Key words for use in RFCs to Indicate 3996 Requirement Levels", BCP 14, RFC 2119, 3997 DOI 10.17487/RFC2119, March 1997, 3998 . 4000 [2] American National Standards Institute (formerly United 4001 States of America Standards Institute), "USA Code for 4002 Information Interchange", ANSI X3.4-1968, 1968. ANSI 4003 X3.4-1968 has been replaced by newer versions with slight 4004 modifications, but the 1968 version remains definitive for 4005 the Internet. 4007 [3] Postel, J., "Simple Mail Transfer Protocol", STD 10, 4008 RFC 821, DOI 10.17487/RFC0821, August 1982, 4009 . 4011 [4] Mockapetris, P., "Domain names - implementation and 4012 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 4013 November 1987, . 4015 [5] Braden, R., Ed., "Requirements for Internet Hosts - 4016 Application and Support", STD 3, RFC 1123, 4017 DOI 10.17487/RFC1123, October 1989, 4018 . 4020 [6] Klensin, J., Freed, N., and K. Moore, "SMTP Service 4021 Extension for Message Size Declaration", STD 10, RFC 1870, 4022 DOI 10.17487/RFC1870, November 1995, 4023 . 4025 [7] Vaudreuil, G., "Enhanced Mail System Status Codes", 4026 RFC 3463, DOI 10.17487/RFC3463, January 2003, 4027 . 4029 [8] Newman, C., "ESMTP and LMTP Transmission Types 4030 Registration", RFC 3848, DOI 10.17487/RFC3848, July 2004, 4031 . 4033 [9] Klyne, G., Nottingham, M., and J. Mogul, "Registration 4034 Procedures for Message Header Fields", BCP 90, RFC 3864, 4035 DOI 10.17487/RFC3864, September 2004, 4036 . 4038 [10] Hinden, R. and S. Deering, "IP Version 6 Addressing 4039 Architecture", RFC 4291, DOI 10.17487/RFC4291, February 4040 2006, . 4042 [11] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 4043 Specifications: ABNF", STD 68, RFC 5234, 4044 DOI 10.17487/RFC5234, January 2008, 4045 . 4047 [12] Resnick, P., Ed., "Internet Message Format", RFC 5322, 4048 DOI 10.17487/RFC5322, October 2008, 4049 . 4051 10.2. Informative References 4053 [13] Crocker, D., "STANDARD FOR THE FORMAT OF ARPA INTERNET 4054 TEXT MESSAGES", STD 11, RFC 822, DOI 10.17487/RFC0822, 4055 August 1982, . 4057 [14] Butler, M., Postel, J., Chase, D., Goldberger, J., and J. 4058 Reynolds, "Post Office Protocol: Version 2", RFC 937, 4059 DOI 10.17487/RFC0937, February 1985, 4060 . 4062 [15] Postel, J. and J. Reynolds, "File Transfer Protocol", 4063 STD 9, RFC 959, DOI 10.17487/RFC0959, October 1985, 4064 . 4066 [16] Partridge, C., "Mail routing and the domain system", 4067 STD 10, RFC 974, DOI 10.17487/RFC0974, January 1986, 4068 . 4070 [17] Partridge, C., "Duplicate messages and SMTP", RFC 1047, 4071 DOI 10.17487/RFC1047, February 1988, 4072 . 4074 [18] Lambert, M., "PCMAIL: A distributed mail system for 4075 personal computers", RFC 1056, DOI 10.17487/RFC1056, June 4076 1988, . 4078 [19] Crispin, M., "Interactive Mail Access Protocol: Version 4079 2", RFC 1176, DOI 10.17487/RFC1176, August 1990, 4080 . 4082 [20] Durand, A. and F. Dupont, "SMTP 521 Reply Code", RFC 1846, 4083 DOI 10.17487/RFC1846, September 1995, 4084 . 4086 [21] Galvin, J., Murphy, S., Crocker, S., and N. Freed, 4087 "Security Multiparts for MIME: Multipart/Signed and 4088 Multipart/Encrypted", RFC 1847, DOI 10.17487/RFC1847, 4089 October 1995, . 4091 [22] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D. 4092 Crocker, "SMTP Service Extensions", STD 10, RFC 1869, 4093 DOI 10.17487/RFC1869, November 1995, 4094 . 4096 [23] Myers, J. and M. Rose, "Post Office Protocol - Version 3", 4097 STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996, 4098 . 4100 [24] De Winter, J., "SMTP Service Extension for Remote Message 4101 Queue Starting", RFC 1985, DOI 10.17487/RFC1985, August 4102 1996, . 4104 [25] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 4105 Extensions (MIME) Part One: Format of Internet Message 4106 Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996, 4107 . 4109 [26] Moore, K., "MIME (Multipurpose Internet Mail Extensions) 4110 Part Three: Message Header Extensions for Non-ASCII Text", 4111 RFC 2047, DOI 10.17487/RFC2047, November 1996, 4112 . 4114 [27] Kille, S., "MIXER (Mime Internet X.400 Enhanced Relay): 4115 Mapping between X.400 and RFC 822/MIME", RFC 2156, 4116 DOI 10.17487/RFC2156, January 1998, 4117 . 4119 [28] Elz, R. and R. Bush, "Clarifications to the DNS 4120 Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997, 4121 . 4123 [29] Freed, N. and K. Moore, "MIME Parameter Value and Encoded 4124 Word Extensions: Character Sets, Languages, and 4125 Continuations", RFC 2231, DOI 10.17487/RFC2231, November 4126 1997, . 4128 [30] Klensin, J., Ed., "Simple Mail Transfer Protocol", 4129 RFC 2821, DOI 10.17487/RFC2821, April 2001, 4130 . 4132 [31] Freed, N., "SMTP Service Extension for Command 4133 Pipelining", STD 60, RFC 2920, DOI 10.17487/RFC2920, 4134 September 2000, . 4136 [32] Freed, N., "Behavior of and Requirements for Internet 4137 Firewalls", RFC 2979, DOI 10.17487/RFC2979, October 2000, 4138 . 4140 [33] Vaudreuil, G., "SMTP Service Extensions for Transmission 4141 of Large and Binary MIME Messages", RFC 3030, 4142 DOI 10.17487/RFC3030, December 2000, 4143 . 4145 [34] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service 4146 Extension for Delivery Status Notifications (DSNs)", 4147 RFC 3461, DOI 10.17487/RFC3461, January 2003, 4148 . 4150 [35] Moore, K. and G. Vaudreuil, "An Extensible Message Format 4151 for Delivery Status Notifications", RFC 3464, 4152 DOI 10.17487/RFC3464, January 2003, 4153 . 4155 [36] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4156 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, 4157 . 4159 [37] Hansen, T., Ed. and A. Melnikov, Ed., "Message Disposition 4160 Notification", STD 85, RFC 8098, DOI 10.17487/RFC8098, 4161 February 2017, . 4163 [38] Schaad, J., Ramsdell, B., and S. Turner, "Secure/ 4164 Multipurpose Internet Mail Extensions (S/MIME) Version 4.0 4165 Message Specification", RFC 8551, DOI 10.17487/RFC8551, 4166 April 2019, . 4168 [39] Nakamura, M. and J. Hagino, "SMTP Operational Experience 4169 in Mixed IPv4/v6 Environments", RFC 3974, 4170 DOI 10.17487/RFC3974, January 2005, 4171 . 4173 [40] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 4174 Resource Identifier (URI): Generic Syntax", STD 66, 4175 RFC 3986, DOI 10.17487/RFC3986, January 2005, 4176 . 4178 [41] Kitterman, S., "Sender Policy Framework (SPF) for 4179 Authorizing Use of Domains in Email, Version 1", RFC 7208, 4180 DOI 10.17487/RFC7208, April 2014, 4181 . 4183 [42] Gellens, R. and J. Klensin, "Message Submission for Mail", 4184 STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011, 4185 . 4187 [43] Fenton, J., "Analysis of Threats Motivating DomainKeys 4188 Identified Mail (DKIM)", RFC 4686, DOI 10.17487/RFC4686, 4189 September 2006, . 4191 [44] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed., 4192 "DomainKeys Identified Mail (DKIM) Signatures", STD 76, 4193 RFC 6376, DOI 10.17487/RFC6376, September 2011, 4194 . 4196 [45] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. 4197 Thayer, "OpenPGP Message Format", RFC 4880, 4198 DOI 10.17487/RFC4880, November 2007, 4199 . 4201 [46] Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced 4202 Mail System Status Codes", BCP 138, RFC 5248, 4203 DOI 10.17487/RFC5248, June 2008, 4204 . 4206 [47] Klensin, J., Freed, N., Rose, M., and D. Crocker, Ed., 4207 "SMTP Service Extension for 8-bit MIME Transport", STD 71, 4208 RFC 6152, DOI 10.17487/RFC6152, March 2011, 4209 . 4211 [48] Klensin, J., "SMTP 521 and 556 Reply Codes", RFC 7504, 4212 DOI 10.17487/RFC7504, June 2015, 4213 . 4215 [49] Levine, J. and M. Delany, "A "Null MX" No Service Resource 4216 Record for Domains That Accept No Mail", RFC 7505, 4217 DOI 10.17487/RFC7505, June 2015, 4218 . 4220 [50] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 4221 DOI 10.17487/RFC5321, October 2008, 4222 . 4224 [51] Klensin, J.C., Ed., Murchison, K., Ed., and E. Sam, Ed., 4225 "Applicability Statement for IETF Core Email Protocols", 6 4226 August 2021, . 4229 [52] Internet Assigned Number Authority (IANA), "IANA Mail 4230 Parameters", 2007, 4231 . 4233 [53] Internet Assigned Number Authority (IANA), "Address 4234 Literal Tags", 2007, 4235 . 4237 [54] RFC Editor, "RFC Errata - RFC 5321", 2019, 4238 . Captured 4239 2019-11-19 4241 [55] IANA, "SMTP Service Extensions", 2021, 4242 . Notes in draft: RFC 4244 Editor: Please adjust date field to reflect whatever you 4245 want for a registry that is updated periodically. IANA: 4246 Please determine if the above URL is a sufficiently stable 4247 reference and adjust as appropriate if it is not. 4249 Appendix A. TCP Transport Service 4251 The TCP connection supports the transmission of 8-bit bytes. The 4252 SMTP data is 7-bit ASCII characters. Each character is transmitted 4253 as an 8-bit byte with the high-order bit cleared to zero. Service 4254 extensions may modify this rule to permit transmission of full 8-bit 4255 data bytes as part of the message body, or, if specifically designed 4256 to do so, in SMTP commands or responses. 4258 Appendix B. Generating SMTP Commands from RFC 822 Header Fields 4260 Some systems use an RFC 822 header section (only) in a mail 4261 submission protocol, or otherwise generate SMTP commands from RFC 822 4262 header fields when such a message is handed to an MTA from a UA. 4263 While the MTA-UA protocol is a private matter, not covered by any 4264 Internet Standard, there are problems with this approach. For 4265 example, there have been repeated problems with proper handling of 4266 "bcc" copies and redistribution lists when information that 4267 conceptually belongs to the mail envelope is not separated early in 4268 processing from header field information (and kept separate). 4270 It is recommended that the UA provide its initial ("submission 4271 client") MTA with an envelope separate from the message itself. 4272 However, if the envelope is not supplied, SMTP commands SHOULD be 4273 generated as follows: 4275 1. Each recipient address from a TO, CC, or BCC header field SHOULD 4276 be copied to a RCPT command (generating multiple message copies 4277 if that is required for queuing or delivery). This includes any 4278 addresses listed in a RFC 822 "group". Any BCC header fields 4279 SHOULD then be removed from the header section. Once this 4280 process is completed, the remaining header fields SHOULD be 4281 checked to verify that at least one TO, CC, or BCC header field 4282 remains. If none do, then a BCC header field with no additional 4283 information SHOULD be inserted as specified in [12]. 4285 2. The return address in the MAIL command SHOULD, if possible, be 4286 derived from the system's identity for the submitting (local) 4287 user, and the "From:" header field otherwise. If there is a 4288 system identity available, it SHOULD also be copied to the Sender 4289 header field if it is different from the address in the From 4290 header field. (Any Sender header field that was already there 4291 SHOULD be removed.) Systems may provide a way for submitters to 4292 override the envelope return address, but may want to restrict 4293 its use to privileged users. This will not prevent mail forgery, 4294 but may lessen its incidence; see Section 7.1. 4296 When an MTA is being used in this way, it bears responsibility for 4297 ensuring that the message being transmitted is valid. The mechanisms 4298 for checking that validity, and for handling (or returning) messages 4299 that are not valid at the time of arrival, are part of the MUA-MTA 4300 interface and not covered by this specification. 4302 A submission protocol based on Standard RFC 822 information alone 4303 MUST NOT be used to gateway a message from a foreign (non-SMTP) mail 4304 system into an SMTP environment. Additional information to construct 4305 an envelope must come from some source in the other environment, 4306 whether supplemental header fields or the foreign system's envelope. 4308 Attempts to gateway messages using only their header "To" and "Cc" 4309 fields have repeatedly caused mail loops and other behavior adverse 4310 to the proper functioning of the Internet mail environment. These 4311 problems have been especially common when the message originates from 4312 an Internet mailing list and is distributed into the foreign 4313 environment using envelope information. When these messages are then 4314 processed by a header-section-only remailer, loops back to the 4315 Internet environment (and the mailing list) are almost inevitable. 4317 Appendix C. Source Routes 4319 Historically, the was a reverse source routing list of 4320 hosts and a source mailbox. The first host in the was 4321 historically the host sending the MAIL command; today, source routes 4322 SHOULD NOT appear in the reverse-path. Similarly, the 4323 may be a source routing lists of hosts and a destination mailbox. 4324 However, in general, the SHOULD contain only a mailbox 4325 and domain name, relying on the domain name system to supply routing 4326 information if required. The use of source routes is deprecated (see 4327 Appendix F.2); while servers MUST be prepared to receive and handle 4328 them as discussed in Section 3.3 and Appendix F.2, clients SHOULD NOT 4329 transmit them and this section is included in the current 4330 specification only to provide context. It has been modified somewhat 4331 from the material in RFC 821 to prevent server actions that might 4332 confuse clients or subsequent servers that do not expect a full 4333 source route implementation. 4335 Historically, for relay purposes, the forward-path may have been a 4336 source route of the form "@ONE,@TWO:JOE@THREE", where ONE, TWO, and 4337 THREE MUST be fully-qualified domain names. This form was used to 4338 emphasize the distinction between an address and a route. The 4339 mailbox (here, JOE@THREE) is an absolute address, and the route is 4340 information about how to get there. The two concepts should not be 4341 confused. 4343 If source routes are used contrary to requirements and 4344 recommendations elsewhere in this specification, RFC 821 and the text 4345 below should be consulted for the mechanisms for constructing and 4346 updating the forward-path. A server that is reached by means of a 4347 source route (e.g., its domain name appears first in the list in the 4348 forward-path) MUST remove its domain name from any forward-paths in 4349 which that domain name appears before forwarding the message and MAY 4350 remove all other source routing information. The reverse-path SHOULD 4351 NOT be updated by servers conforming to this specification. 4353 Notice that the forward-path and reverse-path appear in the SMTP 4354 commands and replies, but not necessarily in the message. That is, 4355 there is no need for these paths and especially this syntax to appear 4356 in the "To:" , "From:", "CC:", etc. fields of the message header 4357 section. Conversely, SMTP servers MUST NOT derive final message 4358 routing information from message header fields. 4360 When the list of hosts is present despite the recommendations and 4361 requirements above, it is a "reverse" source route and indicates that 4362 the mail was relayed through each host on the list (the first host in 4363 the list was the most recent relay). This list is used as a source 4364 route to return non-delivery notices to the sender. If, contrary to 4365 the recommendations here, a relay host adds itself to the beginning 4366 of the list, it MUST use its name as known in the transport 4367 environment to which it is relaying the mail rather than that of the 4368 transport environment from which the mail came (if they are 4369 different). Note that a situation could easily arise in which some 4370 relay hosts add their names to the reverse source route and others do 4371 not, generating discontinuities in the routing list. This is another 4372 reason why servers needing to return a message SHOULD ignore the 4373 source route entirely and simply use the domain as specified in the 4374 Mailbox. 4376 Appendix D. Scenarios 4378 This section presents complete scenarios of several types of SMTP 4379 sessions. In the examples, "C:" indicates what is said by the SMTP 4380 client, and "S:" indicates what is said by the SMTP server. 4382 D.1. A Typical SMTP Transaction Scenario 4384 This SMTP example shows mail sent by Smith at host bar.com, and to 4385 Jones, Green, and Brown at host foo.com. Here we assume that host 4386 bar.com contacts host foo.com directly. The mail is accepted for 4387 Jones and Brown. Green does not have a mailbox at host foo.com. 4389 S: 220 foo.com Simple Mail Transfer Service Ready 4390 C: EHLO bar.com 4391 S: 250-foo.com greets bar.com 4392 S: 250-8BITMIME 4393 S: 250-SIZE 4394 S: 250-DSN 4395 S: 250 HELP 4396 C: MAIL FROM: 4397 S: 250 OK 4398 C: RCPT TO: 4399 S: 250 OK 4400 C: RCPT TO: 4401 S: 550 No such user here 4402 C: RCPT TO: 4403 S: 250 OK 4404 C: DATA 4405 S: 354 Start mail input; end with . 4406 C: Blah blah blah... 4407 C: ...etc. etc. etc. 4408 C: . 4409 S: 250 OK 4410 C: QUIT 4411 S: 221 foo.com Service closing transmission channel 4413 D.2. Aborted SMTP Transaction Scenario 4415 S: 220 foo.com Simple Mail Transfer Service Ready 4416 C: EHLO bar.com 4417 S: 250-foo.com greets bar.com 4418 S: 250-8BITMIME 4419 S: 250-SIZE 4420 S: 250-DSN 4421 S: 250 HELP 4422 C: MAIL FROM: 4423 S: 250 OK 4424 C: RCPT TO: 4425 S: 250 OK 4426 C: RCPT TO: 4427 S: 550 No such user here 4428 C: RSET 4429 S: 250 OK 4430 C: QUIT 4431 S: 221 foo.com Service closing transmission channel 4433 D.3. Relayed Mail Scenario 4435 Step 1 -- Source Host to Relay Host 4436 The source host performs a DNS lookup on XYZ.COM (the destination 4437 address) and finds DNS MX records specifying xyz.com as the best 4438 preference and foo.com as a lower preference. It attempts to open a 4439 connection to xyz.com and fails. It then opens a connection to 4440 foo.com, with the following dialogue: 4442 S: 220 foo.com Simple Mail Transfer Service Ready 4443 C: EHLO bar.com 4444 S: 250-foo.com greets bar.com 4445 S: 250-8BITMIME 4446 S: 250-SIZE 4447 S: 250-DSN 4448 S: 250 HELP 4449 C: MAIL FROM: 4450 S: 250 OK 4451 C: RCPT TO: 4452 S: 250 OK 4453 C: DATA 4454 S: 354 Start mail input; end with . 4455 C: Date: Thu, 21 May 1998 05:33:29 -0700 4456 C: From: John Q. Public 4457 C: Subject: The Next Meeting of the Board 4458 C: To: Jones@xyz.com 4459 C: 4460 C: Bill: 4461 C: The next meeting of the board of directors will be 4462 C: on Tuesday. 4463 C: John. 4464 C: . 4465 S: 250 OK 4466 C: QUIT 4467 S: 221 foo.com Service closing transmission channel 4469 Step 2 -- Relay Host to Destination Host 4470 foo.com, having received the message, now does a DNS lookup on 4471 xyz.com. It finds the same set of MX records, but cannot use the one 4472 that points to itself (or to any other host as a worse preference). 4473 It tries to open a connection to xyz.com itself and succeeds. Then 4474 we have: 4476 S: 220 xyz.com Simple Mail Transfer Service Ready 4477 C: EHLO foo.com 4478 S: 250 xyz.com is on the air 4479 C: MAIL FROM: 4480 S: 250 OK 4481 C: RCPT TO: 4482 S: 250 OK 4483 C: DATA 4484 S: 354 Start mail input; end with . 4485 C: Received: from bar.com by foo.com ; Thu, 21 May 1998 4486 C: 05:33:29 -0700 4487 C: Date: Thu, 21 May 1998 05:33:29 -0700 4488 C: From: John Q. Public 4489 C: Subject: The Next Meeting of the Board 4490 C: To: Jones@xyz.com 4491 C: 4492 C: Bill: 4493 C: The next meeting of the board of directors will be 4494 C: on Tuesday. 4495 C: John. 4496 C: . 4497 S: 250 OK 4498 C: QUIT 4499 S: 221 xyz.com Service closing transmission channel 4501 D.4. Verifying and Sending Scenario 4502 S: 220 foo.com Simple Mail Transfer Service Ready 4503 C: EHLO bar.com 4504 S: 250-foo.com greets bar.com 4505 S: 250-8BITMIME 4506 S: 250-SIZE 4507 S: 250-DSN 4508 S: 250-VRFY 4509 S: 250 HELP 4510 C: VRFY Crispin 4511 S: 250 Mark Crispin 4512 C: MAIL FROM: 4513 S: 250 OK 4514 C: RCPT TO: 4515 S: 250 OK 4516 C: DATA 4517 S: 354 Start mail input; end with . 4518 C: Blah blah blah... 4519 C: ...etc. etc. etc. 4520 C: . 4521 S: 250 OK 4522 C: QUIT 4523 S: 221 foo.com Service closing transmission channel 4525 Appendix E. Other Gateway Issues 4527 In general, gateways between the Internet and other mail systems 4528 SHOULD attempt to preserve any layering semantics across the 4529 boundaries between the two mail systems involved. Gateway- 4530 translation approaches that attempt to take shortcuts by mapping 4531 (such as mapping envelope information from one system to the message 4532 header section or body of another) have generally proven to be 4533 inadequate in important ways. Systems translating between 4534 environments that do not support both envelopes and a header section 4535 and Internet mail must be written with the understanding that some 4536 information loss is almost inevitable. 4538 Appendix F. Deprecated Features of RFC 821 4540 A few features of RFC 821 have proven to be problematic and SHOULD 4541 NOT be used in Internet mail. Some of these features were deprecated 4542 in RFC 2821 in 2001; source routing and two-digit years in dates were 4543 deprecated by RFC 1123 in 1989. Of the domain literal forms, RFC 4544 1123 required support only for the dotted decimal form. With the 4545 possible exception of old, hardware-embedded, applications, there is 4546 no longer any excuse for these features to appear on the contemporary 4547 Internet. 4549 F.1. TURN 4551 This command, described in RFC 821, raises important security issues 4552 since, in the absence of strong authentication of the host requesting 4553 that the client and server switch roles, it can easily be used to 4554 divert mail from its correct destination. Its use is deprecated; 4555 SMTP systems SHOULD NOT use it unless the server can authenticate the 4556 client. 4558 F.2. Source Routing 4560 RFC 821 utilized the concept of explicit source routing to get mail 4561 from one host to another via a series of relays. The requirement to 4562 utilize source routes in regular mail traffic was eliminated by the 4563 introduction of the domain name system "MX" record and the last 4564 significant justification for them was eliminated by the 4565 introduction, in RFC 1123, of a clear requirement that addresses 4566 following an "@" must all be fully-qualified domain names. 4567 Consequently, the only remaining justifications for the use of source 4568 routes are support for very old SMTP clients or MUAs and in mail 4569 system debugging. They can, however, still be useful in the latter 4570 circumstance and for routing mail around serious, but temporary, 4571 problems such as problems with the relevant DNS records. 4573 SMTP servers MUST continue to accept source route syntax as specified 4574 in the main body of this document and in RFC 1123. They MAY, if 4575 necessary, ignore the routes and utilize only the target domain in 4576 the address. If they do utilize the source route, the message MUST 4577 be sent to the first domain shown in the address. In particular, a 4578 server MUST NOT guess at shortcuts within the source route. 4580 Clients SHOULD NOT utilize explicit source routing except under 4581 unusual circumstances, such as debugging or potentially relaying 4582 around firewall or mail system configuration errors. 4584 F.3. HELO 4586 As discussed in Sections 3.1 and 4.1.1, EHLO SHOULD be used rather 4587 than HELO when the server will accept the former. Servers MUST 4588 continue to accept and process HELO in order to support older 4589 clients. 4591 F.4. #-literals 4593 RFC 821 provided for specifying an Internet address as a decimal 4594 integer host number prefixed by a pound sign, "#". In practice, that 4595 form has been obsolete since the introduction of TCP/IP. It is 4596 deprecated and MUST NOT be used. 4598 F.5. Dates and Years 4600 When dates are inserted into messages by SMTP clients or servers 4601 (e.g., in trace header fields), four-digit years MUST BE used. Two- 4602 digit years are deprecated; three-digit years were never permitted in 4603 the Internet mail system. 4605 F.6. Sending versus Mailing 4607 In addition to specifying a mechanism for delivering messages to 4608 user's mailboxes, RFC 821 provided additional, optional, commands to 4609 deliver messages directly to the user's terminal screen. These 4610 commands (SEND, SAML, SOML) were rarely implemented, and changes in 4611 workstation technology and the introduction of other protocols may 4612 have rendered them obsolete even where they are implemented. 4614 Clients SHOULD NOT use SEND, SAML, or SOML commands. If a server 4615 implements them, the implementation model specified in RFC 821 [3] 4616 MUST be used and the command names MUST be published in the response 4617 to the EHLO command. 4619 Appendix G. Other Outstanding Issues 4621 [[RFC Editor: Please remove this section before publication.]] 4623 In December 2019, an issue was raised on the ietf-smtp@ietf.org list 4624 that led to a broad discussion of ways in which existing practice had 4625 diverged from the specifications and recommendations of RFC 5321 in 4626 the more than eleven years since it was published (some of those 4627 issues probably affect the boundary between RFC 5321 and 5322 and 4628 hence the latter as well). In most cases, those divergences call for 4629 revision of the Technical Specification to match the practice, 4630 clarification of the specification text in other ways, or a more 4631 comprehensive explanation of why the practices recommended by the 4632 specification should really be followed. 4634 Those discussions raised two other issues, which were that 4636 * The publication of the Submission Server specification of RFC 6409 4637 in November 2011 may not have been fully reflected in RFC 5321 4638 (despite the even earlier publication of RFC 4409) and 4640 * There may be inconsistencies between the July 2009 Internet Mail 4641 Architecture description of RFC 5598 and the model described in 4642 RFC 5321. The issue called out in Appendix G.3 below may be an 4643 example of one of those inconsistencies. 4645 Those discrepancies should be identified and discussed and decisions 4646 made to fix them (and where) or to ignore them and let them continue. 4648 There has also been discussion on the mailing list, perhaps amounting 4649 to very rough consensus, that any revision of RFC 5321 and/or 5322 4650 should be accompanied by a separate Applicability Statement document 4651 that would make recommendations about applicability or best practices 4652 in particular areas rather than trying to get everything into the two 4653 technical specifications. This appendix does not attempt to identify 4654 which issues should get which treatment. 4656 This work is now (starting in the last half of 2020) being considered 4657 in the EMAILCORE WG. This appendix will act as a temporary record of 4658 issues that should be discussed and decided upon before a revised 4659 SMTP specification (or a related Applicability Statement) is 4660 published, issues that have not been reflected in errata (see 4661 Appendix H.1 below for those covered by errata). 4663 Ticket numbers listed below reference the list in 4664 https://trac.ietf.org/trac/emailcore/report/1 . 4666 G.1. IP Address literals 4668 The specification is unclear about whether IP address literals, 4669 particularly IP address literals used as arguments to the EHLO 4670 command, are required to be accepted or whether they are allowed to 4671 be rejected as part of the general "operational necessity" exception. 4672 Some have suggested that rejection of them is so common as an anti- 4673 spam measure that the use of such literals should be deprecated 4674 entirely in the specification, others that the are still useful and 4675 used and/or that, whatever is said about IP address literals within 4676 an SMTP session (e.g., in MAIL or RCPT commands), they should 4677 continue to be allowed (and required) in EHLO. 4678 Ticket #1. 4680 G.2. Repeated Use of EHLO 4682 While the specification says that an SMTP client's sending EHLO again 4683 after it has been issued (starting an SMTP session and treats it as 4684 if RSET had been sent (closing the session) followed by EHLO, there 4685 are apparently applications, at least some of them involving setting 4686 up of secure connections, in which the second EHLO is required and 4687 does not imply RSET. Does the specification need to be adjusted to 4688 reflect or call out those cases? 4690 After extended discussion in October 2020, it appears that the 4691 easiest fix to these problems is to clarify the conditions for 4692 termination of a mail transaction in Section 3.3 and to clearly 4693 specify the effect of a second (or subsequent) EHLO command in 4694 Section 4.1.4. 4695 See also Appendix G.7.4. 4696 Ticket #2. Both changes have been made in draft-ietf-emailcore- 4697 rfc5321bis-01. 4699 G.3. Meaning of "MTA" and Related Terminology 4701 A terminology issue has come up about what the term "MTA" actually 4702 refers to, a question that became at least slightly more complicated 4703 when we formalized RFC 6409 Submission Servers. Does the document 4704 need to be adjusted to be more clear about this topic? Note that the 4705 answer may interact with the question asked in Section 2 above. 4706 Possibly along the same lines, RFC 2821 changed the RFC 821 4707 terminology from "sender-SMTP" and "receiver-SMTP" to "SMTP client" 4708 and "SMTP server" respectively. As things have evolved, it is 4709 possible that newer terminology is a source of confusion and that the 4710 terminology should be changed back, something that also needs 4711 discussion. 4712 Ticket #3. 4714 G.4. Originator, or Originating System, Authentication 4716 Should RFC 5321bis address authentication and related issues or 4717 should Section 3.9 or other text be reshaped (in addition to or 4718 instead of the comment on that section) to lay a better foundation 4719 for such work, either in the context of mailing lists or more 4720 generally? 4721 This may interact with Erratum 4055 and Ticket #30 below. 4723 G.5. Remove or deprecate the work-around from code 552 to 452 4725 The suggestion in Section 4.5.3.1.10 may have outlived its usefulness 4726 and/or be inconsistent with current practice. Should it be removed 4727 and/or explicitly deprecated? 4728 Ticket #5. 4730 SHOULD requirement removed. 4732 G.6. Clarify where the protocol stands with respect to submission and 4733 TLS issues 4735 1. submission on port 587 4737 2. submission on port 465 4739 3. TLS relay on a port different from 25 (whenever) 4740 4. Recommendations about general use of transport layer (hop by hop) 4741 security, particularly encryption including consideration of RFC 4742 8314. 4744 G.7. Probably-substantive Discussion Topics Identified in Other Ways 4746 The following issues were identified as a group in the opening Note 4747 but called out specifically only in embedded CREF comments in 4748 versions of this draft prior to the first EMAILCORE version. 4750 G.7.1. Issues with 521, 554, and 556 codes 4752 See new Section 4.2.4.2. More text may be needed, there or 4753 elsewhere, about choices of codes in response to initial opening and 4754 to EHLO, especially to deal with selective policy rejections. In 4755 particular, should we more strongly discourage the use of 554 on 4756 initial opening. And should we make up a 421 code (or a new 4yz 4757 code, perhaps 454) code for situations where the server is 4758 temporarily out of service? 4759 Ticket #6. 4761 G.7.2. SMTP Model, terminology, and relationship to RFC 5598 4763 CREF comment in Section 2, CREF comment in Section 2.3.10, and 4764 comments in the introductory portion of Appendix G. 4766 G.7.3. Resolvable FQDNs and private domain names 4768 Multiple CREF comments in Section 2.3.5 4769 Tickets #9, #10 and #41. 4771 Ticket #41 marked "closed no change", per email 2021-0405. 4773 G.7.4. Possible clarification about mail transactions and transaction 4774 state 4776 CREF comment in Section 3.3 and also reference in Section 4.1.4 4777 Ticket #11. 4779 // See correspondence on this ticket 2021-07-06 through 2021-07-09. 4781 G.7.5. Issues with mailing lists, aliases, and forwarding 4783 CREF comment in Section 3.9. May also want to note forwarding as an 4784 email address portability issue. Note that, if changes are made in 4785 this area, they should be kept consistent with the description and 4786 discussion of the 251 and 551 in Section 4.2 and Section 3.5 as well 4787 as Section 3.4 to avoid introducing inconsistencies. In addition, 4788 there are some terminology issues about the use of the term "lists", 4789 identified in erratum 1820, that should be reviewed after any more 4790 substantive changes are made to the relevant sections. 4791 Ticket #12 and Ticket #34 (Ticket #34/ erratum 1820 may be resolved 4792 in -06). 4794 G.7.6. Requirements for domain name and/or IP address in EHLO 4796 Text in Section 4.1.4; change made in -05. 4797 Ticket #19. 4799 G.7.7. Does the 'first digit only' and/or non-listed reply code text 4800 need clarification? 4802 Resolved. Text in Section 4.2 changed 2021-02-08 and CREF comment in 4803 Section 4.3.1 removed. 4805 Perhaps unresolved -- ongoing discussion on mailing list after IETF 4806 110. 4807 Ticket #13. 4809 G.7.8. Size limits 4811 Once a decision is made about line length rules for RFC 5322bis, 4812 review the size limit discussions in this document, particularly the 4813 CREF comment (Note in Draft) at the end of the introductory material 4814 to Section 4.5.3 to be sure this document says what we want it to 4815 say. (See the additional question about minimum quantities, etc., in 4816 Appendix G.7.19.) 4817 Ticket #14 (closed - no action) and maybe Ticket #38 (to A/S). 4819 G.7.9. Discussion of 'blind' copies and RCPT 4821 CREF comment in Section 7.2. May also need to discussion whether 4822 that terminology is politically incorrect and suggest a replacement. 4823 Ticket #15. 4825 G.7.10. Further clarifications needed to source routes? 4827 The current text largely deprecates the use of source routes but 4828 suggests that servers continue to support them. Is additional work 4829 needed in this area? See CREF comment in Appendix C 4830 Ticket #17. 4832 G.7.11. Should 1yz Be Revisited? 4834 RFC 5321 depreciated the "positive preliminary reply" response code 4835 category with first digit "1", so that the first digit of valid SMTP 4836 response codes must be 2, 3, 4, or 5. It has been suggested (see 4837 mail from Hector Santos with Subject "SMTP Reply code 1yz Positive 4838 Preliminary reply", March 5, 2020 12:56 -0500, on the SMTP list) that 4839 these codes should be reinstated to deal with some situations that 4840 became more plausible after 5321 was published. Do we need to take 4841 this back up? 4842 Ticket #18. 4844 G.7.12. Review Timeout Specifications 4846 RFC 5321 (and its predecessors going back to 821) specify minimum 4847 periods for client and server to wait before timing out. Are those 4848 intervals still appropriate in a world of faster processors and 4849 faster networks? Should they be updated and revised? Or should more 4850 qualifying language be added? 4851 Ticket #16. 4853 G.7.13. Possible SEND, SAML, SOML Loose End 4855 Per discussion (and Ticket #20), the text about SEND, SAML, and SOML 4856 has been removed from the main body of the document so that the only 4857 discussion of them now appears in Appendix F.6. Per the editor's 4858 note in that appendix, is any further discussion needed? 4860 G.7.14. Abstract Update 4862 Does the Abstract need to be modified in the light of RFC 6409 or 4863 other changes? 4865 G.7.15. Informative References to MIME and/or Message Submission 4867 Should RFC 2045 (MIME) and/or RFC 6409 (Message Submission) be 4868 referenced at the end of Section 1.2? 4869 Ticket #53. 4871 G.7.16. Mail Transaction Discussion 4873 Does the discussion of mail transactions need more work (see CREF in 4874 Section 3.3.)? 4876 G.7.17. Hop by hop Authentication and/or Encryption 4878 Should this document discuss hop-by-hop authentication or, for that 4879 matter, encryption? (See CREF in Section 2.) 4880 Propose "No, it shouldn't" (20211101 conversation with Todd.) 4881 Ticket #50. 4883 G.7.18. More Text About 554 Given 521, etc. 4885 Does reply code 554 need additional or different explanation in the 4886 light of the addition of the new 521 code and/or the new (in 5321bis 4887 Section 4.2.4.2? (See CREF in Section 4.2.3.) 4889 G.7.19. Minimum Lengths and Quantities 4891 Are the minimum lengths and quantities specified in Section 4.5.3 4892 still appropriate or do they need adjusting? (See CREF at the 4893 beginning of that section.) Also note potential interaction with the 4894 proposed LIMITS SMTP extension (draft-freed-smtp-limits) which may 4895 make this question OBE. 4897 G.8. Enhanced Reply Codes and DSNs 4899 Enhanced Mail System Status Codes (RFC 3463) [7] were added to SMTP 4900 before RFC 5321 was published and are now, together with a 4901 corresponding registry [46], widely deployed and in extensive use in 4902 the network. Similar, the structure and extensions options for 4903 Delivery Status Notifications [35] is implemented, deployed, and in 4904 wide use. Is it time to fold all or part of those mature 4905 specifications into the SMTP spec or at least to mention and 4906 normatively reference them? And, as an aside, do those specs need 4907 work or, if they are kept separate, is it time to move them to 4908 Internet Standard? 4910 At least one of the current references to RFC 3463 indicates that it 4911 SHOULD be used. That presumably makes the reference normative 4912 because one needs that specification to know what the present 4913 document requires. It has been moved in the -03 version of this 4914 draft, but, unless it is move to Internet Standard, it will require 4915 downref treatment. 4917 G.9. Revisiting Quoted Strings 4919 Recent discussions both in and out of the IETF have highlighted 4920 instances of non-compliance with the specification of a Local-part 4921 consisting of a Quoted-string, whether any content of QcontentSMTP 4922 that actually requires special treatment consists of qtextSMTP, 4923 quoted-pairSMTP, or both. Section 4.1.2 (of RFC 5321, repeated 4924 above) ends with a few paragraphs of warnings (essentially a partial 4925 applicability statement), the first of which cautions against 4926 cleverness with either Quoted-string or case sensitivity as a threat 4927 to interoperability. 4929 The Quoted-string portion of that discussion has apparently been 4930 widely not read or ignored. Do we need to do something else? If we 4931 do an Applicability Statement, would it be useful to either reference 4932 the discussion in this document from there or to move the discussion 4933 there and reference it (normatively?) from here? 4935 There has been a separate discussion of empty quoted strings in 4936 addresses, i.e., whether the production should be 4937 required to included at least one non-whitespace character. It is 4938 separate from this issue but would be further impacted or distorted 4939 from the considerations identified in this Section. 4940 Ticket #21. May also interact with Ticket #35. 4942 G.10. Internationalization 4944 RFC 5321 came long before work on internationalization of email 4945 addresses and headers (other than by use of encoded words in MINE) 4946 and specifically before the work of the EAI WG leading to the 4947 SMTPUTF8 specifications, specifically RFCs 6530ff. The second 4948 explanatory paragraph at the end of Section 4.1.2 ("Systems MUST NOT 4949 define mailboxes ...") is an extremely strong prohibition against the 4950 use of non-ASCII characters in SMTP commands and the requirements 4951 about message content in Section 2.3.1 an equally strong one for 4952 content. Would it be appropriate to add something like "in the 4953 absence of relevant extensions" there? Also, given [mis]behavior 4954 seen in the wild, does that paragraph (or an A/S) need an explicit 4955 caution about SMTP servers or clients assuming they can apply the 4956 popular web convention of using %NN sequences as a way to encode non- 4957 ASCII characters ( in RFC 3986) and assuming some later 4958 system will interpret it as they expect? Would it be appropriate to 4959 add an Internationalization Considerations section to the body of 4960 this document if only for the purpose of pointing people elsewhere? 4962 More broadly, while the EAI WG's extensions for non-ASCII headers and 4963 addresses are explicitly out of scope for the EMAILCORE WG (at least 4964 for 5321bis (and 5322bis), those documents make assumptions and 4965 interpretations of the core documents. Are there areas in which 4966 5321bis could and should be clarified to lay a more solid foundation 4967 for the EAI/SMTPUTF8 work and, if so, what are they? 4969 G.11. SMTP Clients, Servers, Senders, and Receivers 4971 RFC 821 used the terms "SMTP-sender" and "SMTP-receiver". In RFC 4972 2821 (and hence in 5321), we switched that to "client" and "server" 4973 (See the discussion in Section 1.2). In part because a relay is a 4974 server and then a client (in some recent practice, even interleaving 4975 the two functions by opening the connection to the next host in line 4976 and sending commands before the incoming transaction is complete), 4977 RFC 5321 continues to use the original terminology in some places. 4978 Should we revisit that usage, possibly even returning to consistent 4979 use of the original terminology? 4981 G.12. Extension Keywords Starting in 'X-' 4983 Section 2.2.2 contains a discussion of SMTP keywords starting in "X". 4984 Given general experience with such things and RFC 6648, is there any 4985 reason to not deprecate that practice entirely and remove that text? 4986 If we do so, should the former Section 4.1.5 be dropped or rewritten 4987 to make clear this is an obsolete practice? 4988 4.1.5 eliminated in rfc5321bis-06. 4989 Ticket #42 (resolved with -06??). 4991 G.13. Deprecating HELO 4993 RFC 5321 (and 2821 before it) very carefully circle around the status 4994 of HELO, even recommending its use as a fallback when EHLO is sent 4995 and a "command not recognized" response is received. We are just a 4996 few months short of 20 years; is it time to deprecate the thing and 4997 clean out some or all of that text? And, given a recent (4Q2020) 4998 discussion on the EMAILCORE list, should EHLO be explicitly bound to 4999 SMTP over TCP with the older transports allowed only with HELO? 5000 While those questions may seem independent, separating them is fairly 5001 hard given the way the text is now constructed. 5003 Resolved 2021-01-19: No change 5004 Ticket #43. 5006 G.14. The FOR Clause in Trace Fields: Semantics, Security 5007 Considerations, and Other Issues 5009 The FOR clause in time-stamp ("Received:") fields is seriously under- 5010 defined. It is optional, the syntax is clear, but its semantics and 5011 use, while perhaps obvious from content and the application of common 5012 sense, have never been defined ("never" going back to 821). Do we 5013 want to better define it? Is there any chance that a definition 5014 would invalid existing, conforming and sensible, implementations? If 5015 we do want to define semantics, draft text and advice as to where it 5016 should go are invited. 5018 (Paragraph added 2021-08-18) 5019 In particular, recentdiscussions point strongly to the need for a 5020 statement to the effect that the value of the for clause must contain 5021 one of the addresses that caused the message to be routed to the 5022 recipient of this message copy (thanks Ned), that no mare than one 5023 address can appear, and that showing one address when there are 5024 multiple RCPT commands may be a security and/or privacy issue (thanks 5025 Ned and Viktor and see ). More detailed or specific 5027 guidance, including case analysis, are probably material for the A/s, 5028 but that is obviously up to the WG. 5030 Note the existing discussions in Section 7.2 and Section 7.6 as they 5031 may need adjustment, or at least cross-references, especially if FOR 5032 is more precisely defined. 5034 There is probably an error in Section 7.6. Its last sentence implies 5035 a possible interaction between messages with multiple recipients and 5036 the FOR clause of trace fields. However, because the syntax of the 5037 FOR clause only allows one Mailbox (or Path), it isn't clear if that 5038 statement is meaningful. Should it be revised to discuss other 5039 situations in which including FOR might not be desirable from a 5040 security or privacy standpoint? (See above -- this paragraph 5041 deliberately not changed in -04). 5042 Ticket #55 5044 G.15. Resistance to Attacks and Operational Necessity 5046 Section 7.8 is often cited as allowing an exception to the rules of 5047 the specification for reasons of operational necessity, not just 5048 attack resistance. I (JcK) believe the broader interpretation was 5049 intended by YAM (the section was new in RFC 5321). Recommendation: 5050 change the title to explicitly include "Local Operational 5051 Requirements" and add text to indicate that attack resistance is not 5052 the only possible source of such requirements. 5054 G.16. Mandatory 8BITMIME 5056 There was extensive discussion on the mailing list in October 2021 5057 about messages with and without 8-bit (i.e., octets with the leading 5058 on) content and a tentative conclusion that support for 8BITMIME 5059 should be required. If that is the WG's conclusion, we need to 5060 figure out what to say and where to say it. 5062 Appendix H. RFC 5321 Errata Summary and Tentative Change Log 5064 [[RFC Editor: Please remove this section before publication.]] 5066 H.1. RFC 5321 Errata Summary 5068 This document addresses the following errata filed against RFC 5321 5069 since its publication in October 2008 [54]. As with the previous 5070 appendix, ticket numbers included below reference 5071 https://trac.ietf.org/trac/emailcore/report/1 . 5072 // [[Note in Draft: Items with comments below have not yet been 5073 // resolved as errata. As of the end of November 2020, none of them 5074 // have been checked and verified by the emailcore WG.]] 5076 1683 ABNF error. Section 4.4 5077 Ticket #23. 5079 4198 Description error. Section 4.2. 5080 RESOLVED, ticket #24, 2020-12-14. 5082 2578 Syntax description error. Section 4.1.2 5084 1543 Wrong code in description Section 3.8 5085 Ticket #26 5087 4315 ABNF - IPv6 Section 4.1.3. 5088 // [5321bis]The IPv6 syntax has been adjusted since 5321 was 5089 // published (the erratum mentions RFC 5952, but RFC 6874 and 5090 draft- 5091 // carpenter-6man-rfc6874bis should also be considered). See the 5092 // rewritten form and the comment in the section cited in the 5093 // previous sentence, at least for the RFC 5952 issues. The 5094 editor 5095 // awaits instructions. See https://www.rfc-editor.org/errata/ 5096 // eid4315 5097 Ticket #27. 5099 5414 ABNF for Quoted-string Section 4.1.2 5100 Ticket #22. 5102 1851 Location of text on unexpected close Section 4.1.1.5. Text 5103 moved per email 2020-12-31. 5104 Ticket #28. 5106 3447 Use of normative language (e.g., more "MUST"s), possible 5107 confusion in some sections Section 4.4. 5108 Ticket #7 5110 // [5321bis]As Barry notes in his verifier comments on the erratum 5111 // (see https://www.rfc-editor.org/errata/eid3447), the comments 5112 and 5113 // suggestions here raise a number of interesting (and difficult) 5114 // issues. One of the issues is that the core of RFCs 5321 (and 5115 // 2821) is text carried over from Jon Postel's RFC 821, a 5116 document 5117 // that was not only written in a different style than the IETF 5118 uses 5119 // today but that was written at a time when no one had dreamt of 5120 RFC 5121 // 2119 or even the IETF itself. It appears to me that trying to 5122 // patch that style might easily result in a document that is 5123 harder 5124 // to read as well as being error prone. If we want to get the 5125 // document entirely into contemporary style, we really should 5126 bite 5127 // the bullet and do a complete rewrite. To respond to a 5128 different 5129 // point in Barry's discussion, I think an explicit statement that 5130 // 5321/5322 and their predecessors differ in places and why would 5131 be 5132 // helpful. Text, and suggestions about where to put it, are 5133 // solicited. A list of differences might be a good idea too, but 5134 // getting it right might be more work than there is available 5135 energy 5136 // to do correctly. 5138 5711 Missing leading spaces in example Appendix D.3. 5140 // [5321bis]Well, this is interesting because the XML is correct 5141 and 5142 // the spaces are there, embedded in artwork. So either the 5143 XML2RFC 5144 // processor at the time took those leading spaces out or the RFC 5145 // Editor improved on the document and the change was not caught 5146 in 5147 // AUTH48, perhaps because rfcdiff ignores white space. We just 5148 need 5149 // to watch for future iterations. 5151 As of 2021-03-15, both the txt and html-ized versions of draft- 5152 ietf-emailcore-rfc5321bis-02 were showing identical output for 5153 both parts of the example, so the problem appears to be OBE at 5154 worst. 5155 Ticket #29 (closed 2021-03-16) 5157 4055 Erratum claims the the description of SPF and DKIM is wrong. 5158 It is not clear what 5321bis should really say about them, but the 5159 current text probably needs work (or dropping, which is what the 5160 proposed erratum suggests). 5161 Text changed; ticket should probably be closed after WG reviews 5162 -04. 5163 Ticket #30 (resolved??). 5165 // [5321bis]Note that rejected errata have _not_ been reviewed to see 5166 // if they contain anything useful that should be discussed again 5167 // with the possibility of rethinking and changing text. Volunteers 5168 // sought. 5170 H.2. Changes from RFC 5321 (published October 2008) to the initial 5171 (-00) version of this draft 5173 * Acknowledgments section (Section 9) trimmed back for new document. 5175 * Introductory paragraph to Appendix F extended to make it clear 5176 that these features were deprecated a long time ago and really 5177 should not be in use any more. 5179 * Adjusted some language to clarify that source routes really, 5180 really, should not be used or depended upon. 5182 * IPv6 address syntax replaced by a copy of the IPv6 URI syntax and 5183 a note added. 5185 * Production index added as a first step in tying all productions to 5186 their sources. As part of the effort to make the document more 5187 easily navigable, table of contents entries have been created for 5188 the individual command descriptions. 5190 * Clarified the relationship between the SMTP "letters, digits, and 5191 hyphens" and DNS "preferred name syntax" (Section 2.3.5). 5193 * Revised the reply code sections to add new 521 and 556 codes, 5194 clarify relationships, and be explicit about the requirement for 5195 clients to rely on first digits rather than the sequences in 5196 Section 4.3.2. 5198 * In conjunction with the above, explicitly obsolete RFCs 1846 and 5199 7504 (but that might not be right -- see email 2021-10-03. 5201 * Incorporated a correction reflecting Errata ID 2578. 5203 * Some small editorial changes made to eliminate redundant 5204 statements that were very close together. Other, equally small, 5205 editorial changes have been made to improve grammar or clarity. 5207 * A few questions, marked "[[5321bis Editor's Note:", or "[[Note in 5208 Draft" have been added for the group to resolve. Other questions, 5209 especially those in the errata summary, are simply included in 5210 narrative comments in CREFs. 5212 * Checked and rationalized "response" (to a command) and "reply 5213 code" terminology. One can talk about a "999 response" but only a 5214 "999 reply code". There is no such thing as a "response code". 5216 * Added note about length limit on mailbox names ("email 5217 addresses"). 5219 * Added an "errata summary" subsection to this change log/ 5220 comparison to 5321 in this Appendix. The entire Appendix will, of 5221 course, disappear at the time of RFC publication unless someone 5222 wants to make a strong case for retaining it. 5224 * Rationalized CREFs to 2821, 5321, 5321bis etc.; added note to 5225 readers below the Abstract. 5227 * Temporarily added a "Note on Reading This Working Draft" after the 5228 Abstract. 5230 H.3. Changes Among Versions of Rfc5321bis 5232 H.3.1. Changes from draft-klensin-rfc5321bis-00 (posted 2012-12-02) to 5233 -01 5235 Substantively, these two versions differ only by suppression of the 5236 CREF and other discussion associated with the evolution from RFC 2821 5237 to RFC 5321. That change includes an update to the document's Note 5238 to Readers, the date, the file name, and the addition of this change 5239 log subsection. 5241 H.3.2. Changes from draft-klensin-rfc5321bis-01 (20191203) to -02 5243 * Minor clarifications to improve text, e.g., addition of NOOP to 5244 the list of non-mail transaction examples in Section 4.1.4. 5246 * Added topics exposed in the ietf-smtp list and the IETF list 5247 "dogfood" discussion during December 2019 and an index listing of 5248 substantive issues identified only in CREFs in the prior draft as 5249 a new Appendix G.. 5251 H.3.3. Changes from draft-klensin-rfc5321bis-02 (2019-12-27) to -03 5253 * Added more text to Appendix G.7.1 to specifically call out the 5254 session-opening policy issues surrounding these codes. 5256 * Added discussion of "1yz" reinstatement in Appendix G.7.11. 5258 * Added discussion of timeouts in Appendix G.7.12. 5260 * Added subsection on Enhanced Status Codes and DSNs to the 5261 outstanding issues list Appendix G.8. 5263 * Replaced reference to RFC 1652 (8BITMIME) with the Internet 5264 Standard version, RFC 6152. 5266 * With help from cketti, clarified the ABNF productions whose 5267 terminals appear in other documents. 5269 * Added discussions of Quoted-string, Internationalization, and 5270 client-server versus sender-receiver terminology to Appendix G. 5272 * Added note to the Abstract. 5274 H.3.4. Changes from draft-klensin-rfc5321bis-03 (2020-07-02) to draft- 5275 ietf-emailcore-rfc5321bis-00 5277 * Added a paragraph about non-null quoted strings to Appendix G.9. 5279 * Added an explicit pointer to email insecurity and TLS to 5280 Appendix G.6. Inspired by Ben Kaduk's comment on the WG Charter, 5281 2020-09-09. 5283 * Converted document from individual to emailcore WG effort. 5285 H.3.5. Changes from draft-ietf-emailcore-rfc5321bis-00 (2020-10-06) to 5286 -01 5288 * Editorial: Corrected "blackslash" to "backslash" 5290 * Rewrote the introduction to Appendix G slightly to reflect the 5291 creation of the EMAILCORE WG. 5293 * Applied fixes for repeated use of EHLO. See Appendix G.2. 5295 * Added two new questions, one about "X" extensions (Appendix G.12) 5296 and one about the status of HELO (Appendix G.13). 5298 * Removed mention of SEND, SAML, SOML from the main body of the text 5299 (Ticket #20). 5301 * Added a warning about side effects to Appendix G.7.5. 5303 * Added ticket numbers to descriptions of issues and changes, 5304 adjusted some text so relationships would be more clear, and added 5305 subsections to the Appendix G and H lists to pick up on tickets 5306 that were not easily identified in those sections of with the 5307 text. 5309 * Made several additions to the Index, including one to deal with 5310 SEND et al., as above. 5312 H.3.6. Changes from draft-ietf-emailcore-rfc5321bis-01 (2020-12-25) to 5313 -02 5315 * Corrected discussion mailing list to point to emailcore@ietf.org 5316 in the introductory note. 5318 * Added new subsection(s) to Appendix G to reflect newly discovered 5319 issues. 5321 * Changed "as discussed in" references in Section 4.5.5 per ticket 5322 #45. 5324 * Corrected a misleading use of the term "mailbox" in Section 3.3. 5326 * Changed descriptions of use of first digit in replies per ticket 5327 #13. See Appendix G.7.7. 5329 * Moved paragraph per ticket #28, erratum 1851. 5331 * Added more clarifying cross-references, clarified some CREFs, and 5332 cleaned out some of those that no longer seemed relevant. 5334 * Removed "updates 1123" is unnecessary and obsolete. 5336 * Updated several references. 5338 H.3.7. Changes from draft-ietf-emailcore-rfc5321bis-02 (2021-02-21) to 5339 -03 5341 * Editorial: Fixed some instances of constructions like "RCPT TO 5342 command". The name of the command is RCPT. Sloppy editing in 5343 2008. 5345 * Added text and cross-references to clarify the role of 452 and 552 5346 in "too many recipients" situations. 5348 * Added Appendix G.15 to discuss changes to better reflect 5349 "operational necessity" issue. 5351 * Added detail for erratum 5711, ticket #29. 5353 * Added new subsections of Appendix G.7 to keep some previously- 5354 unnoted CREF notes from getting lost. Also removed some CREFs 5355 that were notes on changes made before the WG was created or 5356 appeared to no longer have value and trimmed or rewrote some of 5357 the remaining ones. 5359 * More discussion of Ticket #13, See Appendix G.7.7. 5361 * Identified Ticket #41 as closed. See Appendix Appendix G.7.3; 5362 notes removed from Section 2.3.5. 5364 * "SHOULD" requirement for interpreting 552 "too many recipients" 5365 removed from Section 4.5.3.1.10, explanation added, and text 5366 cleaned up. Also removed the parenthetical historical notes on 5367 the return code definitions in Section 4.2. See Appendix G.5. 5368 (Ticket #5) 5370 * Modified Appendix G.8 to add a note about the normative status of 5371 RFC 3463 and moved that reference. 5373 * Several clarifications to initiation and termination of mail 5374 transactions in Section 4.1.4. 5376 * Several additional minor editorial improvements. 5378 * Note for drafts -03 and -04 only, modified somewhat for -05: Some 5379 issues are still outstanding: Notes were posted to the list on 5380 2021-07-09 about tickets #7 (closed?), #10, #14 (closed), #20, 5381 #30, and #42. Even though some comments about them appeared in 5382 the subsequent day or so, there appears to have been insufficient 5383 time for discussions to stabilize sufficiently for changes to be 5384 included in this version of the I-D. 5386 H.3.8. Changes from draft-ietf-emailcore-rfc5321bis-03 (2021-07-10) to 5387 -04 5389 * Clarified that the "period" in . is really the ASCII 5390 one in Section 3.3. 5392 // Editor's note: change treated as Editorial without a ticket. 5393 If 5394 // there are objections, speak up. 5396 * Several other small editorial corrections. 5398 * Added several notes about the possible need to add text to reflect 5399 the presence of MSAs and to clarify whether MUAs send messages 5400 directly to MTAs or whether, in that case, the MUAs are just 5401 incorporating MSA functions. 5403 * Added new text to Appendix G.14 reflecting discussions of the 5404 Received...FOR issue. 5406 * Adjusted discussion of erratum 4315 (Ticket #27) to reflect more 5407 recent IPv6 syntax developments. 5409 * Adjusted discussion of the various "mail not accepted" codes, 5410 rewrote Section 4.2.4.2, annotated and inserted cross-references 5411 in relevant response code descriptions and (tentatively) 5412 identified this document as obsoleting RFC 7505. Editor's guess, 5413 reinforced by a brief conversation with John Levine (lead author 5414 of 7505), is that we should incorporate text as needed and 5415 obsolete it. The changes include replacing the reference to the 5416 "nullMX" I-D with RFC 7505, which I am appalled that neither I nor 5417 anyone else noticed earlier. Cf. Appendix G.7.1, Section 4.2.4.2, 5418 and Ticket #6. 5420 H.3.9. Changes from draft-ietf-emailcore-rfc5321bis-04 (2021-10-03) to 5421 -05 5423 * Took a first step toward rewriting and updating the introductory 5424 material. It is only a first step; suggestions welcome. 5426 * Minor editorial fixes. 5428 * Correct text about domain name checking in Section 4.1.4, probably 5429 fixing ticket #19. See CREF added there. 5431 * Added Appendix G.16 a placeholder for the 8BITMIME discussion and 5432 possible action. 5434 * Additional changes to the description and organization of trace 5435 field materials. Intended to resolve the 5321bis part of Ticket 5436 #7. 5438 * Remaining patch to SEND, etc., discussion in Appendix F.6 applied 5439 and CREF removed. 5441 * Removed discussion of "X-" and edited associated text. The fix 5442 may or may not be sufficient to resolve Ticket #42. 5444 * Verified that the problems of getting four-level sections (e.g., 5445 "4.1.1.1" and other command-specific ones) into the table of 5446 contents and the index reflecting page numbers still exist and 5447 updated the introductory note. 5449 H.3.10. Changes from draft-ietf-emailcore-rfc5321bis-05 (2021-10-24) to 5450 -06 5452 * Finished making changes for "X-" and commands starting in "X". 5453 Changes made in -05 were incomplete. This should allow closing 5454 Ticket #42. 5456 * Removed spurious "for use in delivery notifications" from 3.6.2. 5457 Was just a pasting-type error. 5459 * Changed "In other words" to "In particular" in Section 2.3.5 per 5460 Ticket #10 and July 2021 mailing list discussion. Removed 5461 associated CREF. 5463 * Converted to xml2rfc v3 (thanks to John Levine for doing the hard 5464 parts) and then modified the introductory note accordingly. 5466 * Started reworking the Abstract -- see revised CREF there. 5468 * Rewrote Section 2.3.3 slightly to note the existence of submission 5469 servers and removed the CREF. 5471 * Updated Appendix G.7.17 and slightly modified CREF note in 5472 Section 2 -- proposed to not get 5321bis involved with this 5473 (Ticket #50). 5475 * Rewrote parts of Section 3.9 to clarify text amd respond to Ticket 5476 #34. 5478 * Inserted suggested text info CREF at end of Section 1.2. Comments 5479 welcome. Soon. 5481 Index 5483 A C 5485 A 5486 Argument Syntax 5487 A-d-l Section 4.1.2 5488 ALPHA Section 4.1.2, Paragraph 2, Item 1 5489 Additional-Registered-Clauses Section 4.4.1 5490 Addtl-Link Section 4.4.1 5491 Addtl-Protocol Section 4.4.1 5492 Argument Section 4.1.2 5493 At-domain Section 4.1.2 5494 Atom Section 4.1.2 5495 By-domain Section 4.4.1 5496 CFWS Section 4.1.2, Paragraph 2, Item 2 5497 CRLF Section 4.1.2, Paragraph 2, Item 1 5498 DIGIT Section 4.1.2, Paragraph 2, Item 1 5499 Domain Section 4.1.2 5500 Dot-string Section 4.1.2 5501 Extended-Domain Section 4.4.1 5502 FWS Section 4.1.2, Paragraph 2, Item 2 5503 For Section 4.4.1 5504 Forward-Path Section 4.1.2 5505 From-domain Section 4.4.1 5506 General-address-literal Section 4.1.3 5507 Greeting Section 4.2 5508 HEXDIG Section 4.1.2, Paragraph 2, Item 1 5509 ID Section 4.4.1 5510 IPv4-address-literal Section 4.1.3 5511 IPv6-addr Section 4.1.3 5512 IPv6-address-literal Section 4.1.3 5513 Keyword Section 4.1.2 5514 Ldh-str Section 4.1.2 5515 Let-dig Section 4.1.2 5516 Link Section 4.4.1 5517 Local-part Section 4.1.2 5518 Mail-parameters Section 4.1.2 5519 Mailbox Section 4.1.2 5520 Opt-info Section 4.4.1 5521 Path Section 4.1.2 5522 Protocol Section 4.4.1 5523 QcontentSMTP Section 4.1.2 5524 Quoted-string Section 4.1.2 5525 Rcpt-parameters Section 4.1.2 5526 Reply-code Section 4.2 5527 Reply-line Section 4.2 5528 Return-path-line Section 4.4.1 5529 Reverse-Path Section 4.1.2 5530 SP Section 4.1.2, Paragraph 2, Item 1 5531 Snum Section 4.1.3 5532 Stamp Section 4.4.1 5533 Standardized-tag Section 4.1.3 5534 String Section 4.1.2 5535 TCP-info Section 4.4.1 5536 Time-stamp-line Section 4.4.1 5537 Via Section 4.4.1 5538 With Section 4.4.1 5539 address-literal Section 4.1.2 5540 atext Section 4.1.2, Paragraph 2, Item 2 5541 dcontent Section 4.1.3 5542 esmtp-keyword Section 4.1.2 5543 esmtp-param Section 4.1.2 5544 esmtp-value Section 4.1.2 5545 h16 Section 4.1.3 5546 ls32 Section 4.1.3 5547 qtextSMTP Section 4.1.2 5548 quoted-pairSMTP Section 4.1.2 5549 sub-domain Section 4.1.2 5550 textstring Section 4.2 5552 C 5554 Command Syntax 5555 data Section 4.1.1.4, Paragraph 8, Item 1 5556 ehlo Section 3.2, Paragraph 1; Section 4.1.1.1, Paragraph 1 5557 expn Section 4.1.1.7, Paragraph 4, Item 1 5558 helo Section 4.1.1.1, Paragraph 1 5559 help Section 4.1.1.8, Paragraph 5, Item 1 5560 mail Section 4.1.1.2 5561 noop Section 4.1.1.9, Paragraph 4, Item 1 5562 quit Section 4.1.1.10, Paragraph 5, Item 1 5563 rcpt Section 4.1.1.3, Paragraph 17 5564 rset Section 4.1.1.5, Paragraph 4, Item 1 5565 send, saml, soml Appendix G.7.13, Paragraph 1 5566 vrfy Section 4.1.1.6, Paragraph 4, Item 1 5568 Author's Address 5570 John C. Klensin 5571 1770 Massachusetts Ave, Suite 322 5572 Cambridge, MA 02140 5573 United States of America 5575 Email: john-ietf@jck.com