idnits 2.17.00 (12 Aug 2021) /tmp/idnits52157/draft-ietf-emailcore-rfc5321bis-04.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 RFC5321, but the abstract doesn't seem to directly say this. It does mention RFC5321 though, so this could be OK. -- 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 == Line 270 has weird spacing: '...es from draf...' -- The document date (October 3, 2021) is 229 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 3113 -- Looks like a reference, but probably isn't: 'JcK 20210904' on line 2650 == Unused Reference: '41' is defined on line 4176, but no explicit reference was found in the text == Unused Reference: '43' is defined on line 4185, but no explicit reference was found in the text == Unused Reference: '44' is defined on line 4189, 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) Summary: 2 errors (**), 0 flaws (~~), 6 warnings (==), 14 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 EMAILCORE J. Klensin 3 Internet-Draft October 3, 2021 4 Obsoletes: 5321, 1846, 7504, 7505 (if 5 approved) 6 Intended status: Standards Track 7 Expires: April 6, 2022 9 Simple Mail Transfer Protocol 10 draft-ietf-emailcore-rfc5321bis-04 12 Abstract 14 This document is a specification of the basic protocol for Internet 15 electronic mail transport. It consolidates, updates, and clarifies 16 several previous documents, making all or parts of most of them 17 obsolete. It covers the SMTP extension mechanisms and best practices 18 for the contemporary Internet, but does not provide details about 19 particular extensions. Although SMTP was designed as a mail 20 transport and delivery protocol, this specification also contains 21 information that is important to its use as a "mail submission" 22 protocol for "split-UA" (User Agent) mail reading systems and mobile 23 environments. This document replaces the earlier version with the 24 same title in RFC 5321. 25 [[CREF1: Note in Draft: Except for the last sentence, the above is 26 unchanged from 5321 and may need adjusting in the light of RFC 6409 27 (Message Submission) as an Internet Standard.]] 29 Notes on Reading This Working Draft 31 This working draft is extensively annotated with information about 32 changes made over the decade since RFC 5321 appeared, especially when 33 those changes might be controversial or should get careful review. 34 Anything marked in CREF comments with "[5321bis]" is current. In 35 general, unless those are marked with "[[Note in Draft", in the 36 contents of an "Editor's note", or are in the "Errata Summary" 37 appendix (Appendix H.1, they are just notes on changes that have 38 already been made and where those changes originated. As one can 39 tell from the dates (when they are given), this document has been 40 periodically updated over a very long period of time. 42 As people review or try to use this document, it may be worth paying 43 special attention to the historical discussion in Section 1.2. The 44 decision to merge documents rather than do a complete rewrite was 45 motivated by weighing the risks of inadvertently introducing changes 46 against greater readability and deciding to preserve close 47 approximations to original text and document structures in most 48 cases. One result is that information may not be be organized as the 49 reader might expect. An index is provided to assist in the quest for 50 information. 52 This evolving draft should be discussed on the emailcore@ietf.org 53 list. 55 Technology Note: The table of contents would be much easier to use, 56 especially for locating commands, if the subsections containing the 57 commands were listed. The source XML is marked up with "toc=include" 58 attributes to facilitate that. Unfortunately, there is apparently a 59 bug in the current version of xml2rfc v2, one that also appeared in 60 the version used ot generate RFC 5321, that prevents those 61 subsections from appearing in the TOC. The command names can now be 62 found in the index, but the index is to page numbers, not section 63 numbers. Both problems have been reported. 65 Status of This Memo 67 This Internet-Draft is submitted in full conformance with the 68 provisions of BCP 78 and BCP 79. 70 Internet-Drafts are working documents of the Internet Engineering 71 Task Force (IETF). Note that other groups may also distribute 72 working documents as Internet-Drafts. The list of current Internet- 73 Drafts is at https://datatracker.ietf.org/drafts/current/. 75 Internet-Drafts are draft documents valid for a maximum of six months 76 and may be updated, replaced, or obsoleted by other documents at any 77 time. It is inappropriate to use Internet-Drafts as reference 78 material or to cite them other than as "work in progress." 80 This Internet-Draft will expire on April 6, 2022. 82 Copyright Notice 84 Copyright (c) 2021 IETF Trust and the persons identified as the 85 document authors. All rights reserved. 87 This document is subject to BCP 78 and the IETF Trust's Legal 88 Provisions Relating to IETF Documents 89 (https://trustee.ietf.org/license-info) in effect on the date of 90 publication of this document. Please review these documents 91 carefully, as they describe your rights and restrictions with respect 92 to this document. Code Components extracted from this document must 93 include Simplified BSD License text as described in Section 4.e of 94 the Trust Legal Provisions and are provided without warranty as 95 described in the Simplified BSD License. 97 Table of Contents 99 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 6 100 1.1. Transport of Electronic Mail . . . . . . . . . . . . . . 6 101 1.2. History and Context for This Document . . . . . . . . . . 7 102 1.3. Document Conventions . . . . . . . . . . . . . . . . . . 8 103 2. The SMTP Model . . . . . . . . . . . . . . . . . . . . . . . 8 104 2.1. Basic Structure . . . . . . . . . . . . . . . . . . . . . 9 105 2.2. The Extension Model . . . . . . . . . . . . . . . . . . . 11 106 2.2.1. Background . . . . . . . . . . . . . . . . . . . . . 11 107 2.2.2. Definition and Registration of Extensions . . . . . . 12 108 2.2.3. Special Issues with Extensions . . . . . . . . . . . 13 109 2.3. SMTP Terminology . . . . . . . . . . . . . . . . . . . . 13 110 2.3.1. Mail Objects . . . . . . . . . . . . . . . . . . . . 13 111 2.3.2. Senders and Receivers . . . . . . . . . . . . . . . . 14 112 2.3.3. Mail Agents and Message Stores . . . . . . . . . . . 14 113 2.3.4. Host . . . . . . . . . . . . . . . . . . . . . . . . 14 114 2.3.5. Domain Names . . . . . . . . . . . . . . . . . . . . 15 115 2.3.6. Buffer and State Table . . . . . . . . . . . . . . . 16 116 2.3.7. Commands and Replies . . . . . . . . . . . . . . . . 16 117 2.3.8. Lines . . . . . . . . . . . . . . . . . . . . . . . . 16 118 2.3.9. Message Content and Mail Data . . . . . . . . . . . . 16 119 2.3.10. Originator, Delivery, Relay, and Gateway Systems . . 17 120 2.3.11. Mailbox and Address . . . . . . . . . . . . . . . . . 17 121 2.4. General Syntax Principles and Transaction Model . . . . . 18 122 3. The SMTP Procedures: An Overview . . . . . . . . . . . . . . 19 123 3.1. Session Initiation . . . . . . . . . . . . . . . . . . . 20 124 3.2. Client Initiation . . . . . . . . . . . . . . . . . . . . 20 125 3.3. Mail Transactions . . . . . . . . . . . . . . . . . . . . 21 126 3.4. Forwarding for Address Correction or Updating . . . . . . 24 127 3.5. Commands for Debugging Addresses . . . . . . . . . . . . 24 128 3.5.1. Overview . . . . . . . . . . . . . . . . . . . . . . 25 129 3.5.2. VRFY Normal Response . . . . . . . . . . . . . . . . 27 130 3.5.3. Meaning of VRFY or EXPN Success Response . . . . . . 27 131 3.5.4. Semantics and Applications of EXPN . . . . . . . . . 28 132 3.6. Relaying and Mail Routing . . . . . . . . . . . . . . . . 28 133 3.6.1. Source Routes and Relaying . . . . . . . . . . . . . 28 134 3.6.2. Mail eXchange Records and Relaying . . . . . . . . . 29 135 3.6.3. Message Submission Servers as Relays . . . . . . . . 29 136 3.7. Mail Gatewaying . . . . . . . . . . . . . . . . . . . . . 30 137 3.7.1. Header Fields in Gatewaying . . . . . . . . . . . . . 30 138 3.7.2. Received Lines in Gatewaying . . . . . . . . . . . . 31 139 3.7.3. Addresses in Gatewaying . . . . . . . . . . . . . . . 31 140 3.7.4. Other Header Fields in Gatewaying . . . . . . . . . . 31 141 3.7.5. Envelopes in Gatewaying . . . . . . . . . . . . . . . 32 142 3.8. Terminating Sessions and Connections . . . . . . . . . . 32 143 3.9. Mailing Lists and Aliases . . . . . . . . . . . . . . . . 33 144 3.9.1. Alias . . . . . . . . . . . . . . . . . . . . . . . . 33 145 3.9.2. List . . . . . . . . . . . . . . . . . . . . . . . . 34 146 4. The SMTP Specifications . . . . . . . . . . . . . . . . . . . 34 147 4.1. SMTP Commands . . . . . . . . . . . . . . . . . . . . . . 34 148 4.1.1. Command Semantics and Syntax . . . . . . . . . . . . 34 149 4.1.2. Command Argument Syntax . . . . . . . . . . . . . . . 42 150 4.1.3. Address Literals . . . . . . . . . . . . . . . . . . 45 151 4.1.4. Order of Commands . . . . . . . . . . . . . . . . . . 46 152 4.1.5. Private-Use Commands . . . . . . . . . . . . . . . . 48 153 4.2. SMTP Replies . . . . . . . . . . . . . . . . . . . . . . 48 154 4.2.1. Reply Code Severities and Theory . . . . . . . . . . 50 155 4.2.2. Reply Codes by Function Groups . . . . . . . . . . . 52 156 4.2.3. Reply Codes in Numeric Order . . . . . . . . . . . . 54 157 4.2.4. Some specific code situations and relationships . . . 55 158 4.3. Sequencing of Commands and Replies . . . . . . . . . . . 57 159 4.3.1. Sequencing Overview . . . . . . . . . . . . . . . . . 57 160 4.3.2. Command-Reply Sequences . . . . . . . . . . . . . . . 58 161 4.4. Trace Information . . . . . . . . . . . . . . . . . . . . 60 162 4.5. Additional Implementation Issues . . . . . . . . . . . . 64 163 4.5.1. Minimum Implementation . . . . . . . . . . . . . . . 64 164 4.5.2. Transparency . . . . . . . . . . . . . . . . . . . . 65 165 4.5.3. Sizes and Timeouts . . . . . . . . . . . . . . . . . 66 166 4.5.4. Retry Strategies . . . . . . . . . . . . . . . . . . 70 167 4.5.5. Messages with a Null Reverse-Path . . . . . . . . . . 72 168 5. Address Resolution and Mail Handling . . . . . . . . . . . . 72 169 5.1. Locating the Target Host . . . . . . . . . . . . . . . . 72 170 5.2. IPv6 and MX Records . . . . . . . . . . . . . . . . . . . 75 171 6. Problem Detection and Handling . . . . . . . . . . . . . . . 75 172 6.1. Reliable Delivery and Replies by Email . . . . . . . . . 75 173 6.2. Unwanted, Unsolicited, and "Attack" Messages . . . . . . 76 174 6.3. Loop Detection . . . . . . . . . . . . . . . . . . . . . 77 175 6.4. Compensating for Irregularities . . . . . . . . . . . . . 77 176 7. Security Considerations . . . . . . . . . . . . . . . . . . . 78 177 7.1. Mail Security and Spoofing . . . . . . . . . . . . . . . 79 178 7.2. "Blind" Copies . . . . . . . . . . . . . . . . . . . . . 80 179 7.3. VRFY, EXPN, and Security . . . . . . . . . . . . . . . . 80 180 7.4. Mail Rerouting Based on the 251 and 551 Response 181 Codes . . . . . . . . . . . . . . . . . . . . . . . . . . 81 182 7.5. Information Disclosure in Announcements . . . . . . . . . 81 183 7.6. Information Disclosure in Trace Fields . . . . . . . . . 82 184 7.7. Information Disclosure in Message Forwarding . . . . . . 82 185 7.8. Local Operational Requirement and Resistance to Attacks . 82 186 7.9. Scope of Operation of SMTP Servers . . . . . . . . . . . 82 187 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 83 188 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 84 189 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 85 190 10.1. Normative References . . . . . . . . . . . . . . . . . . 85 191 10.2. Informative References . . . . . . . . . . . . . . . . . 86 192 Appendix A. TCP Transport Service . . . . . . . . . . . . . . . 91 193 Appendix B. Generating SMTP Commands from RFC 822 Header Fields 91 194 Appendix C. Source Routes . . . . . . . . . . . . . . . . . . . 92 195 Appendix D. Scenarios . . . . . . . . . . . . . . . . . . . . . 93 196 D.1. A Typical SMTP Transaction Scenario . . . . . . . . . . . 93 197 D.2. Aborted SMTP Transaction Scenario . . . . . . . . . . . . 94 198 D.3. Relayed Mail Scenario . . . . . . . . . . . . . . . . . . 95 199 D.4. Verifying and Sending Scenario . . . . . . . . . . . . . 96 200 Appendix E. Other Gateway Issues . . . . . . . . . . . . . . . . 97 201 Appendix F. Deprecated Features of RFC 821 . . . . . . . . . . . 97 202 F.1. TURN . . . . . . . . . . . . . . . . . . . . . . . . . . 97 203 F.2. Source Routing . . . . . . . . . . . . . . . . . . . . . 97 204 F.3. HELO . . . . . . . . . . . . . . . . . . . . . . . . . . 98 205 F.4. #-literals . . . . . . . . . . . . . . . . . . . . . . . 98 206 F.5. Dates and Years . . . . . . . . . . . . . . . . . . . . . 98 207 F.6. Sending versus Mailing . . . . . . . . . . . . . . . . . 98 208 Appendix G. Other Outstanding Issues . . . . . . . . . . . . . . 99 209 G.1. IP Address literals . . . . . . . . . . . . . . . . . . . 100 210 G.2. Repeated Use of EHLO . . . . . . . . . . . . . . . . . . 100 211 G.3. Meaning of "MTA" and Related Terminology . . . . . . . . 100 212 G.4. Originator, or Originating System, Authentication . . . . 101 213 G.5. Remove or deprecate the work-around from code 552 to 452 101 214 G.6. Clarify where the protocol stands with respect to 215 submission and TLS issues . . . . . . . . . . . . . . . . 101 216 G.7. Probably-substantive Discussion Topics Identified in 217 Other Ways . . . . . . . . . . . . . . . . . . . . . . . 101 218 G.7.1. Issues with 521, 554, and 556 codes . . . . . . . . . 101 219 G.7.2. SMTP Model, terminology, and relationship to RFC 5598 102 220 G.7.3. Resolvable FQDNs and private domain names . . . . . . 102 221 G.7.4. Possible clarification about mail transactions and 222 transaction state . . . . . . . . . . . . . . . . . . 102 223 G.7.5. Issues with mailing lists, aliases, and forwarding . 102 224 G.7.6. Requirements for domain name and/or IP address in 225 EHLO . . . . . . . . . . . . . . . . . . . . . . . . 102 226 G.7.7. Does the 'first digit only' and/or non-listed reply 227 code text need clarification? . . . . . . . . . . . . 102 228 G.7.8. Size limits . . . . . . . . . . . . . . . . . . . . . 103 229 G.7.9. Discussion of 'blind' copies and RCPT . . . . . . . . 103 230 G.7.10. Further clarifications needed to source routes? . . . 103 231 G.7.11. Should 1yz Be Revisited? . . . . . . . . . . . . . . 103 232 G.7.12. Review Timeout Specifications . . . . . . . . . . . . 103 233 G.7.13. Possible SEND, SAML, SOML Loose End . . . . . . . . . 104 234 G.7.14. Abstract Update . . . . . . . . . . . . . . . . . . . 104 235 G.7.15. Informative References to MIME and/or Message 236 Submission . . . . . . . . . . . . . . . . . . . . . 104 237 G.7.16. Mail Transaction Discussion . . . . . . . . . . . . . 104 238 G.7.17. Hop-by-hop Authentication and/or Encryption . . . . . 104 239 G.7.18. More Text About 554 Given 521, etc. . . . . . . . . . 104 240 G.7.19. Minimum Lengths and Quantities . . . . . . . . . . . 104 242 G.8. Enhanced Reply Codes and DSNs . . . . . . . . . . . . . . 104 243 G.9. Revisiting Quoted Strings . . . . . . . . . . . . . . . . 105 244 G.10. Internationalization . . . . . . . . . . . . . . . . . . 105 245 G.11. SMTP Clients, Servers, Senders, and Receivers . . . . . . 106 246 G.12. Extension Keywords Starting in 'X-' . . . . . . . . . . . 106 247 G.13. Deprecating HELO . . . . . . . . . . . . . . . . . . . . 106 248 G.14. The FOR Clause in Trace Fields: Semantics, Security 249 Considerations, and Other Issues . . . . . . . . . . . . 107 250 G.15. Resistance to Attacks and Operational Necessity . . . . . 108 251 Appendix H. RFC 5321 Errata Summary and Tentative Change Log . . 108 252 H.1. RFC 5321 Errata Summary . . . . . . . . . . . . . . . . . 108 253 H.2. Changes from RFC 5321 (published October 2008) to the 254 initial (-00) version of this draft . . . . . . . . . . . 110 255 H.3. Changes Among Versions of Rfc5321bis . . . . . . . . . . 111 256 H.3.1. Changes from draft-klensin-rfc5321bis-00 (posted 257 2012-12-02) to -01 . . . . . . . . . . . . . . . . . 111 258 H.3.2. Changes from draft-klensin-rfc5321bis-01 (20191203) 259 to -02 . . . . . . . . . . . . . . . . . . . . . . . 111 260 H.3.3. Changes from draft-klensin-rfc5321bis-02 (2019-12-27) 261 to -03 . . . . . . . . . . . . . . . . . . . . . . . 111 262 H.3.4. Changes from draft-klensin-rfc5321bis-03 (2020-07-02) 263 to draft-ietf-emailcore-rfc5321bis-00 . . . . . . . . 112 264 H.3.5. Changes from draft-ietf-emailcore-rfc5321bis-00 265 (2020-10-06) to -01 . . . . . . . . . . . . . . . . . 112 266 H.3.6. Changes from draft-ietf-emailcore-rfc5321bis-01 267 (2020-12-25) to -02 . . . . . . . . . . . . . . . . . 113 268 H.3.7. Changes from draft-ietf-emailcore-rfc5321bis-02 269 (2021-02-21) to -03 . . . . . . . . . . . . . . . . . 113 270 H.3.8. Changes from draft-ietf-emailcore-rfc5321bis-03 271 (2021-07-10) to -04 . . . . . . . . . . . . . . . . . 114 272 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 273 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 117 275 1. Introduction 277 1.1. Transport of Electronic Mail 279 The objective of the Simple Mail Transfer Protocol (SMTP) is to 280 transfer mail reliably and efficiently. 282 SMTP is independent of the particular transmission subsystem and 283 requires only a reliable ordered data stream channel. While this 284 document specifically discusses transport over TCP, other transports 285 are possible. Appendices to RFC 821 [3] describe some of them. 287 An important feature of SMTP is its capability to transport mail 288 across multiple networks, usually referred to as "SMTP mail relaying" 289 (see Section 3.6). A network consists of the mutually-TCP-accessible 290 hosts on the public Internet, the mutually-TCP-accessible hosts on a 291 firewall-isolated TCP/IP Intranet, or hosts in some other LAN or WAN 292 environment utilizing a non-TCP transport-level protocol. Using 293 SMTP, a process can transfer mail to another process on the same 294 network or to some other network via a relay or gateway process 295 accessible to both networks. 297 In this way, a mail message may pass through a number of intermediate 298 relay or gateway hosts on its path from sender to ultimate recipient. 299 The Mail eXchanger mechanisms of the domain name system (RFC 1035 300 [4], RFC 974 [16], and Section 5 of this document) are used to 301 identify the appropriate next-hop destination for a message being 302 transported. 304 1.2. History and Context for This Document 306 This document is a specification of the basic protocol for the 307 Internet electronic mail transport. It consolidates, updates and 308 clarifies, but does not add new or change existing functionality of 309 the following: 311 o the original SMTP (Simple Mail Transfer Protocol) specification of 312 RFC 821 [3], 314 o domain name system requirements and implications for mail 315 transport from RFC 1035 [4] and RFC 974 [16], 317 o the clarifications and applicability statements in RFC 1123 [5], 319 o the new error codes added by RFC 1846 [20] and later by RFC 7504 320 [48], obsoleting both of those documents, and 322 o material drawn from the SMTP Extension mechanisms in RFC 1869 323 [22]. 325 o Editorial and clarification changes to RFC 2821 [30] to bring that 326 specification to Draft Standard. 328 It obsoletes RFC 821, RFC 974, RFC 1869, and RFC 2821 and updates RFC 329 1123 (replacing the mail transport materials of RFC 1123). However, 330 RFC 821 specifies some features that were not in significant use in 331 the Internet by the mid-1990s and (in appendices) some additional 332 transport models. Those sections are omitted here in the interest of 333 clarity and brevity; readers needing them should refer to RFC 821. 335 It also includes some additional material from RFC 1123 that required 336 amplification. This material has been identified in multiple ways, 337 mostly by tracking flaming on various lists and newsgroups and 338 problems of unusual readings or interpretations that have appeared as 339 the SMTP extensions have been deployed. Where this specification 340 moves beyond consolidation and actually differs from earlier 341 documents, it supersedes them technically as well as textually. 343 [[CREF2: JcK: 202107219: does the text that follows need rewriting? 344 See comment in Abstract. ]] 345 Although SMTP was designed as a mail transport and delivery protocol, 346 this specification also contains information that is important to its 347 use as a "mail submission" protocol, as recommended for Post Office 348 Protocol (POP) (RFC 937 [14], RFC 1939 [23]) and IMAP (RFC 3501 349 [36]). In general, the separate mail submission protocol specified 350 in RFC 6409 [42] is now preferred to direct use of SMTP; more 351 discussion of that subject appears in that document. 353 Section 2.3 provides definitions of terms specific to this document. 354 Except when the historical terminology is necessary for clarity, this 355 document uses the current 'client' and 'server' terminology to 356 identify the sending and receiving SMTP processes, respectively. 358 A companion document, RFC 5322 [12], discusses message header 359 sections and bodies and specifies formats and structures for them. 360 [[CREF3: [rfc5321bis 20210317] Would this be an appropriate place to 361 mention RFC 2045 (MIME) and/or RFC 6409 (Message Submission)?]] 363 1.3. Document Conventions 365 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 366 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 367 document are to be interpreted as described in RFC 2119 [1]. As each 368 of these terms was intentionally and carefully chosen to improve the 369 interoperability of email, each use of these terms is to be treated 370 as a conformance requirement. 372 Because this document has a long history and to avoid the risk of 373 various errors and of confusing readers and documents that point to 374 this one, most examples and the domain names they contain are 375 preserved from RFC 2821. Readers are cautioned that these are 376 illustrative examples that should not actually be used in either code 377 or configuration files. 379 2. The SMTP Model 381 [[CREF4: [5321bis] [[Editor's Note: There have been extensive and 382 repeated discussions on the SMTP and IETF lists about whether this 383 document should say something about hop-by-hop (MTA-to-MTA) SMTP 384 authentication and, if so, what?? Note that end to end message 385 authentication is almost certainly out of scope for SMTP.]]]] 387 2.1. Basic Structure 389 The SMTP design can be pictured as: 391 +----------+ +----------+ 392 +------+ | | | | 393 | User |<-->| | SMTP | | 394 +------+ | Client- |Commands/Replies| Server- | 395 +------+ | SMTP |<-------------->| SMTP | +------+ 396 | File |<-->| | and Mail | |<-->| File | 397 |System| | | | | |System| 398 +------+ +----------+ +----------+ +------+ 399 SMTP client SMTP server 401 When an SMTP client has a message to transmit, it establishes a two- 402 way transmission channel to an SMTP server. The responsibility of an 403 SMTP client is to transfer mail messages to one or more SMTP servers, 404 or report its failure to do so. 406 The means by which a mail message is presented to an SMTP client, and 407 how that client determines the identifier(s) ("names") of the 408 domain(s) to which mail messages are to be transferred, are local 409 matters. They are not addressed by this document. In some cases, 410 the designated domain(s), or those determined by an SMTP client, will 411 identify the final destination(s) of the mail message. In other 412 cases, common with SMTP clients associated with implementations of 413 the POP (RFC 937 [14], RFC 1939 [23]) or IMAP (RFC 3501 [36]) 414 protocols, or when the SMTP client is inside an isolated transport 415 service environment, the domain determined will identify an 416 intermediate destination through which all mail messages are to be 417 relayed. SMTP clients that transfer all traffic regardless of the 418 target domains associated with the individual messages, or that do 419 not maintain queues for retrying message transmissions that initially 420 cannot be completed, may otherwise conform to this specification but 421 are not considered fully-capable. Fully-capable SMTP 422 implementations, including the relays used by these less capable 423 ones, and their destinations, are expected to support all of the 424 queuing, retrying, and alternate address functions discussed in this 425 specification. In many situations and configurations, the less- 426 capable clients discussed above SHOULD be using the message 427 submission protocol (RFC 6409 [42]) rather than SMTP. 429 The means by which an SMTP client, once it has determined a target 430 domain, determines the identity of an SMTP server to which a copy of 431 a message is to be transferred, and then performs that transfer, are 432 covered by this document. To effect a mail transfer to an SMTP 433 server, an SMTP client establishes a two-way transmission channel to 434 that SMTP server. An SMTP client determines the address of an 435 appropriate host running an SMTP server by resolving a destination 436 domain name to either an intermediate Mail eXchanger host or a final 437 target host. 439 An SMTP server may be either the ultimate destination or an 440 intermediate "relay" (that is, it may assume the role of an SMTP 441 client after receiving the message) or "gateway" (that is, it may 442 transport the message further using some protocol other than SMTP). 443 SMTP commands are generated by the SMTP client and sent to the SMTP 444 server. SMTP replies are sent from the SMTP server to the SMTP 445 client in response to the commands. 447 In other words, message transfer can occur in a single connection 448 between the original SMTP-sender and the final SMTP-recipient, or can 449 occur in a series of hops through intermediary systems. In either 450 case, once the server has issued a success response at the end of the 451 mail data, a formal handoff of responsibility for the message occurs: 452 the protocol requires that a server MUST accept responsibility for 453 either delivering the message or properly reporting the failure to do 454 so (see Sections 6.1, 6.2, and 7.8, below). 456 Once the transmission channel is established and initial handshaking 457 is completed, the SMTP client normally initiates a mail transaction. 458 Such a transaction consists of a series of commands to specify the 459 originator and destination of the mail and transmission of the 460 message content (including any lines in the header section or other 461 structure) itself. When the same message is sent to multiple 462 recipients, this protocol encourages the transmission of only one 463 copy of the data for all recipients at the same destination (or 464 intermediate relay) host. 466 The server responds to each command with a reply; replies may 467 indicate that the command was accepted, that additional commands are 468 expected, or that a temporary or permanent error condition exists. 469 Commands specifying the sender or recipients may include server- 470 permitted SMTP service extension requests, as discussed in 471 Section 2.2. The dialog is purposely lock-step, one-at-a-time, 472 although this can be modified by mutually agreed upon extension 473 requests such as command pipelining (RFC 2920 [31]). 475 Once a given mail message has been transmitted, the client may either 476 request that the connection be shut down or may initiate other mail 477 transactions. In addition, an SMTP client may use a connection to an 478 SMTP server for ancillary services such as verification of email 479 addresses or retrieval of mailing list subscriber addresses. 481 As suggested above, this protocol provides mechanisms for the 482 transmission of mail. Historically, this transmission normally 483 occurred directly from the sending user's host to the receiving 484 user's host when the two hosts are connected to the same transport 485 service. When they are not connected to the same transport service, 486 transmission occurs via one or more relay SMTP servers. A very 487 common case in the Internet today involves submission of the original 488 message to an intermediate, "message submission" server, which is 489 similar to a relay but has some additional properties; such servers 490 are discussed in Section 2.3.10 and at some length in RFC 6409 [42]. 491 An intermediate host that acts as either an SMTP relay or as a 492 gateway into some other transmission environment is usually selected 493 through the use of the domain name service (DNS) Mail eXchanger 494 mechanism. Explicit "source" routing (see Section 5 and Appendix C 495 and Appendix F.2) SHOULD NOT be used. 497 2.2. The Extension Model 499 2.2.1. Background 501 In an effort that started in 1990, approximately a decade after RFC 502 821 was completed, the protocol was modified with a "service 503 extensions" model that permits the client and server to agree to 504 utilize shared functionality beyond the original SMTP requirements. 505 The SMTP extension mechanism defines a means whereby an extended SMTP 506 client and server may recognize each other, and the server can inform 507 the client as to the service extensions that it supports. 509 Contemporary SMTP implementations MUST support the basic extension 510 mechanisms. For instance, servers MUST support the EHLO command even 511 if they do not implement any specific extensions and clients SHOULD 512 preferentially utilize EHLO rather than HELO. (However, for 513 compatibility with older conforming implementations, SMTP clients and 514 servers MUST support the original HELO mechanisms as a fallback.) 515 Unless the different characteristics of HELO must be identified for 516 interoperability purposes, this document discusses only EHLO. 518 SMTP is widely deployed and high-quality implementations have proven 519 to be very robust. However, the Internet community now considers 520 some services to be important that were not anticipated when the 521 protocol was first designed. If support for those services is to be 522 added, it must be done in a way that permits older implementations to 523 continue working acceptably. The extension framework consists of: 525 o The SMTP command EHLO, superseding the earlier HELO, 527 o a registry of SMTP service extensions, 529 o additional parameters to the SMTP MAIL and RCPT commands, and 530 o optional replacements for commands defined in this protocol, such 531 as for DATA in non-ASCII transmissions (RFC 3030 [33]). 533 SMTP's strength comes primarily from its simplicity. Experience with 534 many protocols has shown that protocols with few options tend towards 535 ubiquity, whereas protocols with many options tend towards obscurity. 537 Each and every extension, regardless of its benefits, must be 538 carefully scrutinized with respect to its implementation, deployment, 539 and interoperability costs. In many cases, the cost of extending the 540 SMTP service will likely outweigh the benefit. 542 2.2.2. Definition and Registration of Extensions 544 The IANA maintains a registry of SMTP service extensions [53]. A 545 corresponding EHLO keyword value is associated with each extension. 546 Each service extension registered with the IANA must be defined in a 547 formal Standards-Track or IESG-approved Experimental protocol 548 document. The definition must include: 550 o the textual name of the SMTP service extension; 552 o the EHLO keyword value associated with the extension; 554 o the syntax and possible values of parameters associated with the 555 EHLO keyword value; 557 o any additional SMTP verbs associated with the extension 558 (additional verbs will usually be, but are not required to be, the 559 same as the EHLO keyword value); 561 o any new parameters the extension associates with the MAIL or RCPT 562 verbs; 564 o a description of how support for the extension affects the 565 behavior of a server and client SMTP; and 567 o the increment by which the extension is increasing the maximum 568 length of the commands MAIL and/or RCPT, over that specified in 569 this Standard. 571 In addition, any EHLO keyword value starting with an upper or lower 572 case "X" refers to a local SMTP service extension used exclusively 573 through bilateral agreement. Keywords beginning with "X" MUST NOT be 574 used in a registered service extension. Conversely, keyword values 575 presented in the EHLO response that do not begin with "X" MUST 576 correspond to a Standard, Standards-Track, or IESG-approved 577 Experimental SMTP service extension registered with IANA. A 578 conforming server MUST NOT offer non-"X"-prefixed keyword values that 579 are not described in a registered extension. 581 Additional verbs and parameter names are bound by the same rules as 582 EHLO keywords; specifically, verbs beginning with "X" are local 583 extensions that may not be registered or standardized. Conversely, 584 verbs not beginning with "X" must always be registered. 586 2.2.3. Special Issues with Extensions 588 Extensions that change fairly basic properties of SMTP operation are 589 permitted. The text in other sections of this document must be 590 understood in that context. In particular, extensions can change the 591 minimum limits specified in Section 4.5.3, can change the ASCII 592 character set requirement as mentioned above, or can introduce some 593 optional modes of message handling. 595 In particular, if an extension implies that the delivery path 596 normally supports special features of that extension, and an 597 intermediate SMTP system finds a next hop that does not support the 598 required extension, it MAY choose, based on the specific extension 599 and circumstances, to requeue the message and try later and/or try an 600 alternate MX host. If this strategy is employed, the timeout to fall 601 back to an unextended format (if one is available) SHOULD be less 602 than the normal timeout for bouncing as undeliverable (e.g., if 603 normal timeout is three days, the requeue timeout before attempting 604 to transmit the mail without the extension might be one day). 606 2.3. SMTP Terminology 608 2.3.1. Mail Objects 610 SMTP transports a mail object. A mail object contains an envelope 611 and content. 613 The SMTP envelope is sent as a series of SMTP protocol units 614 (described in Section 3). It consists of an originator address (to 615 which error reports should be directed), one or more recipient 616 addresses, and optional protocol extension material. Historically, 617 variations on the reverse-path (originator) address specification 618 command (MAIL) could be used to specify alternate delivery modes, 619 such as immediate display; those variations have now been deprecated 620 (see Appendix F and Appendix F.6). 622 The SMTP content is sent in the SMTP DATA protocol unit and has two 623 parts: the header section and the body. If the content conforms to 624 other contemporary standards, the header section consists of a 625 collection of header fields, each consisting of a header name, a 626 colon, and data, structured as in the message format specification 627 (RFC 5322 [12]); the body, if structured, is defined according to 628 MIME (RFC 2045 [25]). The content is textual in nature, expressed 629 using the US-ASCII repertoire [2]. Although SMTP extensions (such as 630 "8BITMIME", RFC 6152 [47]) may relax this restriction for the content 631 body, the content header fields are always encoded using the US-ASCII 632 repertoire. Two MIME extensions (RFC 2047 [26] and RFC 2231 [29]) 633 define an algorithm for representing header values outside the US- 634 ASCII repertoire, while still encoding them using the US-ASCII 635 repertoire. 637 2.3.2. Senders and Receivers 639 In RFC 821, the two hosts participating in an SMTP transaction were 640 described as the "SMTP-sender" and "SMTP-receiver". This document 641 has been changed to reflect current industry terminology and hence 642 refers to them as the "SMTP client" (or sometimes just "the client") 643 and "SMTP server" (or just "the server"), respectively. Since a 644 given host may act both as server and client in a relay situation, 645 "receiver" and "sender" terminology is still used where needed for 646 clarity. 648 2.3.3. Mail Agents and Message Stores 650 Additional mail system terminology became common after RFC 821 was 651 published and, where convenient, is used in this specification. In 652 particular, SMTP servers and clients provide a mail transport service 653 and therefore act as "Mail Transfer Agents" (MTAs). "Mail User 654 Agents" (MUAs or UAs) are normally thought of as the sources and 655 targets of mail. At the source, an MUA might collect mail to be 656 transmitted from a user and hand it off to an MTA; the final 657 ("delivery") MTA would be thought of as handing the mail off to an 658 MUA (or at least transferring responsibility to it, e.g., by 659 depositing the message in a "message store"). However, while these 660 terms are used with at least the appearance of great precision in 661 other environments, the implied boundaries between MUAs and MTAs 662 often do not accurately match common, and conforming, practices with 663 Internet mail. Hence, the reader should be cautious about inferring 664 the strong relationships and responsibilities that might be implied 665 if these terms were used elsewhere 666 .[[CREF5: JcK 20210729: Does the above need to be rewritten to 667 identify the MSA role? ]] 669 2.3.4. Host 671 For the purposes of this specification, a host is a computer system 672 attached to the Internet (or, in some cases, to a private TCP/IP 673 network) and supporting the SMTP protocol. Hosts are known by names 674 (see the next section); they SHOULD NOT be identified by numerical 675 addresses, i.e., by address literals as described in Section 4.1.2. 677 2.3.5. Domain Names 679 A domain name (or often just a "domain") consists of one or more 680 components, separated by dots if more than one appears. In the case 681 of a top-level domain used by itself in an email address, a single 682 string is used without any dots. This makes the requirement, 683 described in more detail below, that only fully-qualified domain 684 names appear in SMTP transactions on the public Internet, 685 particularly important where top-level domains are involved. These 686 components ("labels" in DNS terminology, RFC 1035 [4]) are restricted 687 for SMTP purposes to consist of a sequence of letters, digits, and 688 hyphens drawn from the ASCII character set [2] and conforming to what 689 RFC 1035 Section 2.3.1 calls the "preferred name syntax". Domain 690 names are used as names of hosts and of other entities in the domain 691 name hierarchy. For example, a domain may refer to an alias (label 692 of a CNAME RR) or the label of Mail eXchanger records to be used to 693 deliver mail instead of representing a host name. See RFC 1035 [4] 694 and Section 5 of this specification. 696 The domain name, as described in this document and in RFC 1035 [4], 697 is the entire, fully-qualified name (often referred to as an "FQDN"). 698 A domain name that is not in FQDN form is no more than a local alias. 699 Local aliases MUST NOT appear in any SMTP transaction. 701 Only resolvable, fully-qualified domain names (FQDNs) are permitted 702 when domain names are used in SMTP. In other words, names that can 703 be resolved to MX RRs or address (i.e., A or AAAA) RRs (as discussed 704 in Section 5) are permitted, as are CNAME RRs whose targets can be 705 resolved, in turn, to MX or address RRs. 706 [[CREF6: [[5321bis Editor's Note: it is not clear whether "In other 707 words" really meant "for example" or it is was intended that the only 708 labels permitted are those that own records in one of the above RR 709 types]]]] 710 Local nicknames or unqualified names MUST NOT be used. There are two 711 exceptions to the rule requiring FQDNs: 713 o The domain name given in the EHLO command MUST be either a primary 714 host name (a domain name that resolves to an address RR) or, if 715 the host has no name, an address literal, as described in 716 Section 4.1.3 and discussed further in the EHLO discussion of 717 Section 4.1.4. 719 o The reserved mailbox name "postmaster" may be used in a RCPT 720 command without domain qualification (see Section 4.1.1.3) and 721 MUST be accepted if so used. 723 2.3.6. Buffer and State Table 725 SMTP sessions are stateful, with both parties carefully maintaining a 726 common view of the current state. In this document, we model this 727 state by a virtual "buffer" and a "state table" on the server that 728 may be used by the client to, for example, "clear the buffer" or 729 "reset the state table", causing the information in the buffer to be 730 discarded and the state to be returned to some previous state. 732 2.3.7. Commands and Replies 734 SMTP commands and, unless altered by a service extension, message 735 data, are transmitted from the sender to the receiver via the 736 transmission channel in "lines". 738 An SMTP reply is an acknowledgment (positive or negative) sent in 739 "lines" from receiver to sender via the transmission channel in 740 response to a command. The general form of a reply is a numeric 741 completion code (indicating failure or success) usually followed by a 742 text string. The codes are for use by programs and the text is 743 usually intended for human users. RFC 3463 [7], specifies further 744 structuring of the reply strings, including the use of supplemental 745 and more specific completion codes (see also RFC 5248 [46]). 747 2.3.8. Lines 749 Lines consist of zero or more data characters terminated by the 750 sequence ASCII character "CR" (hex value 0D) followed immediately by 751 ASCII character "LF" (hex value 0A). This termination sequence is 752 denoted as in this document. Conforming implementations MUST 753 NOT recognize or generate any other character or character sequence 754 as a line terminator. Limits MAY be imposed on line lengths by 755 servers (see Section 4). 757 In addition, the appearance of "bare" "CR" or "LF" characters in text 758 (i.e., either without the other) has a long history of causing 759 problems in mail implementations and applications that use the mail 760 system as a tool. SMTP client implementations MUST NOT transmit 761 these characters except when they are intended as line terminators 762 and then MUST, as indicated above, transmit them only as a 763 sequence. 765 2.3.9. Message Content and Mail Data 767 The terms "message content" and "mail data" are used interchangeably 768 in this document to describe the material transmitted after the DATA 769 command is accepted and before the end of data indication is 770 transmitted. Message content includes the message header section and 771 the possibly structured message body. In the absence of extensions, 772 both are required to be ASCII (see Section 2.3.1). The MIME 773 specification (RFC 2045 [25]) provides the standard mechanisms for 774 structured message bodies. 776 2.3.10. Originator, Delivery, Relay, and Gateway Systems 778 This specification makes a distinction among four types of SMTP 779 systems, based on the role those systems play in transmitting 780 electronic mail. An "originating" system (sometimes called an SMTP 781 originator) introduces mail into the Internet or, more generally, 782 into a transport service environment. A "delivery" SMTP system is 783 one that receives mail from a transport service environment and 784 passes it to a mail user agent or deposits it in a message store that 785 a mail user agent is expected to subsequently access. A "relay" SMTP 786 system (usually referred to just as a "relay") receives mail from an 787 SMTP client and transmits it, without modification to the message 788 data other than adding trace information, to another SMTP server for 789 further relaying or for delivery. 791 A "gateway" SMTP system (usually referred to just as a "gateway") 792 receives mail from a client system in one transport environment and 793 transmits it to a server system in another transport environment. 794 Differences in protocols or message semantics between the transport 795 environments on either side of a gateway may require that the gateway 796 system perform transformations to the message that are not permitted 797 to SMTP relay systems. For the purposes of this specification, 798 firewalls that rewrite addresses should be considered as gateways, 799 even if SMTP is used on both sides of them (see RFC 2979 [32]). 800 [[CREF7: [5321bis] [[Note in draft/Placeholder: There has been a 801 request to expand this section, possibly into a more extensive model 802 of Internet mail. Comments from others solicited. In particular, 803 does RFC 5598 make that suggestion OBE?]] ]] 805 2.3.11. Mailbox and Address 807 As used in this specification, an "address" is a character string 808 that identifies a user to whom mail will be sent or a location into 809 which mail will be deposited. The term "mailbox" refers to that 810 depository. The two terms are typically used interchangeably unless 811 the distinction between the location in which mail is placed (the 812 mailbox) and a reference to it (the address) is important. An 813 address normally consists of user and domain specifications. The 814 standard mailbox naming convention is defined to be "local- 815 part@domain"; contemporary usage permits a much broader set of 816 applications than simple "user names". Consequently, and due to a 817 long history of problems when intermediate hosts have attempted to 818 optimize transport by modifying them, the local-part MUST be 819 interpreted and assigned semantics only by the host specified in the 820 domain part of the address. 822 2.4. General Syntax Principles and Transaction Model 824 SMTP commands and replies have a rigid syntax. All commands begin 825 with a command verb. All replies begin with a three digit numeric 826 code. In some commands and replies, arguments are required following 827 the verb or reply code. Some commands do not accept arguments (after 828 the verb), and some reply codes are followed, sometimes optionally, 829 by free form text. In both cases, where text appears, it is 830 separated from the verb or reply code by a space character. Complete 831 definitions of commands and replies appear in Section 4. 833 Verbs and argument values (e.g., "TO:" or "to:" in the RCPT command 834 and extension name keywords) are not case sensitive, with the sole 835 exception in this specification of a mailbox local-part (SMTP 836 Extensions may explicitly specify case-sensitive elements). That is, 837 a command verb, an argument value other than a mailbox local-part, 838 and free form text MAY be encoded in upper case, lower case, or any 839 mixture of upper and lower case with no impact on its meaning. The 840 local-part of a mailbox MUST BE treated as case sensitive. 841 Therefore, SMTP implementations MUST take care to preserve the case 842 of mailbox local-parts. In particular, for some hosts, the user 843 "smith" is different from the user "Smith". However, exploiting the 844 case sensitivity of mailbox local-parts impedes interoperability and 845 is discouraged. Mailbox domains follow normal DNS rules and are 846 hence not case sensitive. 848 A few SMTP servers, in violation of this specification (and RFC 821) 849 require that command verbs be encoded by clients in upper case. 850 Implementations MAY wish to employ this encoding to accommodate those 851 servers. 853 The argument clause consists of a variable-length character string 854 ending with the end of the line, i.e., with the character sequence 855 . The receiver will take no action until this sequence is 856 received. 858 The syntax for each command is shown with the discussion of that 859 command. Common elements and parameters are shown in Section 4.1.2. 861 Commands and replies are composed of characters from the ASCII 862 character set [2]. When the transport service provides an 8-bit byte 863 (octet) transmission channel, each 7-bit character is transmitted, 864 right justified, in an octet with the high-order bit cleared to zero. 865 More specifically, the unextended SMTP service provides 7-bit 866 transport only. An originating SMTP client that has not successfully 867 negotiated an appropriate extension with a particular server (see the 868 next paragraph) MUST NOT transmit messages with information in the 869 high-order bit of octets. If such messages are transmitted in 870 violation of this rule, receiving SMTP servers MAY clear the high- 871 order bit or reject the message as invalid. In general, a relay SMTP 872 SHOULD assume that the message content it has received is valid and, 873 assuming that the envelope permits doing so, relay it without 874 inspecting that content. Of course, if the content is mislabeled and 875 the data path cannot accept the actual content, this may result in 876 the ultimate delivery of a severely garbled message to the recipient. 877 Delivery SMTP systems MAY reject such messages, or return them as 878 undeliverable, rather than deliver them. In the absence of a server- 879 offered extension explicitly permitting it, a sending SMTP system is 880 not permitted to send envelope commands in any character set other 881 than US-ASCII. Receiving systems SHOULD reject such commands, 882 normally using "500 syntax error - invalid character" replies. 884 8-bit message content transmission MAY be requested of the server by 885 a client using extended SMTP facilities, notably the "8BITMIME" 886 extension, RFC 6152 [47]. 8BITMIME SHOULD be supported by SMTP 887 servers. However, it MUST NOT be construed as authorization to 888 transmit unrestricted 8-bit material, nor does 8BITMIME authorize 889 transmission of any envelope material in other than ASCII. 8BITMIME 890 MUST NOT be requested by senders for material with the high bit on 891 that is not in MIME format with an appropriate content-transfer 892 encoding; servers MAY reject such messages. 894 The metalinguistic notation used in this document corresponds to the 895 "Augmented BNF" used in other Internet mail system documents. The 896 reader who is not familiar with that syntax should consult the ABNF 897 specification in RFC 5234 [11]. Metalanguage terms used in running 898 text are surrounded by pointed brackets (e.g., ) for clarity. 899 The reader is cautioned that the grammar expressed in the 900 metalanguage is not comprehensive. There are many instances in which 901 provisions in the text constrain or otherwise modify the syntax or 902 semantics implied by the grammar. 904 3. The SMTP Procedures: An Overview 906 This section contains descriptions of the procedures used in SMTP: 907 session initiation, mail transaction, forwarding mail, verifying 908 mailbox names and expanding mailing lists, and opening and closing 909 exchanges. Comments on relaying, a note on mail domains, and a 910 discussion of changing roles are included at the end of this section. 911 Several complete scenarios are presented in Appendix D. 913 3.1. Session Initiation 915 An SMTP session is initiated when a client opens a connection to a 916 server and the server responds with an opening message. 918 SMTP server implementations MAY include identification of their 919 software and version information in the connection greeting reply 920 after the 220 code, a practice that permits more efficient isolation 921 and repair of any problems. Implementations MAY make provision for 922 SMTP servers to disable the software and version announcement where 923 it causes security concerns. While some systems also identify their 924 contact point for mail problems, this is not a substitute for 925 maintaining the required "postmaster" address (see Section 4). 927 The SMTP protocol allows a server to formally reject a mail session 928 while still allowing the initial connection as follows: a 521 929 response MAY be given in the initial connection opening message 930 instead of the 220. A server taking this approach MUST still wait 931 for the client to send a QUIT (see Section 4.1.1.10) before closing 932 the connection and SHOULD respond to any intervening commands with 933 "503 bad sequence of commands". Since an attempt to make an SMTP 934 connection to such a system is probably in error, a server returning 935 a 521 [[CREF8: (or 554?]] response on connection opening SHOULD 936 provide enough information in the reply text to facilitate debugging 937 of the sending system. See Section 4.2.4.2. 939 3.2. Client Initiation 941 Once the server has sent the greeting (welcoming) message and the 942 client has received it, the client normally sends the EHLO command to 943 the server, indicating the client's identity. In addition to opening 944 the session, use of EHLO indicates that the client is able to process 945 service extensions and requests that the server provide a list of the 946 extensions it supports. Older SMTP systems that are unable to 947 support service extensions, and contemporary clients that do not 948 require service extensions in the mail session being initiated, MAY 949 use HELO instead of EHLO. Servers MUST NOT return the extended EHLO- 950 style response to a HELO command. For a particular connection 951 attempt, if the server returns a "command not recognized" response to 952 EHLO, the client SHOULD be able to fall back and send HELO. 954 In the EHLO command, the host sending the command identifies itself; 955 the command may be interpreted as saying "Hello, I am " (and, 956 in the case of EHLO, "and I support service extension requests"). 958 3.3. Mail Transactions 960 There are three steps to SMTP mail transactions. The transaction 961 starts with a MAIL command that gives the sender identification. (In 962 general, the MAIL command may be sent only when no mail transaction 963 is in progress; see Section 4.1.4.) A series of one or more RCPT 964 commands follows, giving the receiver information. Then, a DATA 965 command initiates transfer of the mail data and is terminated by the 966 "end of mail" data indicator, which also confirms (and terminates) 967 the transaction. 969 Mail transactions are also terminated by the RSET command 970 (Section 4.1.1.5), the sending of an EHLO command (Section 3.2), or 971 the sending of a QUIT command (Section 3.8) which terminates both any 972 active mail transaction and the SMTP connection. 974 The first step in the procedure is the MAIL command. 976 MAIL FROM: [SP ] 978 This command tells the SMTP-receiver that a new mail transaction is 979 starting and to reset all its state tables and buffers, including any 980 recipients or mail data. The portion of the first or 981 only argument contains the source mailbox (between "<" and ">" 982 brackets), which can be used to report errors (see Section 4.2 for a 983 discussion of error reporting). If accepted, the SMTP server returns 984 a "250 OK" reply. If the mailbox specification is not acceptable for 985 some reason, the server MUST return a reply indicating whether the 986 failure is permanent (i.e., will occur again if the client tries to 987 send the same address again) or temporary (i.e., the address might be 988 accepted if the client tries again later). Despite the apparent 989 scope of this requirement, there are circumstances in which the 990 acceptability of the reverse-path may not be determined until one or 991 more forward-paths (in RCPT commands) can be examined. In those 992 cases, the server MAY reasonably accept the reverse-path (with a 250 993 reply) and then report problems after the forward-paths are received 994 and examined. Normally, failures produce 550 or 553 replies. 996 Historically, the was permitted to contain more than 997 just a mailbox; however, contemporary systems SHOULD NOT use source 998 routing (see Appendix C). 1000 The optional are associated with negotiated SMTP 1001 service extensions (see Section 2.2). 1003 The second step in the procedure is the RCPT command. This step of 1004 the procedure can be repeated any number of times. 1006 RCPT TO: [ SP ] 1008 The first or only argument to this command includes a forward-path 1009 (normally a mailbox local-part and domain, always surrounded by "<" 1010 and ">" brackets) identifying one recipient. If accepted, the SMTP 1011 server returns a "250 OK" reply and stores the forward-path. If the 1012 recipient is known not to be a deliverable address, the SMTP server 1013 returns a 550 reply, typically with a string such as "no such user - 1014 " and the mailbox name (other circumstances and reply codes are 1015 possible). 1017 The can contain more than just a mailbox. 1018 Historically, the was permitted to contain a source 1019 routing list of hosts and the destination mailbox; however, 1020 contemporary SMTP clients SHOULD NOT utilize source routes (see 1021 Appendix C). Servers MUST be prepared to encounter a list of source 1022 routes in the forward-path, but they SHOULD ignore the routes or MAY 1023 decline to support the relaying they imply. Similarly, servers MAY 1024 decline to accept mail that is destined for other hosts or systems. 1025 These restrictions make a server useless as a relay for clients that 1026 do not support full SMTP functionality. Consequently, restricted- 1027 capability clients MUST NOT assume that any SMTP server on the 1028 Internet can be used as their mail processing (relaying) site. If a 1029 RCPT command appears without a previous MAIL command, the server MUST 1030 return a 503 "Bad sequence of commands" response. The optional 1031 are associated with negotiated SMTP service 1032 extensions (see Section 2.2). [[CREF9: [5321bis]: this section would 1033 be improved by being more specific about where mail transactions 1034 begin and end and then talking about "transaction state" here, rather 1035 than specific prior commands. --JcK]] 1037 Since it has been a common source of errors, it is worth noting that 1038 spaces are not permitted on either side of the colon following FROM 1039 in the MAIL command or TO in the RCPT command. The syntax is exactly 1040 as given above. 1042 The third step in the procedure is the DATA command (or some 1043 alternative specified in a service extension). 1045 DATA 1047 If accepted, the SMTP server returns a 354 Intermediate reply and 1048 considers all succeeding lines up to but not including the end of 1049 mail data indicator to be the message text. When the end of text is 1050 successfully received and stored, the SMTP-receiver sends a "250 OK" 1051 reply. 1053 Since the mail data is sent on the transmission channel, the end of 1054 mail data must be indicated so that the command and reply dialog can 1055 be resumed. SMTP indicates the end of the mail data by sending a 1056 line containing only a "." (period or full stop, hex 2E). A 1057 transparency procedure is used to prevent this from interfering with 1058 the user's text (see Section 4.5.2). 1060 The end of mail data indicator also confirms the mail transaction and 1061 tells the SMTP server to now process the stored recipients and mail 1062 data. If accepted, the SMTP server returns a "250 OK" reply. The 1063 DATA command can fail at only two points in the protocol exchange: 1065 If there was no MAIL, or no RCPT, command, or all such commands were 1066 rejected, the server MAY return a "command out of sequence" (503) or 1067 "no valid recipients" (554) reply in response to the DATA command. 1068 If one of those replies (or any other 5yz reply) is received, the 1069 client MUST NOT send the message data; more generally, message data 1070 MUST NOT be sent unless a 354 reply is received. 1072 If the verb is initially accepted and the 354 reply issued, the DATA 1073 command should fail only if the mail transaction was incomplete (for 1074 example, no recipients), if resources were unavailable (including, of 1075 course, the server unexpectedly becoming unavailable), or if the 1076 server determines that the message should be rejected for policy or 1077 other reasons. 1079 However, in practice, some servers do not perform recipient 1080 verification until after the message text is received. These servers 1081 SHOULD treat a failure for one or more recipients as a "subsequent 1082 failure" and return a mail message as discussed in Section 6 and, in 1083 particular, in Section 6.1. Using a "550 mailbox not found" (or 1084 equivalent) reply code after the data are accepted makes it difficult 1085 or impossible for the client to determine which recipients failed. 1087 When the RFC 822 format ([13], [12]) is being used, the mail data 1088 include the header fields such as those named Date, Subject, To, Cc, 1089 and From. Server SMTP systems SHOULD NOT reject messages based on 1090 perceived defects in the RFC 822 or MIME (RFC 2045 [25]) message 1091 header section or message body. In particular, they MUST NOT reject 1092 messages in which the numbers of Resent-header fields do not match or 1093 Resent-to appears without Resent-from and/or Resent-date. 1095 Mail transaction commands MUST be used in the order discussed above. 1097 3.4. Forwarding for Address Correction or Updating 1099 Forwarding support is most often required to consolidate and simplify 1100 addresses within, or relative to, some enterprise and less frequently 1101 to establish addresses to link a person's prior address with a 1102 current one. Silent forwarding of messages (without server 1103 notification to the sender), for security or non-disclosure purposes, 1104 is common in the contemporary Internet. 1106 In both the enterprise and the "new address" cases, information 1107 hiding (and sometimes security) considerations argue against exposure 1108 of the "final" address through the SMTP protocol as a side effect of 1109 the forwarding activity. This may be especially important when the 1110 final address may not even be reachable by the sender. Consequently, 1111 the "forwarding" mechanisms described in Section 3.2 of RFC 821, and 1112 especially the 251 (corrected destination) and 551 reply codes from 1113 RCPT must be evaluated carefully by implementers and, when they are 1114 available, by those configuring systems (see also Section 7.4). 1116 In particular: 1118 o Servers MAY forward messages when they are aware of an address 1119 change. When they do so, they MAY either provide address-updating 1120 information with a 251 code, or may forward "silently" and return 1121 a 250 code. However, if a 251 code is used, they MUST NOT assume 1122 that the client will actually update address information or even 1123 return that information to the user. 1125 Alternately, 1127 o Servers MAY reject messages or return them as non-deliverable when 1128 they cannot be delivered precisely as addressed. When they do so, 1129 they MAY either provide address-updating information with a 551 1130 code, or may reject the message as undeliverable with a 550 code 1131 and no address-specific information. However, if a 551 code is 1132 used, they MUST NOT assume that the client will actually update 1133 address information or even return that information to the user. 1135 SMTP server implementations that support the 251 and/or 551 reply 1136 codes SHOULD provide configuration mechanisms so that sites that 1137 conclude that they would undesirably disclose information can disable 1138 or restrict their use. 1140 3.5. Commands for Debugging Addresses 1141 3.5.1. Overview 1143 SMTP provides commands to verify a user name or obtain the content of 1144 a mailing list. This is done with the VRFY and EXPN commands, which 1145 have character string arguments. Implementations SHOULD support VRFY 1146 and EXPN (however, see Section 3.5.2 and Section 7.3). 1148 For the VRFY command, the string is a user name or a user name and 1149 domain (see below). If a normal (i.e., 250) response is returned, 1150 the response MAY include the full name of the user and MUST include 1151 the mailbox of the user. It MUST be in either of the following 1152 forms: 1154 User Name 1155 local-part@domain 1157 When a name that is the argument to VRFY could identify more than one 1158 mailbox, the server MAY either note the ambiguity or identify the 1159 alternatives. In other words, any of the following are legitimate 1160 responses to VRFY: 1162 553 User ambiguous 1164 or 1166 553- Ambiguous; Possibilities are 1167 553-Joe Smith 1168 553-Harry Smith 1169 553 Melvin Smith 1171 or 1173 553-Ambiguous; Possibilities 1174 553- 1175 553- 1176 553 1178 Under normal circumstances, a client receiving a 553 reply would be 1179 expected to expose the result to the user. Use of exactly the forms 1180 given, and the "user ambiguous" or "ambiguous" keywords, possibly 1181 supplemented by extended reply codes, such as those described in RFC 1182 3463 [7], will facilitate automated translation into other languages 1183 as needed. Of course, a client that was highly automated or that was 1184 operating in another language than English might choose to try to 1185 translate the response to return some other indication to the user 1186 than the literal text of the reply, or to take some automated action 1187 such as consulting a directory service for additional information 1188 before reporting to the user. 1190 For the EXPN command, the string identifies a mailing list, and the 1191 successful (i.e., 250) multiline response MAY include the full name 1192 of the users and MUST give the mailboxes on the mailing list. 1194 In some hosts, the distinction between a mailing list and an alias 1195 for a single mailbox is a bit fuzzy, since a common data structure 1196 may hold both types of entries, and it is possible to have mailing 1197 lists containing only one mailbox. If a request is made to apply 1198 VRFY to a mailing list, a positive response MAY be given if a message 1199 so addressed would be delivered to everyone on the list, otherwise an 1200 error SHOULD be reported (e.g., "550 That is a mailing list, not a 1201 user" or "252 Unable to verify members of mailing list"). If a 1202 request is made to expand a user name, the server MAY return a 1203 positive response consisting of a list containing one name, or an 1204 error MAY be reported (e.g., "550 That is a user name, not a mailing 1205 list"). 1207 In the case of a successful multiline reply (normal for EXPN), 1208 exactly one mailbox is to be specified on each line of the reply. 1209 The case of an ambiguous request is discussed above. 1211 "User name" is a fuzzy term and has been used deliberately. An 1212 implementation of the VRFY or EXPN commands MUST include at least 1213 recognition of local mailboxes as "user names". However, since 1214 current Internet practice often results in a single host handling 1215 mail for multiple domains, hosts, especially hosts that provide this 1216 functionality, SHOULD accept the "local-part@domain" form as a "user 1217 name"; hosts MAY also choose to recognize other strings as "user 1218 names". 1220 The case of expanding a mailbox list requires a multiline reply, such 1221 as: 1223 C: EXPN Example-People 1224 S: 250-Jon Postel 1225 S: 250-Fred Fonebone 1226 S: 250 Sam Q. Smith 1228 or 1230 C: EXPN Executive-Washroom-List 1231 S: 550 Access Denied to You. 1233 The character string arguments of the VRFY and EXPN commands cannot 1234 be further restricted due to the variety of implementations of the 1235 user name and mailbox list concepts. On some systems, it may be 1236 appropriate for the argument of the EXPN command to be a file name 1237 for a file containing a mailing list, but again there are a variety 1238 of file naming conventions in the Internet. Similarly, historical 1239 variations in what is returned by these commands are such that the 1240 response SHOULD be interpreted very carefully, if at all, and SHOULD 1241 generally only be used for diagnostic purposes. 1243 3.5.2. VRFY Normal Response 1245 When normal (2yz or 551) responses are returned from a VRFY or EXPN 1246 request, the reply MUST include the name using a "" construction, where "domain" is a fully-qualified 1248 domain name. In circumstances exceptional enough to justify 1249 violating the intent of this specification, free-form text MAY be 1250 returned. In order to facilitate parsing by both computers and 1251 people, addresses SHOULD appear in pointed brackets. When addresses, 1252 rather than free-form debugging information, are returned, EXPN and 1253 VRFY MUST return only valid domain addresses that are usable in SMTP 1254 RCPT commands. Consequently, if an address implies delivery to a 1255 program or other system, the mailbox name used to reach that target 1256 MUST be given. Paths (explicit source routes) MUST NOT be returned 1257 by VRFY or EXPN. 1259 Server implementations SHOULD support both VRFY and EXPN. For 1260 security reasons, implementations MAY provide local installations a 1261 way to disable either or both of these commands through configuration 1262 options or the equivalent (see Section 7.3). When these commands are 1263 supported, they are not required to work across relays when relaying 1264 is supported. Since they were both optional in RFC 821, but VRFY was 1265 made mandatory in RFC 1123 [5], if EXPN is supported, it MUST be 1266 listed as a service extension in an EHLO response. VRFY MAY be 1267 listed as a convenience but, since support for it is required, SMTP 1268 clients are not required to check for its presence on the extension 1269 list before using it. 1271 3.5.3. Meaning of VRFY or EXPN Success Response 1273 A server MUST NOT return a 250 code in response to a VRFY or EXPN 1274 command unless it has actually verified the address. In particular, 1275 a server MUST NOT return 250 if all it has done is to verify that the 1276 syntax given is valid. In that case, 502 (Command not implemented) 1277 or 500 (Syntax error, command unrecognized) SHOULD be returned. As 1278 stated elsewhere, implementation (in the sense of actually validating 1279 addresses and returning information) of VRFY and EXPN are strongly 1280 recommended. Hence, implementations that return 500 or 502 for VRFY 1281 are not in full compliance with this specification. 1283 There may be circumstances where an address appears to be valid but 1284 cannot reasonably be verified in real time, particularly when a 1285 server is acting as a mail exchanger for another server or domain. 1287 "Apparent validity", in this case, would normally involve at least 1288 syntax checking and might involve verification that any domains 1289 specified were ones to which the host expected to be able to relay 1290 mail. In these situations, reply code 252 SHOULD be returned. These 1291 cases parallel the discussion of RCPT verification in Section 2.1. 1292 Similarly, the discussion in Section 3.4 applies to the use of reply 1293 codes 251 and 551 with VRFY (and EXPN) to indicate addresses that are 1294 recognized but that would be forwarded or rejected were mail received 1295 for them. Implementations generally SHOULD be more aggressive about 1296 address verification in the case of VRFY than in the case of RCPT, 1297 even if it takes a little longer to do so. 1299 3.5.4. Semantics and Applications of EXPN 1301 EXPN is often very useful in debugging and understanding problems 1302 with mailing lists and multiple-target-address aliases. Some systems 1303 have attempted to use source expansion of mailing lists as a means of 1304 eliminating duplicates. The propagation of aliasing systems with 1305 mail on the Internet for hosts (typically with MX and CNAME DNS 1306 records), for mailboxes (various types of local host aliases), and in 1307 various proxying arrangements has made it nearly impossible for these 1308 strategies to work consistently, and mail systems SHOULD NOT attempt 1309 them. 1311 3.6. Relaying and Mail Routing 1313 3.6.1. Source Routes and Relaying 1315 In general, the availability of Mail eXchanger records in the domain 1316 name system (RFC 1035 [4], RFC 974 [16]) makes the use of explicit 1317 source routes in the Internet mail system unnecessary. Many 1318 historical problems with the interpretation of explicit source routes 1319 have made their use undesirable. SMTP clients SHOULD NOT generate 1320 explicit source routes except under unusual circumstances. SMTP 1321 servers MAY decline to act as mail relays or to accept addresses that 1322 specify source routes. When route information is encountered, SMTP 1323 servers MAY ignore the route information and simply send to the final 1324 destination specified as the last element in the route and SHOULD do 1325 so. There has been an invalid practice of using names that do not 1326 appear in the DNS as destination names, with the senders counting on 1327 the intermediate hosts specified in source routing to resolve any 1328 problems. If source routes are stripped, this practice will cause 1329 failures. This is one of several reasons why SMTP clients MUST NOT 1330 generate invalid source routes or depend on serial resolution of 1331 names in such routes. 1333 When source routes are not used, the process described in RFC 821 for 1334 constructing a reverse-path from the forward-path is not applicable 1335 and the reverse-path at the time of delivery will simply be the 1336 address that appeared in the MAIL command. 1338 3.6.2. Mail eXchange Records and Relaying 1340 A relay SMTP server is usually the target of a DNS MX record that 1341 designates it, rather than the final delivery system. The relay 1342 server may accept or reject the task of relaying the mail in the same 1343 way it accepts or rejects mail for a local user. If it accepts the 1344 task, it then becomes an SMTP client, establishes a transmission 1345 channel to the next SMTP server specified in the DNS (according to 1346 the rules in Section 5), and sends it the mail. If it declines to 1347 relay mail to a particular address for policy reasons, a 550 response 1348 SHOULD be returned. 1350 [[CREF10: Text below reflects proposed replacement of the paragraph ( 1351 edited version of suggestion by D Crocker 2021-03-17 17:23 email). 1352 Cf. Ticket #30:]] 1353 This specification does not deal with the verification of return 1354 paths. Server efforts to verify a return path and actions to be 1355 taken under various circumstances are outside the scope of this 1356 specification.for use in delivery notifications. 1358 3.6.3. Message Submission Servers as Relays 1360 Many mail-sending clients exist, especially in conjunction with 1361 facilities that receive mail via POP3 or IMAP, that have limited 1362 capability to support some of the requirements of this specification, 1363 such as the ability to queue messages for subsequent delivery 1364 attempts. For these clients, it is common practice to make private 1365 arrangements to send all messages to a single server for processing 1366 and subsequent distribution. SMTP, as specified here, is not ideally 1367 suited for this role. A standardized mail submission protocol has 1368 been developed that is gradually superseding practices based on SMTP 1369 (see RFC 6409 [42]). In any event, because these arrangements are 1370 private and fall outside the scope of this specification, they are 1371 not described here. 1373 It is important to note that MX records can point to SMTP servers 1374 that act as gateways into other environments, not just SMTP relays 1375 and final delivery systems; see Sections 3.7 and 5. 1377 If an SMTP server has accepted the task of relaying the mail and 1378 later finds that the destination is incorrect or that the mail cannot 1379 be delivered for some other reason, then it MUST construct an 1380 "undeliverable mail" notification message and send it to the 1381 originator of the undeliverable mail (as indicated by the reverse- 1382 path). Formats specified for non-delivery reports by other standards 1383 (see, for example, RFC 3461 [34] and RFC 3464 [35]) SHOULD be used if 1384 possible. 1386 This notification message must be from the SMTP server at the relay 1387 host or the host that first determines that delivery cannot be 1388 accomplished. Of course, SMTP servers MUST NOT send notification 1389 messages about problems transporting notification messages. One way 1390 to prevent loops in error reporting is to specify a null reverse-path 1391 in the MAIL command of a notification message. When such a message 1392 is transmitted, the reverse-path MUST be set to null (see 1393 Section 4.5.5 for additional discussion). A MAIL command with a null 1394 reverse-path appears as follows: 1396 MAIL FROM:<> 1398 As discussed in Section 6.4, a relay SMTP has no need to inspect or 1399 act upon the header section or body of the message data and MUST NOT 1400 do so except to add its own "Received:" header field (Section 4.4) 1401 and, optionally, to attempt to detect looping in the mail system (see 1402 Section 6.3). Of course, this prohibition also applies to any 1403 modifications of these header fields or text (see also Section 7.9). 1405 3.7. Mail Gatewaying 1407 While the relay function discussed above operates within the Internet 1408 SMTP transport service environment, MX records or various forms of 1409 explicit routing may require that an intermediate SMTP server perform 1410 a translation function between one transport service and another. As 1411 discussed in Section 2.3.10, when such a system is at the boundary 1412 between two transport service environments, we refer to it as a 1413 "gateway" or "gateway SMTP". 1415 Gatewaying mail between different mail environments, such as 1416 different mail formats and protocols, is complex and does not easily 1417 yield to standardization. However, some general requirements may be 1418 given for a gateway between the Internet and another mail 1419 environment. 1421 3.7.1. Header Fields in Gatewaying 1423 Header fields MAY be rewritten when necessary as messages are 1424 gatewayed across mail environment boundaries. This may involve 1425 inspecting the message body or interpreting the local-part of the 1426 destination address in spite of the prohibitions in Section 6.4. 1428 Other mail systems gatewayed to the Internet often use a subset of 1429 the RFC 822 header section or provide similar functionality with a 1430 different syntax, but some of these mail systems do not have an 1431 equivalent to the SMTP envelope. Therefore, when a message leaves 1432 the Internet environment, it may be necessary to fold the SMTP 1433 envelope information into the message header section. A possible 1434 solution would be to create new header fields to carry the envelope 1435 information (e.g., "X-SMTP-MAIL:" and "X-SMTP-RCPT:"); however, this 1436 would require changes in mail programs in foreign environments and 1437 might risk disclosure of private information (see Section 7.2). 1439 3.7.2. Received Lines in Gatewaying 1441 When forwarding a message into or out of the Internet environment, a 1442 gateway MUST prepend a Received: line, but it MUST NOT alter in any 1443 way a Received: line that is already in the header section. 1445 "Received:" header fields of messages originating from other 1446 environments may not conform exactly to this specification. However, 1447 the most important use of Received: lines is for debugging mail 1448 faults, and this debugging can be severely hampered by well-meaning 1449 gateways that try to "fix" a Received: line. As another consequence 1450 of trace header fields arising in non-SMTP environments, receiving 1451 systems MUST NOT reject mail based on the format of a trace header 1452 field and SHOULD be extremely robust in the light of unexpected 1453 information or formats in those header fields. 1455 The gateway SHOULD indicate the environment and protocol in the "via" 1456 clauses of Received header field(s) that it supplies. 1458 3.7.3. Addresses in Gatewaying 1460 From the Internet side, the gateway SHOULD accept all valid address 1461 formats in SMTP commands and in the RFC 822 header section, and all 1462 valid RFC 822 messages. Addresses and header fields generated by 1463 gateways MUST conform to applicable standards (including this one and 1464 RFC 5322 [12]). Gateways are, of course, subject to the same rules 1465 for handling source routes as those described for other SMTP systems 1466 in Section 3.3. 1468 3.7.4. Other Header Fields in Gatewaying 1470 The gateway MUST ensure that all header fields of a message that it 1471 forwards into the Internet mail environment meet the requirements for 1472 Internet mail. In particular, all addresses in "From:", "To:", 1473 "Cc:", etc., header fields MUST be transformed (if necessary) to 1474 satisfy the standard header syntax of RFC 5322 [12], MUST reference 1475 only fully-qualified domain names, and MUST be effective and useful 1476 for sending replies. The translation algorithm used to convert mail 1477 from the Internet protocols to another environment's protocol SHOULD 1478 ensure that error messages from the foreign mail environment are 1479 delivered to the reverse-path from the SMTP envelope, not to an 1480 address in the "From:", "Sender:", or similar header fields of the 1481 message. 1483 3.7.5. Envelopes in Gatewaying 1485 Similarly, when forwarding a message from another environment into 1486 the Internet, the gateway SHOULD set the envelope return path in 1487 accordance with an error message return address, if supplied by the 1488 foreign environment. If the foreign environment has no equivalent 1489 concept, the gateway must select and use a best approximation, with 1490 the message originator's address as the default of last resort. 1492 3.8. Terminating Sessions and Connections 1494 An SMTP connection is terminated when the client sends a QUIT 1495 command. The server responds with a positive reply code, after which 1496 it closes the connection. 1498 An SMTP server MUST NOT intentionally close the connection under 1499 normal operational circumstances (see Section 7.8) except: 1501 o After receiving a QUIT command and responding with a 221 reply. 1503 o After detecting the need to shut down the SMTP service and 1504 returning a 421 reply code. This reply code can be issued after 1505 the server receives any command or, if necessary, asynchronously 1506 from command receipt (on the assumption that the client will 1507 receive it after the next command is issued). 1509 o After a timeout, as specified in Section 4.5.3.2, occurs waiting 1510 for the client to send a command or data. 1512 In particular, a server that closes connections in response to 1513 commands that are not understood is in violation of this 1514 specification. Servers are expected to be tolerant of unknown 1515 commands, issuing a 500 reply and awaiting further instructions from 1516 the client. 1518 An SMTP server that is forcibly shut down via external means SHOULD 1519 attempt to send a line containing a 421 reply code to the SMTP client 1520 before exiting. The SMTP client will normally read the 421 reply 1521 code after sending its next command. 1523 SMTP clients that experience a connection close, reset, or other 1524 communications failure due to circumstances not under their control 1525 (in violation of the intent of this specification but sometimes 1526 unavoidable) SHOULD, to maintain the robustness of the mail system, 1527 treat the mail transaction as if a 421 response had been received and 1528 act accordingly. 1530 There are circumstances, contrary to the intent of this 1531 specification, in which an SMTP server may receive an indication that 1532 the underlying TCP connection has been closed or reset. To preserve 1533 the robustness of the mail system, SMTP servers SHOULD be prepared 1534 for this condition and SHOULD treat it as if a QUIT had been received 1535 before the connection disappeared. 1537 3.9. Mailing Lists and Aliases 1539 [[CREF11: [5321bis] If "alias and list models" are explained 1540 elsewhere, cross reference". Also note that this section appears to 1541 prohibit an exploder from adding List-* headers. That needs to be 1542 finessed.]] 1543 An SMTP-capable host SHOULD support both the alias and the list 1544 models of address expansion for multiple delivery. When a message is 1545 delivered or forwarded to each address of an expanded list form, the 1546 return address in the envelope ("MAIL FROM:") MUST be changed to be 1547 the address of a person or other entity who administers the list. 1548 However, in this case, the message header section (RFC 5322 [12]) 1549 MUST be left unchanged; in particular, the "From" field of the header 1550 section is unaffected. 1552 An important mail facility is a mechanism for multi-destination 1553 delivery of a single message, by transforming (or "expanding" or 1554 "exploding") a pseudo-mailbox address into a list of destination 1555 mailbox addresses. When a message is sent to such a pseudo-mailbox 1556 (sometimes called an "exploder"), copies are forwarded or 1557 redistributed to each mailbox in the expanded list. Servers SHOULD 1558 simply utilize the addresses on the list; application of heuristics 1559 or other matching rules to eliminate some addresses, such as that of 1560 the originator, is strongly discouraged. We classify such a pseudo- 1561 mailbox as an "alias" or a "list", depending upon the expansion 1562 rules. 1564 3.9.1. Alias 1566 To expand an alias, the recipient mailer simply replaces the pseudo- 1567 mailbox address in the envelope with each of the expanded addresses 1568 in turn; the rest of the envelope and the message body are left 1569 unchanged. The message is then delivered or forwarded to each 1570 expanded address. 1572 3.9.2. List 1574 A mailing list may be said to operate by "redistribution" rather than 1575 by "forwarding". To expand a list, the recipient mailer replaces the 1576 pseudo-mailbox address in the envelope with each of the expanded 1577 addresses in turn. The return (backward-pointing) address in the 1578 envelope is changed so that all error messages generated by the final 1579 deliveries will be returned to a list administrator, not to the 1580 message originator, who generally has no control over the contents of 1581 the list and will typically find error messages annoying. Note that 1582 the key difference between handling aliases (Section 3.9.1) and 1583 forwarding (this subsection) is the change to the backward-pointing 1584 address in this case. When a list constrains its processing to the 1585 very limited set of modifications and actions described here, it is 1586 attempting to emulate an MTA; such lists can be treated as a 1587 continuation in email transit. 1589 There exist mailing lists that perform additional, sometimes 1590 extensive, modifications to a message and its envelope. Such mailing 1591 lists need to be viewed as full MUAs, which accept a delivery and 1592 post a new message. 1594 4. The SMTP Specifications 1596 4.1. SMTP Commands 1598 4.1.1. Command Semantics and Syntax 1600 The SMTP commands define the mail transfer or the mail system 1601 function requested by the user. SMTP commands are character strings 1602 terminated by . The commands themselves are alphabetic 1603 characters terminated by if parameters follow and 1604 otherwise. (In the interest of improved interoperability, SMTP 1605 receivers SHOULD tolerate trailing white space before the terminating 1606 .) The syntax of the local part of a mailbox MUST conform to 1607 receiver site conventions and the syntax specified in Section 4.1.2. 1608 The SMTP commands are discussed below. The SMTP replies are 1609 discussed in Section 4.2. 1611 A mail transaction involves several data objects that are 1612 communicated as arguments to different commands. The reverse-path is 1613 the argument of the MAIL command, the forward-path is the argument of 1614 the RCPT command, and the mail data is the argument of the DATA 1615 command. These arguments or data objects must be transmitted and 1616 held, pending the confirmation communicated by the end of mail data 1617 indication that finalizes the transaction. The model for this is 1618 that distinct buffers are provided to hold the types of data objects; 1619 that is, there is a reverse-path buffer, a forward-path buffer, and a 1620 mail data buffer. Specific commands cause information to be appended 1621 to a specific buffer, or cause one or more buffers to be cleared. 1623 Several commands (RSET, DATA, QUIT) are specified as not permitting 1624 parameters. In the absence of specific extensions offered by the 1625 server and accepted by the client, clients MUST NOT send such 1626 parameters and servers SHOULD reject commands containing them as 1627 having invalid syntax. 1629 4.1.1.1. Extended HELLO (EHLO) or HELLO (HELO) 1631 These commands are used to identify the SMTP client to the SMTP 1632 server. The argument clause contains the fully-qualified domain name 1633 of the SMTP client, if one is available. In situations in which the 1634 SMTP client system does not have a meaningful domain name (e.g., when 1635 its address is dynamically allocated and no reverse mapping record is 1636 available), the client SHOULD send an address literal (see 1637 Section 4.1.3). 1639 RFC 2821, and some earlier informal practices, encouraged following 1640 the literal by information that would help to identify the client 1641 system. That convention was not widely supported, and many SMTP 1642 servers considered it an error. In the interest of interoperability, 1643 it is probably wise for servers to be prepared for this string to 1644 occur, but SMTP clients SHOULD NOT send it. 1646 The SMTP server identifies itself to the SMTP client in the 1647 connection greeting reply and in the response to this command. 1649 A client SMTP SHOULD start an SMTP session by issuing the EHLO 1650 command. If the SMTP server supports the SMTP service extensions, it 1651 will give a successful response, a failure response, or an error 1652 response. If the SMTP server, in violation of this specification, 1653 does not support any SMTP service extensions, it will generate an 1654 error response. Older client SMTP systems MAY, as discussed above, 1655 use HELO (as specified in RFC 821) instead of EHLO, and servers MUST 1656 support the HELO command and reply properly to it. In any event, a 1657 client MUST issue HELO or EHLO before starting a mail transaction. 1659 These commands, and a "250 OK" reply to one of them, confirm that 1660 both the SMTP client and the SMTP server are in the initial state, 1661 that is, there is no transaction in progress and all state tables and 1662 buffers are cleared. 1664 Syntax: 1666 ehlo = "EHLO" SP ( Domain / address-literal ) CRLF 1667 helo = "HELO" SP Domain CRLF 1669 Normally, the response to EHLO will be a multiline reply. Each line 1670 of the response contains a keyword and, optionally, one or more 1671 parameters. Following the normal syntax for multiline replies, these 1672 keywords follow the code (250) and a hyphen for all but the last 1673 line, and the code and a space for the last line. The syntax for a 1674 positive response, using the ABNF notation and terminal symbols of 1675 RFC 5234 [11], is: 1677 ehlo-ok-rsp = ( "250" SP Domain [ SP ehlo-greet ] CRLF ) 1678 / ( "250-" Domain [ SP ehlo-greet ] CRLF 1679 *( "250-" ehlo-line CRLF ) 1680 "250" SP ehlo-line CRLF ) 1682 ehlo-greet = 1*(%d0-9 / %d11-12 / %d14-127) 1683 ; string of any characters other than CR or LF 1685 ehlo-line = ehlo-keyword *( SP ehlo-param ) 1687 ehlo-keyword = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-") 1688 ; additional syntax of ehlo-params depends on 1689 ; ehlo-keyword 1691 ehlo-param = 1*(%d33-126) 1692 ; any CHAR excluding and all 1693 ; control characters (US-ASCII 0-31 and 127 1694 ; inclusive) 1696 Although EHLO keywords may be specified in upper, lower, or mixed 1697 case, they MUST always be recognized and processed in a case- 1698 insensitive manner. This is simply an extension of practices 1699 specified in RFC 821 and Section 2.4. 1701 The EHLO response MUST contain keywords (and associated parameters if 1702 required) for all commands not listed as "required" in Section 4.5.1 1703 excepting only private-use commands as described in Section 4.1.5. 1704 Private-use commands MAY be listed. 1706 4.1.1.2. MAIL (MAIL) 1708 This command is used to initiate a mail transaction in which the mail 1709 data is delivered to an SMTP server that may, in turn, deliver it to 1710 one or more mailboxes or pass it on to another system (possibly using 1711 SMTP). The argument clause contains a reverse-path and may contain 1712 optional parameters. In general, the MAIL command may be sent only 1713 when no mail transaction is in progress, see Section 4.1.4. 1715 The reverse-path consists of the sender mailbox. Historically, that 1716 mailbox might optionally have been preceded by a list of hosts, but 1717 that behavior is now deprecated (see Appendix C). In some types of 1718 reporting messages for which a reply is likely to cause a mail loop 1719 (for example, mail delivery and non-delivery notifications), the 1720 reverse-path may be null (see Section 3.6). 1722 This command clears the reverse-path buffer, the forward-path buffer, 1723 and the mail data buffer, and it inserts the reverse-path information 1724 from its argument clause into the reverse-path buffer. 1726 If service extensions were negotiated, the MAIL command may also 1727 carry parameters associated with a particular service extension. 1729 Syntax: 1731 mail = "MAIL FROM:" Reverse-path 1732 [SP Mail-parameters] CRLF 1734 4.1.1.3. RECIPIENT (RCPT) 1736 This command is used to identify an individual recipient of the mail 1737 data; multiple recipients are specified by multiple uses of this 1738 command. The argument clause contains a forward-path and may contain 1739 optional parameters. 1741 The forward-path normally consists of the required destination 1742 mailbox. Sending systems SHOULD NOT generate the optional list of 1743 hosts known as a source route. Receiving systems MUST recognize 1744 source route syntax but SHOULD strip off the source route 1745 specification and utilize the domain name associated with the mailbox 1746 as if the source route had not been provided. 1748 Similarly, relay hosts SHOULD strip or ignore source routes, and 1749 names MUST NOT be copied into the reverse-path. When mail reaches 1750 its ultimate destination (the forward-path contains only a 1751 destination mailbox), the SMTP server inserts it into the destination 1752 mailbox in accordance with its host mail conventions. 1754 This command appends its forward-path argument to the forward-path 1755 buffer; it does not change the reverse-path buffer nor the mail data 1756 buffer. 1758 For example, mail received at relay host xyz.com with envelope 1759 commands 1761 MAIL FROM: 1762 RCPT TO:<@hosta.int,@jkl.org:userc@d.bar.org> 1764 will normally be sent directly on to host d.bar.org with envelope 1765 commands 1767 MAIL FROM: 1768 RCPT TO: 1770 As provided in Appendix C, xyz.com MAY also choose to relay the 1771 message to hosta.int, using the envelope commands 1773 MAIL FROM: 1774 RCPT TO:<@hosta.int,@jkl.org:userc@d.bar.org> 1776 or to jkl.org, using the envelope commands 1778 MAIL FROM: 1779 RCPT TO:<@jkl.org:userc@d.bar.org> 1781 Attempting to use relaying this way is now strongly discouraged. 1782 Since hosts are not required to relay mail at all, xyz.com MAY also 1783 reject the message entirely when the RCPT command is received, using 1784 a 550 code (since this is a "policy reason"). 1786 If service extensions were negotiated, the RCPT command may also 1787 carry parameters associated with a particular service extension 1788 offered by the server. The client MUST NOT transmit parameters other 1789 than those associated with a service extension offered by the server 1790 in its EHLO response. 1792 Syntax: 1794 rcpt = "RCPT TO:" ( "" / "" / 1795 Forward-path ) [SP Rcpt-parameters] CRLF 1797 Note that, in a departure from the usual rules for 1798 local-parts, the "Postmaster" string shown above is 1799 treated as case-insensitive. 1801 4.1.1.4. DATA (DATA) 1803 The receiver normally sends a 354 response to DATA, and then treats 1804 the lines (strings ending in sequences, as described in 1805 Section 2.3.7) following the command as mail data from the sender. 1806 This command causes the mail data to be appended to the mail data 1807 buffer. The mail data may contain any of the 128 ASCII character 1808 codes, although experience has indicated that use of control 1809 characters other than SP, HT, CR, and LF may cause problems and 1810 SHOULD be avoided when possible. 1812 The mail data are terminated by a line containing only a period, that 1813 is, the character sequence ".", where the first is 1814 actually the terminator of the previous line (see Section 4.5.2). 1815 This is the end of mail data indication. The first of this 1816 terminating sequence is also the that ends the final line of 1817 the data (message text) or, if there was no mail data, ends the DATA 1818 command itself (the "no mail data" case does not conform to this 1819 specification since it would require that neither the trace header 1820 fields required by this specification nor the message header section 1821 required by RFC 5322 [12] be transmitted). An extra MUST NOT 1822 be added, as that would cause an empty line to be added to the 1823 message. The only exception to this rule would arise if the message 1824 body were passed to the originating SMTP-sender with a final "line" 1825 that did not end in ; in that case, the originating SMTP system 1826 MUST either reject the message as invalid or add in order to 1827 have the receiving SMTP server recognize the "end of data" condition. 1829 The custom of accepting lines ending only in , as a concession to 1830 non-conforming behavior on the part of some UNIX systems, has proven 1831 to cause more interoperability problems than it solves, and SMTP 1832 server systems MUST NOT do this, even in the name of improved 1833 robustness. In particular, the sequence "." (bare line 1834 feeds, without carriage returns) MUST NOT be treated as equivalent to 1835 . as the end of mail data indication. 1837 Receipt of the end of mail data indication requires the server to 1838 process the stored mail transaction information. This processing 1839 consumes the information in the reverse-path buffer, the forward-path 1840 buffer, and the mail data buffer, and on the completion of this 1841 command these buffers are cleared. If the processing is successful, 1842 the receiver MUST send an OK reply. If the processing fails, the 1843 receiver MUST send a failure reply. The SMTP model does not allow 1844 for partial failures at this point: either the message is accepted by 1845 the server for delivery and a positive response is returned or it is 1846 not accepted and a failure reply is returned. In sending a positive 1847 "250 OK" completion reply to the end of data indication, the receiver 1848 takes full responsibility for the message (see Section 6.1). Errors 1849 that are diagnosed subsequently MUST be reported in a mail message, 1850 as discussed in Section 4.4. 1852 When the SMTP server accepts a message either for relaying or for 1853 final delivery, it inserts a trace record (also referred to 1854 interchangeably as a "time stamp line" or "Received" line) at the top 1855 of the mail data. This trace record indicates the identity of the 1856 host that sent the message, the identity of the host that received 1857 the message (and is inserting this time stamp), and the date and time 1858 the message was received. Relayed messages will have multiple time 1859 stamp lines. Details for formation of these lines, including their 1860 syntax, is specified in Section 4.4. 1862 Additional discussion about the operation of the DATA command appears 1863 in Section 3.3. 1865 Syntax: 1867 data = "DATA" CRLF 1869 4.1.1.5. RESET (RSET) 1871 This command specifies that the current mail transaction will be 1872 aborted. Any stored sender, recipients, and mail data MUST be 1873 discarded, and all buffers and state tables cleared. The receiver 1874 MUST send a "250 OK" reply to a RSET command with no arguments. A 1875 reset command may be issued by the client at any time. It is 1876 effectively equivalent to a NOOP (i.e., it has no effect) if issued 1877 immediately after EHLO, before EHLO is issued in the session, after 1878 an end of data indicator has been sent and acknowledged, or 1879 immediately before a QUIT. An SMTP server MUST NOT close the 1880 connection as the result of receiving a RSET; that action is reserved 1881 for QUIT (see Section 4.1.1.10). 1883 Since EHLO implies some additional processing and response by the 1884 server, RSET will normally be more efficient than reissuing that 1885 command, even though the formal semantics are the same. 1887 Syntax: 1889 rset = "RSET" CRLF 1891 4.1.1.6. VERIFY (VRFY) 1893 This command asks the receiver to confirm that the argument 1894 identifies a user or mailbox. If it is a user name, information is 1895 returned as specified in Section 3.5. 1897 This command has no effect on the reverse-path buffer, the forward- 1898 path buffer, or the mail data buffer. 1900 Syntax: 1902 vrfy = "VRFY" SP String CRLF 1904 4.1.1.7. EXPAND (EXPN) 1906 This command asks the receiver to confirm that the argument 1907 identifies a mailing list, and if so, to return the membership of 1908 that list. If the command is successful, a reply is returned 1909 containing information as described in Section 3.5. This reply will 1910 have multiple lines except in the trivial case of a one-member list. 1912 This command has no effect on the reverse-path buffer, the forward- 1913 path buffer, or the mail data buffer, and it may be issued at any 1914 time. 1916 Syntax: 1918 expn = "EXPN" SP String CRLF 1920 4.1.1.8. HELP (HELP) 1922 This command causes the server to send helpful information to the 1923 client. The command MAY take an argument (e.g., any command name) 1924 and return more specific information as a response. 1926 This command has no effect on the reverse-path buffer, the forward- 1927 path buffer, or the mail data buffer, and it may be issued at any 1928 time. 1930 SMTP servers SHOULD support HELP without arguments and MAY support it 1931 with arguments. 1933 Syntax: 1935 help = "HELP" [ SP String ] CRLF 1937 4.1.1.9. NOOP (NOOP) 1939 This command does not affect any parameters or previously entered 1940 commands. It specifies no action other than that the receiver send a 1941 "250 OK" reply. 1943 This command has no effect on the reverse-path buffer, the forward- 1944 path buffer, or the mail data buffer, and it may be issued at any 1945 time. If a parameter string is specified, servers SHOULD ignore it. 1947 Syntax: 1949 noop = "NOOP" [ SP String ] CRLF 1951 4.1.1.10. QUIT (QUIT) 1953 This command specifies that the receiver MUST send a "221 OK" reply, 1954 and then close the transmission channel. 1956 The receiver MUST NOT intentionally close the transmission channel 1957 until it receives and replies to a QUIT command (even if there was an 1958 error). The sender MUST NOT intentionally close the transmission 1959 channel until it sends a QUIT command, and it SHOULD wait until it 1960 receives the reply (even if there was an error response to a previous 1961 command). If the connection is closed prematurely due to violations 1962 of the above or system or network failure, the server MUST cancel any 1963 pending transaction, but not undo any previously completed 1964 transaction, and generally MUST act as if the command or transaction 1965 in progress had received a temporary error (i.e., a 4yz response). 1967 The QUIT command may be issued at any time. Any current uncompleted 1968 mail transaction will be aborted. 1970 Syntax: 1972 quit = "QUIT" CRLF 1974 4.1.1.11. Mail-Parameter and Rcpt-Parameter Error Responses 1976 If the server SMTP does not recognize or cannot implement one or more 1977 of the parameters associated with a particular MAIL or RCPT command, 1978 it will return code 555. 1980 If, for some reason, the server is temporarily unable to accommodate 1981 one or more of the parameters associated with a MAIL or RCPT command, 1982 and if the definition of the specific parameter does not mandate the 1983 use of another code, it should return code 455. 1985 Errors specific to particular parameters and their values will be 1986 specified in the parameter's defining RFC. 1988 4.1.2. Command Argument Syntax 1990 The syntax of the argument clauses of the above commands (using the 1991 syntax specified in RFC 5234 [11] where applicable) is given below. 1992 Some of the productions given below are used only in conjunction with 1993 source routes as described in Appendix C. Some terminals not defined 1994 in this document, but are defined elsewhere, specifically: 1996 In the "core" syntax in Appendix B of RFC 5234 [11]: ALPHA , CRLF 1997 , DIGIT , HEXDIG , and SP 1998 In the message format syntax in RFC 5322 [12]: atext , CFWS , and 1999 FWS . 2001 Reverse-path = Path / "<>" 2003 Forward-path = Path 2005 Path = "<" [ A-d-l ":" ] Mailbox ">" 2007 A-d-l = At-domain *( "," At-domain ) 2008 ; Note that this form, the so-called "source 2009 ; route", MUST BE accepted, SHOULD NOT be 2010 ; generated, and SHOULD be ignored. 2012 At-domain = "@" Domain 2014 Mail-parameters = esmtp-param *(SP esmtp-param) 2016 Rcpt-parameters = esmtp-param *(SP esmtp-param) 2018 esmtp-param = esmtp-keyword ["=" esmtp-value] 2020 esmtp-keyword = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-") 2022 esmtp-value = 1*(%d33-60 / %d62-126) 2023 ; any CHAR excluding "=", SP, and control 2024 ; characters. If this string is an email address, 2025 ; i.e., a Mailbox, then the "xtext" syntax [34] 2026 ; SHOULD be used. 2028 Keyword = Ldh-str 2030 Argument = Atom 2032 Domain = sub-domain *("." sub-domain) 2034 sub-domain = Let-dig [Ldh-str] 2036 Let-dig = ALPHA / DIGIT 2038 Ldh-str = *( ALPHA / DIGIT / "-" ) Let-dig 2040 address-literal = "[" ( IPv4-address-literal / 2041 IPv6-address-literal / 2042 General-address-literal ) "]" 2043 ; See Section 4.1.3 2045 Mailbox = Local-part "@" ( Domain / address-literal ) 2046 Local-part = Dot-string / Quoted-string 2047 ; MAY be case-sensitive 2049 Dot-string = Atom *("." Atom) 2051 Atom = 1*atext 2053 Quoted-string = DQUOTE 1*QcontentSMTP DQUOTE 2055 QcontentSMTP = qtextSMTP / quoted-pairSMTP 2057 quoted-pairSMTP = %d92 %d32-126 2058 ; i.e., backslash followed by any ASCII 2059 ; graphic (including itself) or SPace 2061 qtextSMTP = %d32-33 / %d35-91 / %d93-126 2062 ; i.e., within a quoted string, any 2063 ; ASCII graphic or space is permitted 2064 ; without backslash-quoting except 2065 ; double-quote and the backslash itself. 2067 String = Atom / Quoted-string 2069 While the above definition for Local-part is relatively permissive, 2070 for maximum interoperability, a host that expects to receive mail 2071 SHOULD avoid defining mailboxes where the Local-part requires (or 2072 uses) the Quoted-string form or where the Local-part is case- 2073 sensitive. For any purposes that require generating or comparing 2074 Local-parts (e.g., to specific mailbox names), all quoted forms MUST 2075 be treated as equivalent, and the sending system SHOULD transmit the 2076 form that uses the minimum quoting possible. 2078 Systems MUST NOT define mailboxes in such a way as to require the use 2079 in SMTP of non-ASCII characters (octets with the high order bit set 2080 to one) or ASCII "control characters" (decimal value 0-31 and 127). 2081 These characters MUST NOT be used in MAIL or RCPT commands or other 2082 commands that require mailbox names. 2084 Note that the backslash, "\", is a quote character, which is used to 2085 indicate that the next character is to be used literally (instead of 2086 its normal interpretation). For example, "Joe\,Smith" indicates a 2087 single nine-character user name string with the comma being the 2088 fourth character of that string. 2090 To promote interoperability and consistent with long-standing 2091 guidance about conservative use of the DNS in naming and applications 2092 (e.g., see Section 2.3.1 of the base DNS document, RFC 1035 [4]), 2093 characters outside the set of alphabetic characters, digits, and 2094 hyphen MUST NOT appear in domain name labels for SMTP clients or 2095 servers. In particular, the underscore character is not permitted. 2096 SMTP servers that receive a command in which invalid character codes 2097 have been employed, and for which there are no other reasons for 2098 rejection, MUST reject that command with a 501 response (this rule, 2099 like others, could be overridden by appropriate SMTP extensions). 2101 4.1.3. Address Literals 2103 Sometimes a host is not known to the domain name system and 2104 communication (and, in particular, communication to report and repair 2105 the error) is blocked. To bypass this barrier, a special literal 2106 form of the address is allowed as an alternative to a domain name. 2107 For IPv4 addresses, this form uses four small decimal integers 2108 separated by dots and enclosed by brackets such as [123.255.37.2], 2109 which indicates an (IPv4) Internet Address in sequence-of-octets 2110 form. For IPv6 and other forms of addressing that might eventually 2111 be standardized, the form consists of a standardized "tag" that 2112 identifies the address syntax, a colon, and the address itself, in a 2113 format specified as part of the relevant standards (i.e., RFC 4291 2114 [10] for IPv6). 2115 [[CREF12: [5321bis] Proposed erratum 4315 (2015-03-27) suggests yet 2116 another modification to the IPv6 address literal syntax, based on 2117 part on RFC 5952. We should consider whether those, or other, 2118 modifications are appropriate and/or whether, given both the issues 2119 of spam/malware and servers supporting multiple domains, it it time 2120 to deprecate mailboxes containing address literals entirely (EHLO 2121 fields may be a different issue). If we are going to allow IPv6 2122 address literals, it may be time to incorporate something by 2123 reference rather than including specific syntax here (RFC 5952 is 14 2124 pages long and does not contain any ABNF).]] 2126 Specifically: 2128 IPv4-address-literal = Snum 3("." Snum) 2130 IPv6-address-literal = "IPv6:" IPv6-addr 2132 General-address-literal = Standardized-tag ":" 1*dcontent 2134 Standardized-tag = Ldh-str 2135 ; Standardized-tag MUST be specified in a 2136 ; Standards-Track RFC and registered with IANA 2138 dcontent = %d33-90 / ; Printable US-ASCII 2139 %d94-126 ; excl. "[", "\", "]" 2141 Snum = 1*3DIGIT 2142 ; representing a decimal integer 2143 ; value in the range 0 through 255 2145 IPv6-addr = 6( h16 ":" ) ls32 2146 / "::" 5( h16 ":" ) ls32 2147 / [ h16 ] "::" 4( h16 ":" ) ls32 2148 / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32 2149 / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32 2150 / [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32 2151 / [ *4( h16 ":" ) h16 ] "::" ls32 2152 / [ *5( h16 ":" ) h16 ] "::" h16 2153 / [ *6( h16 ":" ) h16 ] "::" 2154 ; This definition is consistent with the one for 2155 ; URIs [40]. 2157 ls32 = ( h16 ":" h16 ) / IPv4address 2158 ; least-significant 32 bits of address 2160 h16 = 1*4HEXDIG 2161 ; 16 bits of address represented in hexadecimal 2163 4.1.4. Order of Commands 2165 There are restrictions on the order in which these commands may be 2166 used. 2168 A session that will contain mail transactions MUST first be 2169 initialized by the use of the EHLO command. An SMTP server SHOULD 2170 accept commands for non-mail transactions (e.g., VRFY, EXPN, or NOOP) 2171 without this initialization. 2173 An EHLO command MAY be issued by a client later in the session. If 2174 it is issued after the session begins and the EHLO command is 2175 acceptable to the SMTP server, the SMTP server MUST clear all buffers 2176 and reset the state exactly as if a RSET command had been issued 2177 (specifically, it terminates any mail transaction that was in 2178 progress, see Section 3.3). In other words, the sequence of RSET 2179 followed immediately by EHLO is redundant, but not harmful other than 2180 in the performance cost of executing unnecessary commands. However 2181 the response to an additional EHLO command MAY be different from that 2182 from prior ones; the client MUST rely only on the responses from the 2183 most recent EHLO command. 2185 If the EHLO command is not acceptable to the SMTP server, 501, 500, 2186 502, or 550 failure replies MUST be returned as appropriate. The 2187 SMTP server MUST stay in the same state after transmitting these 2188 replies that it was in before the EHLO was received. 2190 The SMTP client MUST, if possible, ensure that the domain parameter 2191 to the EHLO command is a primary host name as specified for this 2192 command in Section 2.3.5. If this is not possible (e.g., when the 2193 client's address is dynamically assigned and the client does not have 2194 an obvious name), an address literal SHOULD be substituted for the 2195 domain name. 2197 An SMTP server MAY verify that the domain name argument in the EHLO 2198 command actually corresponds to the IP address of the client. 2199 [[CREF13: [5321bis] [[Note in draft -- proposed change to "An SMTP 2200 server MAY verify that the domain name argument in the EHLO command 2201 has an address record matching the IP address of the client." ]] 2202 However, if the verification fails, the server MUST NOT refuse to 2203 accept a message on that basis. Information captured in the 2204 verification attempt is for logging and tracing purposes. Note that 2205 this prohibition applies to the matching of the parameter to its IP 2206 address only; see Section 7.9 for a more extensive discussion of 2207 rejecting incoming connections or mail messages. 2209 [[CREF14: JcK 20210822: Whatever is done wit the above, it will 2210 ultimately need a pointer to more discussion in the A/S.]] 2212 The NOOP, HELP, EXPN, VRFY, and RSET commands can be used at any time 2213 during a session, or without previously initializing a session. SMTP 2214 servers SHOULD process these normally (that is, not return a 503 2215 code) even if no EHLO command has yet been received; clients SHOULD 2216 open a session with EHLO before sending these commands. 2218 If these rules are followed, the example in RFC 821 that shows "550 2219 access denied to you" in response to an EXPN command is incorrect 2220 unless an EHLO command precedes the EXPN or the denial of access is 2221 based on the client's IP address or other authentication or 2222 authorization-determining mechanisms. 2224 A mail transaction begins with a MAIL command and then consists of 2225 one or more RCPT commands, and a DATA command, in that order. A mail 2226 transaction may be aborted by the RSET, a new EHLO, or the QUIT 2227 command. 2229 SMTP extensions (see Section 2.2) may create additional commands that 2230 initiate, abort, or end the transaction.More generally, any new 2231 command MUST clearly document any effect it has on the transaction 2232 state. 2234 There may be zero or more transactions in a session. MAIL MUST NOT 2235 be sent if a mail transaction is already open, i.e., it should be 2236 sent only if no mail transaction had been started in the session, or 2237 if the previous one successfully concluded with a successful DATA 2238 command, or if the previous one was aborted, e.g., with a RSET or new 2239 EHLO. [[CREF15: [5321bis] See comment about changing this convoluted 2240 discussion to talk about 'mail transaction' above. --Jck (and see 2241 Ticket #11 correspondence with Alexey 2021-07-06)]] 2243 If the transaction beginning command argument is not acceptable, a 2244 501 failure reply MUST be returned and the SMTP server MUST stay in 2245 the same state. If the commands in a transaction are out of order to 2246 the degree that they cannot be processed by the server, a 503 failure 2247 reply MUST be returned and the SMTP server MUST stay in the same 2248 state. 2250 The last command in a session MUST be the QUIT command. The QUIT 2251 command SHOULD be used by the client SMTP to request connection 2252 closure, even when no session opening command was sent and accepted. 2254 4.1.5. Private-Use Commands 2256 As specified in Section 2.2.2, commands starting in "X" may be used 2257 by bilateral agreement between the client (sending) and server 2258 (receiving) SMTP agents. An SMTP server that does not recognize such 2259 a command is expected to reply with "500 Command not recognized". An 2260 extended SMTP server MAY list the feature names associated with these 2261 private commands in the response to the EHLO command. 2263 Commands sent or accepted by SMTP systems that do not start with "X" 2264 MUST conform to the requirements of Section 2.2.2. 2266 4.2. SMTP Replies 2268 Replies to SMTP commands serve to ensure the synchronization of 2269 requests and actions in the process of mail transfer and to guarantee 2270 that the SMTP client always knows the state of the SMTP server. 2271 Every command MUST generate exactly one reply. 2273 The details of the command-reply sequence are described in 2274 Section 4.3. 2276 An SMTP reply consists of a three digit number (transmitted as three 2277 numeric characters) followed by some text unless specified otherwise 2278 in this document. The number is for use by automata to determine 2279 what state to enter next; the text is for the human user. The three 2280 digits contain enough encoded information that the SMTP client need 2281 not examine the text and may either discard it or pass it on to the 2282 user, as appropriate. Exceptions are as noted elsewhere in this 2283 document. In particular, the 220, 221, 251, 421, and 551 reply codes 2284 are associated with message text that must be parsed and interpreted 2285 by machines. In the general case, the text may be receiver dependent 2286 and context dependent, so there are likely to be varying texts for 2287 each reply code. A discussion of the theory of reply codes is given 2288 in Section 4.2.1. Formally, a reply is defined to be the sequence: a 2289 three-digit code, , one line of text, and , or a multiline 2290 reply (as defined in the same section). Since, in violation of this 2291 specification, the text is sometimes not sent, clients that do not 2292 receive it SHOULD be prepared to process the code alone (with or 2293 without a trailing space character). Only the EHLO, EXPN, and HELP 2294 commands are expected to result in multiline replies in normal 2295 circumstances; however, multiline replies are allowed for any 2296 command. 2298 In ABNF, server responses are: 2300 Greeting = ( "220 " (Domain / address-literal) 2301 [ SP textstring ] CRLF ) / 2302 ( "220-" (Domain / address-literal) 2303 [ SP textstring ] CRLF 2304 *( "220-" [ textstring ] CRLF ) 2305 "220" [ SP textstring ] CRLF ) 2307 textstring = 1*(%d09 / %d32-126) ; HT, SP, Printable US-ASCII 2309 Reply-line = *( Reply-code "-" [ textstring ] CRLF ) 2310 Reply-code [ SP textstring ] CRLF 2312 Reply-code = %x32-35 %x30-35 %x30-39 2314 where "Greeting" appears only in the 220 response that announces that 2315 the server is opening its part of the connection. (Other possible 2316 server responses upon connection follow the syntax of Reply-line.) 2318 An SMTP server SHOULD send only the reply codes listed in this 2319 document or additions to the list as discussed below. An SMTP server 2320 SHOULD use the text shown in the examples whenever appropriate. 2322 An SMTP client MUST determine its actions only by the reply code, not 2323 by the text (except for the "change of address" 251 and 551 and, if 2324 necessary, 220, 221, and 421 replies); in the general case, any text, 2325 including no text at all (although senders SHOULD NOT send bare 2326 codes), MUST be acceptable. The space (blank) following the reply 2327 code is considered part of the text. A Sender-SMTP MUST first test 2328 the whole 3 digit reply code it receives, as well as any accompanying 2329 supplemental codes or information (see RFC 3463 [7] and RFC 5248 2330 [46]). If the full reply code is not recognized, and the additional 2331 information is not recognized or missing, the Sender-SMTP MUST use 2332 the first digit (severity indication) of a reply code it receives. 2334 The list of codes that appears below MUST NOT be construed as 2335 permanent. While the addition of new codes should be a rare and 2336 significant activity, with supplemental information in the textual 2337 part of the response (including enhanced status codes [7] and the 2338 successors to that specification) being preferred, new codes may be 2339 added as the result of new Standards or Standards-Track 2340 specifications. Consequently, a sender-SMTP MUST be prepared to 2341 handle codes not specified in this document and MUST do so by 2342 interpreting the first digit only. 2344 In the absence of extensions negotiated with the client, SMTP servers 2345 MUST NOT send reply codes whose first digits are other than 2, 3, 4, 2346 or 5. Clients that receive such out-of-range codes SHOULD normally 2347 treat them as fatal errors and terminate the mail transaction. 2349 4.2.1. Reply Code Severities and Theory 2351 The three digits of the reply each have a special significance. The 2352 first digit denotes whether the response is good, bad, or incomplete. 2353 An unsophisticated SMTP client, or one that receives an unexpected 2354 code, will be able to determine its next action (proceed as planned, 2355 redo, retrench, etc.) by examining this first digit. An SMTP client 2356 that wants to know approximately what kind of error occurred (e.g., 2357 mail system error, command syntax error) may examine the second 2358 digit. The third digit and any supplemental information that may be 2359 present is reserved for the finest gradation of information. 2361 There are four values for the first digit of the reply code: 2363 2yz Positive Completion reply 2364 The requested action has been successfully completed. A new 2365 request may be initiated. 2367 3yz Positive Intermediate reply 2368 The command has been accepted, but the requested action is being 2369 held in abeyance, pending receipt of further information. The 2370 SMTP client should send another command specifying this 2371 information. This reply is used in command sequence groups (i.e., 2372 in DATA). 2374 4yz Transient Negative Completion reply 2375 The command was not accepted, and the requested action did not 2376 occur. However, the error condition is temporary, and the action 2377 may be requested again. The sender should return to the beginning 2378 of the command sequence (if any). It is difficult to assign a 2379 meaning to "transient" when two different sites (receiver- and 2380 sender-SMTP agents) must agree on the interpretation. Each reply 2381 in this category might have a different time value, but the SMTP 2382 client SHOULD try again. A rule of thumb to determine whether a 2383 reply fits into the 4yz or the 5yz category (see below) is that 2384 replies are 4yz if they can be successful if repeated without any 2385 change in command form or in properties of the sender or receiver 2386 (that is, the command is repeated identically and the receiver 2387 does not put up a new implementation). 2389 5yz Permanent Negative Completion reply 2390 The command was not accepted and the requested action did not 2391 occur. The SMTP client SHOULD NOT repeat the exact request (in 2392 the same sequence). Even some "permanent" error conditions can be 2393 corrected, so the human user may want to direct the SMTP client to 2394 reinitiate the command sequence by direct action at some point in 2395 the future (e.g., after the spelling has been changed, or the user 2396 has altered the account status). 2398 It is worth noting that the file transfer protocol (FTP) [15] uses a 2399 very similar code architecture and that the SMTP codes are based on 2400 the FTP model. However, SMTP uses a one-command, one-response model 2401 (while FTP is asynchronous) and FTP's 1yz codes are not part of the 2402 SMTP model. 2404 The second digit encodes responses in specific categories: 2406 x0z Syntax: These replies refer to syntax errors, syntactically 2407 correct commands that do not fit any functional category, and 2408 unimplemented or superfluous commands. 2410 x1z Information: These are replies to requests for information, such 2411 as status or help. 2413 x2z Connections: These are replies referring to the transmission 2414 channel. 2416 x3z Unspecified. 2418 x4z Unspecified. 2420 x5z Mail system: These replies indicate the status of the receiver 2421 mail system vis-a-vis the requested transfer or other mail system 2422 action. 2424 The third digit gives a finer gradation of meaning in each category 2425 specified by the second digit. The list of replies illustrates this. 2426 Each reply text is recommended rather than mandatory, and may even 2427 change according to the command with which it is associated. On the 2428 other hand, the reply codes must strictly follow the specifications 2429 in this section. Receiver implementations should not invent new 2430 codes for slightly different situations from the ones described here, 2431 but rather adapt codes already defined. 2433 For example, a command such as NOOP, whose successful execution does 2434 not offer the SMTP client any new information, will return a 250 2435 reply. The reply is 502 when the command requests an unimplemented 2436 non-site-specific action. A refinement of that is the 504 reply for 2437 a command that is implemented, but that requests an unimplemented 2438 parameter. 2440 The reply text may be longer than a single line; in these cases the 2441 complete text must be marked so the SMTP client knows when it can 2442 stop reading the reply. This requires a special format to indicate a 2443 multiple line reply. 2445 The format for multiline replies requires that every line, except the 2446 last, begin with the reply code, followed immediately by a hyphen, 2447 "-" (also known as minus), followed by text. The last line will 2448 begin with the reply code, followed immediately by , optionally 2449 some text, and . As noted above, servers SHOULD send the 2450 if subsequent text is not sent, but clients MUST be prepared for it 2451 to be omitted. 2453 For example: 2455 250-First line 2456 250-Second line 2457 250-234 Text beginning with numbers 2458 250 The last line 2460 In a multiline reply, the reply code on each of the lines MUST be the 2461 same. It is reasonable for the client to rely on this, so it can 2462 make processing decisions based on the code in any line, assuming 2463 that all others will be the same. In a few cases, there is important 2464 data for the client in the reply "text". The client will be able to 2465 identify these cases from the current context. 2467 4.2.2. Reply Codes by Function Groups 2469 500 Syntax error, command unrecognized (This may include errors such 2470 as command line too long) 2472 501 Syntax error in parameters or arguments 2474 502 Command not implemented (see Section 4.2.4.1) 2476 503 Bad sequence of commands 2477 504 Command parameter not implemented 2479 211 System status, or system help reply 2481 214 Help message (Information on how to use the receiver or the 2482 meaning of a particular non-standard command; this reply is useful 2483 only to the human user) 2485 220 Service ready 2487 221 Service closing transmission channel 2489 421 Service not available, closing transmission channel 2490 (This may be a reply to any command if the service knows it must 2491 shut down) 2493 521 No mail service here. 2495 556 No mail service at this domain. 2497 250 Requested mail action okay, completed 2499 251 User not local; will forward to (See Section 3.4) 2501 252 Cannot VRFY user, but will accept message and attempt delivery 2502 (See Section 3.5.3) 2504 455 Server unable to accommodate parameters 2506 555 MAIL FROM/RCPT TO parameters not recognized or not implemented 2508 450 Requested mail action not taken: mailbox unavailable (e.g., 2509 mailbox busy or temporarily blocked for policy reasons) 2511 550 Requested action not taken: mailbox unavailable (e.g., mailbox 2512 not found, no access, or command rejected for policy reasons) 2514 451 Requested action aborted: error in processing 2516 551 User not local; please try (See Section 3.4) 2518 452 Requested action not taken: insufficient system storage 2519 (preferred code for "too many recipients", see Section 4.5.3.1.10) 2521 552 Requested mail action aborted: exceeded storage allocation. 2523 553 Requested action not taken: mailbox name not allowed (e.g., 2524 mailbox syntax incorrect) 2526 354 Start mail input; end with . 2528 554 Transaction failed (Or, historically in the case of a 2529 connection-opening response, "No SMTP service here". 521 is now 2530 preferred for that function at connection-opening if the server 2531 never accepts mail.) 2532 [[CREF16: [5321bis] [[Note in Draft: Revise above statement in the 2533 light of new 521 code?? -- revised with rfc5321bis-04]] ]] 2535 4.2.3. Reply Codes in Numeric Order 2537 211 System status, or system help reply 2539 214 Help message (Information on how to use the receiver or the 2540 meaning of a particular non-standard command; this reply is useful 2541 only to the human user) 2543 220 Service ready 2545 221 Service closing transmission channel 2547 250 Requested mail action okay, completed 2549 251 User not local; will forward to (See Section 3.4) 2551 252 Cannot VRFY user, but will accept message and attempt delivery 2552 (See Section 3.5.3) 2554 354 Start mail input; end with . 2556 421 Service not available, closing transmission channel 2557 (This may be a reply to any command if the service knows it must 2558 shut down) 2560 450 Requested mail action not taken: mailbox unavailable (e.g., 2561 mailbox busy or temporarily blocked for policy reasons) 2563 451 Requested action aborted: local error in processing 2565 452 Requested action not taken: insufficient system storage (also 2566 preferred code for "too many recipients", see Section 4.5.3.1.10) 2568 455 Server unable to accommodate parameters 2570 500 Syntax error, command unrecognized (This may include errors such 2571 as command line too long) 2573 501 Syntax error in parameters or arguments 2575 502 Command not implemented (see Section 4.2.4.1) 2577 503 Bad sequence of commands 2579 504 Command parameter not implemented 2581 521 No mail service (See Section 4.2.4.2.) 2583 550 Requested action not taken: mailbox unavailable (e.g., mailbox 2584 not found, no access, or command rejected for policy reasons) 2586 551 User not local; please try (See Section 3.4) 2588 552 Requested mail action aborted: exceeded storage allocation. 2590 553 Requested action not taken: mailbox name not allowed (e.g., 2591 mailbox syntax incorrect) 2593 554 Transaction failed (Or, in the case of a connection-opening 2594 response, "No SMTP service here" although 521 is now preferred for 2595 the latter. See Section 4.2.4.2.) 2597 555 MAIL FROM/RCPT TO parameters not recognized or not implemented 2599 556 No mail service at this domain. (See Section 4.2.4.2.) 2601 4.2.4. Some specific code situations and relationships 2603 4.2.4.1. Reply Code 502 2605 Questions have been raised as to when reply code 502 (Command not 2606 implemented) SHOULD be returned in preference to other codes. 502 2607 SHOULD be used when the command is actually recognized by the SMTP 2608 server, but not implemented. If the command is not recognized, code 2609 500 SHOULD be returned. Extended SMTP systems MUST NOT list 2610 capabilities in response to EHLO for which they will return 502 (or 2611 500) replies. 2613 4.2.4.2. "No mail accepted" situations and the 521, 554, and 556 codes 2615 Codes 521, 554, and 556 are all used to report different types of "no 2616 mail accepted" situations. They differ as follows. 521 is an 2617 indication from a system answering on the SMTP port that it does not 2618 support SMTP service (a so-called "dummy server" as discussed in RFC 2619 7504 [48] and elsewhere). Obviously, it requires that system exist 2620 and that a connection can be made successfully to it. Because a 2621 system that does not accept any mail cannot meaningfully accept a 2622 RCPT command, any commands (other than QUIT) issued after an SMTP 2623 server has issued a 521 reply are client (sender) errors. 2625 When a domain does not intend to accept mail and wishes to publish 2626 that fact rather than being subjected to connection attempts, the 2627 best way to accomplish that is to use the "Null MX" convention. This 2628 is done by advertising a single MX RR (see Section 3.3.9 of (RFC 1035 2629 [4]) with an RDATA section consisting of preference number 0 and a 2630 zero-length label, written in master files as ".", as the exchange 2631 domain, to denote that there exists no mail exchanger for that 2632 domain. Reply code 556 is then used by a message submission or 2633 intermediate SMTP system (see Section 1.1) to report that it cannot 2634 forward the message further because it knows from the DNS entry that 2635 the recipient domain does not accept mail. If, despite publishing 2636 the DNS entry, the server domain chooses to respond on the SMTP port, 2637 it SHOULD respond with the 566 code as well. The details of the Null 2638 MX convention were first defined in RFC 7505 [49]; see that document 2639 for additional discussion of the rationale for that convention. 2641 Reply code 554 would normally be used in response to a RCPT command 2642 (or extension command with similar intent) when the SMTP system 2643 identifies a domain that it can (or has) determined never accepts 2644 mail. Other codes, including 554 and the temporary 450, are used for 2645 more transient situations and situations in which an SMTP server 2646 cannot or will not deliver to (or accept mail for) a particular 2647 system or mailbox for policy reasons rather than ones directly 2648 related to SMTP processing. 2650 [[CREF17: [JcK 20210904]: do we want/need to discuss temporary server 2651 outages? And is the discussion above sufficient to obsolete RFC 7505 2652 or do we need either more text or some pretense to claim to update 2653 it.]] 2655 4.2.4.3. Reply Codes after DATA and the Subsequent . 2657 When an SMTP server returns a positive completion status (2yz code) 2658 after the DATA command is completed with ., it accepts 2659 responsibility for: 2661 o delivering the message (if the recipient mailbox exists), or 2663 o if attempts to deliver the message fail due to transient 2664 conditions, retrying delivery some reasonable number of times at 2665 intervals as specified in Section 4.5.4. 2667 o if attempts to deliver the message fail due to permanent 2668 conditions, or if repeated attempts to deliver the message fail 2669 due to transient conditions, returning appropriate notification to 2670 the sender of the original message (using the address in the SMTP 2671 MAIL command). 2673 When an SMTP server returns a temporary error status (4yz) code after 2674 the DATA command is completed with ., it MUST NOT make a 2675 subsequent attempt to deliver that message. The SMTP client retains 2676 responsibility for the delivery of that message and may either return 2677 it to the user or requeue it for a subsequent attempt (see 2678 Section 4.5.4.1). 2680 The user who originated the message SHOULD be able to interpret the 2681 return of a transient failure status (by mail message or otherwise) 2682 as a non-delivery indication, just as a permanent failure would be 2683 interpreted. If the client SMTP successfully handles these 2684 conditions, the user will not receive such a reply. 2686 When an SMTP server returns a permanent error status (5yz) code after 2687 the DATA command is completed with ., it MUST NOT make 2688 any subsequent attempt to deliver the message. As with temporary 2689 error status codes, the SMTP client retains responsibility for the 2690 message, but SHOULD NOT again attempt delivery to the same server 2691 without user review of the message and response and appropriate 2692 intervention. 2694 4.3. Sequencing of Commands and Replies 2696 4.3.1. Sequencing Overview 2698 The communication between the sender and receiver is an alternating 2699 dialogue, controlled by the sender. As such, the sender issues a 2700 command and the receiver responds with a reply. Unless other 2701 arrangements are negotiated through service extensions, the sender 2702 MUST wait for this response before sending further commands. One 2703 important reply is the connection greeting. Normally, a receiver 2704 will send a 220 "Service ready" reply when the connection is 2705 completed. The sender SHOULD wait for this greeting message before 2706 sending any commands. 2708 Note: all the greeting-type replies have the official name (the 2709 fully-qualified primary domain name) of the server host as the first 2710 word following the reply code. Sometimes the host will have no 2711 meaningful name. See Section 4.1.3 for a discussion of alternatives 2712 in these situations. 2714 For example, 2716 220 ISIF.USC.EDU Service ready 2718 or 2720 220 mail.example.com SuperSMTP v 6.1.2 Service ready 2722 or 2724 220 [10.0.0.1] Clueless host service ready 2726 The table below lists alternative success and failure replies for 2727 each command. These SHOULD be strictly adhered to. A receiver MAY 2728 substitute text in the replies, but the meanings and actions implied 2729 by the code numbers and by the specific command reply sequence MUST 2730 be preserved. However, in order to provide robustness as SMTP is 2731 extended and evolves, the discussion in Section 4.2.1 still applies: 2732 all SMTP clients MUST be prepared to accept any code that conforms to 2733 the discussion in that section and MUST be prepared to interpret it 2734 on the basis of its first digit only. 2736 4.3.2. Command-Reply Sequences 2738 Each command is listed with its usual possible replies. The prefixes 2739 used before the possible replies are "I" for intermediate, "S" for 2740 success, and "E" for error. Since some servers may generate other 2741 replies under special circumstances, and to allow for future 2742 extension, SMTP clients SHOULD, when possible, interpret only the 2743 first digit of the reply and MUST be prepared to deal with 2744 unrecognized reply codes by interpreting the first digit only. 2745 Unless extended using the mechanisms described in Section 2.2, SMTP 2746 servers MUST NOT transmit reply codes to an SMTP client that are 2747 other than three digits or that do not start in a digit between 2 and 2748 5 inclusive. 2750 These sequencing rules and, in principle, the codes themselves, can 2751 be extended or modified by SMTP extensions offered by the server and 2752 accepted (requested) by the client. However, if the target is more 2753 precise granularity in the codes, rather than codes for completely 2754 new purposes, the system described in RFC 3463 [7] SHOULD be used in 2755 preference to the invention of new codes. 2757 In addition to the codes listed below, any SMTP command can return 2758 any of the following codes if the corresponding unusual circumstances 2759 are encountered: 2761 500 For the "command line too long" case or if the command name was 2762 not recognized. Note that producing a "command not recognized" 2763 error in response to the required subset of these commands is a 2764 violation of this specification. Similarly, producing a "command 2765 too long" message for a command line shorter than 512 characters 2766 would violate the provisions of Section 4.5.3.1.4. 2768 501 Syntax error in command or arguments. In order to provide for 2769 future extensions, commands that are specified in this document as 2770 not accepting arguments (DATA, RSET, QUIT) SHOULD return a 501 2771 message if arguments are supplied in the absence of EHLO- 2772 advertised extensions. 2774 421 Service shutting down and closing transmission channel 2776 Specific sequences are: 2778 CONNECTION ESTABLISHMENT 2780 S: 220 2781 E: 521, 554, 556 2783 EHLO or HELO 2785 S: 250 2786 E: 504 (a conforming implementation could return this code only 2787 in fairly obscure cases), 550, 502 (permitted only with an old- 2788 style server that does not support EHLO) 2790 MAIL 2792 S: 250 2793 E: 552, 451, 452, 550, 553, 503, 455, 555 2795 RCPT 2797 S: 250, 251 (but see Section 3.4 for discussion of 251 and 551) 2798 E: 550, 551, 552 (obsolete for "too many recipients; see 2799 Section 4.5.3.1.10, 553, 450, 451, 452, 503, 455, 555 2801 DATA 2803 I: 354 -> data -> S: 250 2804 E: 552, 554, 451, 452 2806 E: 450, 550 (rejections for policy reasons) 2808 E: 503, 554 2810 RSET 2812 S: 250 2814 VRFY 2816 S: 250, 251, 252 2817 E: 550, 551, 553, 502, 504 2819 EXPN 2821 S: 250, 252 2822 E: 550, 500, 502, 504 2824 HELP 2826 S: 211, 214 2827 E: 502, 504 2829 NOOP 2831 S: 250 2833 QUIT 2835 S: 221 2837 4.4. Trace Information 2839 When an SMTP server receives a message for delivery or further 2840 processing, it MUST insert trace (often referred to as "time stamp" 2841 or "Received" information) at the beginning of the message content, 2842 as discussed in Section 4.1.1.4. 2844 This line MUST be structured as follows: 2846 o The FROM clause, which MUST be supplied in an SMTP environment, 2847 SHOULD contain both (1) the name of the source host as presented 2848 in the EHLO command and (2) an address literal containing the IP 2849 address of the source, determined from the TCP connection. 2851 o The ID clause MAY contain an "@" as suggested in RFC 822, but this 2852 is not required. 2854 o If the FOR clause appears, it MUST contain exactly one 2855 entry, even when multiple RCPT commands have been given. Multiple 2856 s raise some security issues and have been deprecated, see 2857 Section 7.2. 2859 An Internet mail program MUST NOT change or delete a Received: line 2860 that was previously added to the message header section. SMTP 2861 servers MUST prepend Received lines to messages; they MUST NOT change 2862 the order of existing lines or insert Received lines in any other 2863 location. 2865 As the Internet grows, comparability of Received header fields is 2866 important for detecting problems, especially slow relays. SMTP 2867 servers that create Received header fields SHOULD use explicit 2868 offsets in the dates (e.g., -0800), rather than time zone names of 2869 any type. Local time (with an offset) SHOULD be used rather than UT 2870 when feasible. This formulation allows slightly more information 2871 about local circumstances to be specified. If UT is needed, the 2872 receiver need merely do some simple arithmetic to convert the values. 2873 Use of UT loses information about the time zone-location of the 2874 server. If it is desired to supply a time zone name, it SHOULD be 2875 included in a comment. 2877 When the delivery SMTP server makes the "final delivery" of a 2878 message, it inserts a return-path line at the beginning of the mail 2879 data. This use of return-path is required; mail systems MUST support 2880 it. The return-path line preserves the information in the from the MAIL command. Here, final delivery means the message 2882 has left the SMTP environment. Normally, this would mean it had been 2883 delivered to the destination user or an associated mail drop, but in 2884 some cases it may be further processed and transmitted by another 2885 mail system. 2887 It is possible for the mailbox in the return path to be different 2888 from the actual sender's mailbox, for example, if error responses are 2889 to be delivered to a special error handling mailbox rather than to 2890 the message sender. When mailing lists are involved, this 2891 arrangement is common and useful as a means of directing errors to 2892 the list maintainer rather than the message originator. 2894 The text above implies that the final mail data will begin with a 2895 return path line, followed by one or more time stamp lines. These 2896 lines will be followed by the rest of the mail data: first the 2897 balance of the mail header section and then the body (RFC 5322 [12]). 2899 It is sometimes difficult for an SMTP server to determine whether or 2900 not it is making final delivery since forwarding or other operations 2901 may occur after the message is accepted for delivery. Consequently, 2902 any further (forwarding, gateway, or relay) systems MAY remove the 2903 return path and rebuild the MAIL command as needed to ensure that 2904 exactly one such line appears in a delivered message. 2906 A message-originating SMTP system SHOULD NOT send a message that 2907 already contains a Return-path header field. SMTP servers performing 2908 a relay function MUST NOT inspect the message data, and especially 2909 not to the extent needed to determine if Return-path header fields 2910 are present. SMTP servers making final delivery MAY remove Return- 2911 path header fields before adding their own. 2913 The primary purpose of the Return-path is to designate the address to 2914 which messages indicating non-delivery or other mail system failures 2915 are to be sent. For this to be unambiguous, exactly one return path 2916 SHOULD be present when the message is delivered. Systems using RFC 2917 822 syntax with non-SMTP transports SHOULD designate an unambiguous 2918 address, associated with the transport envelope, to which error 2919 reports (e.g., non-delivery messages) should be sent. 2921 Historical note: Text in RFC 822 that appears to contraindicate the 2922 use of the Return-path header field (or the envelope reverse-path 2923 address from the MAIL command) if the destination for error messages 2924 is not applicable on the Internet. The reverse-path address (as 2925 copied into the Return-path) MUST be used as the target of any mail 2926 containing delivery error messages. 2928 In particular: 2930 o a gateway from SMTP -> elsewhere SHOULD insert a return-path 2931 header field, unless it is known that the "elsewhere" transport 2932 also uses Internet domain addresses and maintains the envelope 2933 sender address separately. 2935 o a gateway from elsewhere -> SMTP SHOULD delete any return-path 2936 header field present in the message, and either copy that 2937 information to the SMTP envelope or combine it with information 2938 present in the envelope of the other transport system to construct 2939 the reverse-path argument to the MAIL command in the SMTP 2940 envelope. 2942 The server must give special treatment to cases in which the 2943 processing following the end of mail data indication is only 2944 partially successful. This could happen if, after accepting several 2945 recipients and the mail data, the SMTP server finds that the mail 2946 data could be successfully delivered to some, but not all, of the 2947 recipients. In such cases, the response to the DATA command MUST be 2948 an OK reply. However, the SMTP server MUST compose and send an 2949 "undeliverable mail" notification message to the originator of the 2950 message. 2952 A single notification listing all of the failed recipients or 2953 separate notification messages MUST be sent for each failed 2954 recipient. For economy of processing by the sender, the former 2955 SHOULD be used when possible. Note that the key difference between 2956 handling aliases (Section 3.9.1) and forwarding (this subsection) is 2957 the change to the backward-pointing address in this case. All 2958 notification messages about undeliverable mail MUST be sent using the 2959 MAIL command and MUST use a null return path as discussed in 2960 Section 3.6. 2962 The time stamp line and the return path line are formally defined as 2963 follows (the definitions for "FWS" and "CFWS" appear in RFC 5322 2964 [12]): 2966 Return-path-line = "Return-Path:" FWS Reverse-path 2968 Time-stamp-line = "Received:" FWS Stamp 2970 Stamp = From-domain By-domain Opt-info [CFWS] ";" 2971 FWS date-time 2972 ; where "date-time" is as defined in RFC 5322 [12] 2973 ; but the "obs-" forms, especially two-digit 2974 ; years, are prohibited in SMTP and MUST NOT be used. 2976 From-domain = "FROM" FWS Extended-Domain 2978 By-domain = CFWS "BY" FWS Extended-Domain 2980 Extended-Domain = Domain / 2981 ( Domain FWS "(" TCP-info ")" ) / 2982 ( address-literal FWS "(" TCP-info ")" ) 2984 TCP-info = address-literal / ( Domain FWS address-literal ) 2985 ; Information derived by server from TCP connection 2986 ; not client EHLO. 2988 Opt-info = [Via] [With] [ID] [For] 2989 [Additional-Registered-Clauses] 2991 Via = CFWS "VIA" FWS Link 2993 With = CFWS "WITH" FWS Protocol 2994 ID = CFWS "ID" FWS ( Atom / msg-id ) 2995 ; msg-id is defined in RFC 5322 [12] 2997 For = CFWS "FOR" FWS ( Path / Mailbox ) 2999 Additional-Registered-Clauses = 1* (CFWS Atom FWS String) 3000 [[CREF18: [5321bis] 5321 errata #1683, 20090215, ]] 3001 ; Additional standard clauses may be added in this 3002 ; location by future standards and registration with 3003 ; IANA. SMTP servers SHOULD NOT use unregistered 3004 ; names. See Section 8. 3006 Link = "TCP" / Addtl-Link 3008 Addtl-Link = Atom 3009 ; Additional standard names for links are 3010 ; registered with the Internet Assigned Numbers 3011 ; Authority (IANA). "Via" is primarily of value 3012 ; with non-Internet transports. SMTP servers 3013 ; SHOULD NOT use unregistered names. 3015 Protocol = "ESMTP" / "SMTP" / Attdl-Protocol 3017 Addtl-Protocol = Atom 3018 ; Additional standard names for protocols are 3019 ; registered with the Internet Assigned Numbers 3020 ; Authority (IANA) in the "mail parameters" 3021 ; registry [8]. SMTP servers SHOULD NOT 3022 ; use unregistered names. 3024 4.5. Additional Implementation Issues 3026 4.5.1. Minimum Implementation 3028 In order to make SMTP workable, the following minimum implementation 3029 MUST be provided by all receivers. The following commands MUST be 3030 supported to conform to this specification: 3032 EHLO 3033 HELO 3034 MAIL 3035 RCPT 3036 DATA 3037 RSET 3038 NOOP 3039 QUIT 3040 VRFY 3042 Any system that includes an SMTP server supporting mail relaying or 3043 delivery MUST support the reserved mailbox "postmaster" as a case- 3044 insensitive local name. This postmaster address is not strictly 3045 necessary if the server always returns 554 on connection opening (as 3046 described in Section 3.1). The requirement to accept mail for 3047 postmaster implies that RCPT commands that specify a mailbox for 3048 postmaster at any of the domains for which the SMTP server provides 3049 mail service, as well as the special case of "RCPT TO:" 3050 (with no domain specification), MUST be supported. 3052 SMTP systems are expected to make every reasonable effort to accept 3053 mail directed to Postmaster from any other system on the Internet. 3054 In extreme cases -- such as to contain a denial of service attack or 3055 other breach of security -- an SMTP server may block mail directed to 3056 Postmaster. However, such arrangements SHOULD be narrowly tailored 3057 so as to avoid blocking messages that are not part of such attacks. 3059 4.5.2. Transparency 3061 Without some provision for data transparency, the character sequence 3062 "." ends the mail text and cannot be sent by the user. 3063 In general, users are not aware of such "forbidden" sequences. To 3064 allow all user composed text to be transmitted transparently, the 3065 following procedures are used: 3067 o Before sending a line of mail text, the SMTP client checks the 3068 first character of the line. If it is a period, one additional 3069 period is inserted at the beginning of the line. 3071 o When a line of mail text is received by the SMTP server, it checks 3072 the line. If the line is composed of a single period, it is 3073 treated as the end of mail indicator. If the first character is a 3074 period and there are other characters on the line, the first 3075 character is deleted. 3077 The mail data may contain any of the 128 ASCII characters. All 3078 characters are to be delivered to the recipient's mailbox, including 3079 spaces, vertical and horizontal tabs, and other control characters. 3080 If the transmission channel provides an 8-bit byte (octet) data 3081 stream, the 7-bit ASCII codes are transmitted, right justified, in 3082 the octets, with the high-order bits cleared to zero. See 3083 Section 3.6 for special treatment of these conditions in SMTP systems 3084 serving a relay function. 3086 In some systems, it may be necessary to transform the data as it is 3087 received and stored. This may be necessary for hosts that use a 3088 different character set than ASCII as their local character set, that 3089 store data in records rather than strings, or which use special 3090 character sequences as delimiters inside mailboxes. If such 3091 transformations are necessary, they MUST be reversible, especially if 3092 they are applied to mail being relayed. 3094 4.5.3. Sizes and Timeouts 3096 4.5.3.1. Size Limits and Minimums 3098 There are several objects that have required minimum/maximum sizes. 3099 Every implementation MUST be able to receive objects of at least 3100 these sizes. Objects larger than these sizes SHOULD be avoided when 3101 possible. However, some Internet mail constructs such as encoded 3102 X.400 addresses (RFC 2156 [27]) will often require larger objects. 3103 Clients MAY attempt to transmit these, but MUST be prepared for a 3104 server to reject them if they cannot be handled by it. To the 3105 maximum extent possible, implementation techniques that impose no 3106 limits on the length of these objects should be used. 3108 Extensions to SMTP may involve the use of characters that occupy more 3109 than a single octet each. This section therefore specifies lengths 3110 in octets where absolute lengths, rather than character counts, are 3111 intended. 3113 [[CREF19: [5321bis] [[Note in Draft: Klensin 20191126: Given the 3114 controversy on the SMTP mailing list between 20191123 and now about 3115 maximum lengths, is the above adequate or is further tuning of the 3116 limit text below needed? ]]]] 3118 4.5.3.1.1. Local-part 3120 The maximum total length of a user name or other local-part is 64 3121 octets. 3123 4.5.3.1.2. Domain 3125 The maximum total length of a domain name or number is 255 octets. 3127 4.5.3.1.3. Path 3129 The maximum total length of a reverse-path or forward-path is 256 3130 octets (including the punctuation and element separators). 3132 4.5.3.1.4. Command Line 3134 The maximum total length of a command line including the command word 3135 and the is 512 octets. SMTP extensions may be used to 3136 increase this limit. 3138 4.5.3.1.5. Reply Line 3140 The maximum total length of a reply line including the reply code and 3141 the is 512 octets. More information may be conveyed through 3142 multiple-line replies. 3144 4.5.3.1.6. Text Line 3146 The maximum total length of a text line including the is 1000 3147 octets (not counting the leading dot duplicated for transparency). 3148 This number may be increased by the use of SMTP Service Extensions. 3150 4.5.3.1.7. Message Content 3152 The maximum total length of a message content (including any message 3153 header section as well as the message body) MUST BE at least 64K 3154 octets. Since the introduction of Internet Standards for multimedia 3155 mail (RFC 2045 [25]), message lengths on the Internet have grown 3156 dramatically, and message size restrictions should be avoided if at 3157 all possible. SMTP server systems that must impose restrictions 3158 SHOULD implement the "SIZE" service extension of RFC 1870 [6], and 3159 SMTP client systems that will send large messages SHOULD utilize it 3160 when possible. 3162 4.5.3.1.8. Recipient Buffer 3164 The minimum total number of recipients that MUST be buffered is 100 3165 recipients. Rejection of messages (for excessive recipients) with 3166 fewer than 100 RCPT commands is a violation of this specification. 3167 The general principle that relaying SMTP server MUST NOT, and 3168 delivery SMTP servers SHOULD NOT, perform validation tests on message 3169 header fields suggests that messages SHOULD NOT be rejected based on 3170 the total number of recipients shown in header fields. A server that 3171 imposes a limit on the number of recipients MUST behave in an orderly 3172 fashion, such as rejecting additional addresses over its limit rather 3173 than silently discarding addresses previously accepted. A client 3174 that needs to deliver a message containing over 100 RCPT commands 3175 SHOULD be prepared to transmit in 100-recipient "chunks" if the 3176 server declines to accept more than 100 recipients in a single 3177 message. 3179 4.5.3.1.9. Treatment When Limits Exceeded 3181 Errors due to exceeding these limits may be reported by using the 3182 reply codes. Some examples of reply codes are: 3184 500 Line too long. 3186 or 3188 501 Path too long 3190 or 3192 452 Too many recipients (see below) 3194 or 3196 552 Too much mail data (historically also used for too many 3197 recipients (see below). 3199 4.5.3.1.10. Too Many Recipients Code 3201 RFC 821 [3] incorrectly listed the error where an SMTP server 3202 exhausts its implementation limit on the number of RCPT commands 3203 ("too many recipients") as having reply code 552. The correct reply 3204 code for this condition is 452. At the time RFC 5321 was written, 3205 the use of response code 552 by servers was sufficiently common that 3206 client implementation were advised to simply treat it as if 452 had 3207 been sent. That advice is no longer necessary or useful. 3209 When a conforming SMTP server encounters this condition, it has at 3210 least 100 successful RCPT commands in its recipient buffer. If the 3211 server is able to accept the message, then at least these 100 3212 addresses will be removed from the SMTP client's queue. When the 3213 client attempts retransmission of those addresses that received 452 3214 responses, at least 100 of these will be able to fit in the SMTP 3215 server's recipient buffer. Each retransmission attempt that is able 3216 to deliver anything will be able to dispose of at least 100 of these 3217 recipients. 3219 If an SMTP server has an implementation limit on the number of RCPT 3220 commands and this limit is exhausted, it MUST use a response code of 3221 452. If the server has a configured site-policy limitation on the 3222 number of RCPT commands, it MAY instead use a 5yz response code. In 3223 particular, if the intent is to prohibit messages with more than a 3224 site-specified number of recipients, rather than merely limit the 3225 number of recipients in a given mail transaction, it would be 3226 reasonable to return a 503 response to any DATA command received 3227 subsequent to the 452 code or to simply return the 503 after DATA 3228 without returning any previous negative response. 3230 4.5.3.2. Timeouts 3232 An SMTP client MUST provide a timeout mechanism. It MUST use per- 3233 command timeouts rather than somehow trying to time the entire mail 3234 transaction. Timeouts SHOULD be easily reconfigurable, preferably 3235 without recompiling the SMTP code. To implement this, a timer is set 3236 for each SMTP command and for each buffer of the data transfer. The 3237 latter means that the overall timeout is inherently proportional to 3238 the size of the message. 3240 Based on extensive experience with busy mail-relay hosts, the minimum 3241 per-command timeout values SHOULD be as follows: 3243 4.5.3.2.1. Initial 220 Message: 5 Minutes 3245 An SMTP client process needs to distinguish between a failed TCP 3246 connection and a delay in receiving the initial 220 greeting message. 3247 Many SMTP servers accept a TCP connection but delay delivery of the 3248 220 message until their system load permits more mail to be 3249 processed. 3251 4.5.3.2.2. MAIL Command: 5 Minutes 3253 4.5.3.2.3. RCPT Command: 5 Minutes 3255 A longer timeout is required if processing of mailing lists and 3256 aliases is not deferred until after the message was accepted. 3258 4.5.3.2.4. DATA Initiation: 2 Minutes 3260 This is while awaiting the "354 Start Input" reply to a DATA command. 3262 4.5.3.2.5. Data Block: 3 Minutes 3264 This is while awaiting the completion of each TCP SEND call 3265 transmitting a chunk of data. 3267 4.5.3.2.6. DATA Termination: 10 Minutes. 3269 This is while awaiting the "250 OK" reply. When the receiver gets 3270 the final period terminating the message data, it typically performs 3271 processing to deliver the message to a user mailbox. A spurious 3272 timeout at this point would be very wasteful and would typically 3273 result in delivery of multiple copies of the message, since it has 3274 been successfully sent and the server has accepted responsibility for 3275 delivery. See Section 6.1 for additional discussion. 3277 4.5.3.2.7. Server Timeout: 5 Minutes. 3279 An SMTP server SHOULD have a timeout of at least 5 minutes while it 3280 is awaiting the next command from the sender. 3282 4.5.4. Retry Strategies 3284 The common structure of a host SMTP implementation includes user 3285 mailboxes, one or more areas for queuing messages in transit, and one 3286 or more daemon processes for sending and receiving mail. The exact 3287 structure will vary depending on the needs of the users on the host 3288 and the number and size of mailing lists supported by the host. We 3289 describe several optimizations that have proved helpful, particularly 3290 for mailers supporting high traffic levels. 3292 Any queuing strategy MUST include timeouts on all activities on a 3293 per-command basis. A queuing strategy MUST NOT send error messages 3294 in response to error messages under any circumstances. 3296 4.5.4.1. Sending Strategy 3298 The general model for an SMTP client is one or more processes that 3299 periodically attempt to transmit outgoing mail. In a typical system, 3300 the program that composes a message has some method for requesting 3301 immediate attention for a new piece of outgoing mail, while mail that 3302 cannot be transmitted immediately MUST be queued and periodically 3303 retried by the sender. A mail queue entry will include not only the 3304 message itself but also the envelope information. 3306 The sender MUST delay retrying a particular destination after one 3307 attempt has failed. In general, the retry interval SHOULD be at 3308 least 30 minutes; however, more sophisticated and variable strategies 3309 will be beneficial when the SMTP client can determine the reason for 3310 non-delivery. 3312 Retries continue until the message is transmitted or the sender gives 3313 up; the give-up time generally needs to be at least 4-5 days. It MAY 3314 be appropriate to set a shorter maximum number of retries for non- 3315 delivery notifications and equivalent error messages than for 3316 standard messages. The parameters to the retry algorithm MUST be 3317 configurable. 3319 A client SHOULD keep a list of hosts it cannot reach and 3320 corresponding connection timeouts, rather than just retrying queued 3321 mail items. 3323 Experience suggests that failures are typically transient (the target 3324 system or its connection has crashed), favoring a policy of two 3325 connection attempts in the first hour the message is in the queue, 3326 and then backing off to one every two or three hours. 3328 The SMTP client can shorten the queuing delay in cooperation with the 3329 SMTP server. For example, if mail is received from a particular 3330 address, it is likely that mail queued for that host can now be sent. 3331 Application of this principle may, in many cases, eliminate the 3332 requirement for an explicit "send queues now" function such as ETRN, 3333 RFC 1985 [24]. 3335 The strategy may be further modified as a result of multiple 3336 addresses per host (see below) to optimize delivery time versus 3337 resource usage. 3339 An SMTP client may have a large queue of messages for each 3340 unavailable destination host. If all of these messages were retried 3341 in every retry cycle, there would be excessive Internet overhead and 3342 the sending system would be blocked for a long period. Note that an 3343 SMTP client can generally determine that a delivery attempt has 3344 failed only after a timeout of several minutes, and even a one-minute 3345 timeout per connection will result in a very large delay if retries 3346 are repeated for dozens, or even hundreds, of queued messages to the 3347 same host. 3349 At the same time, SMTP clients SHOULD use great care in caching 3350 negative responses from servers. In an extreme case, if EHLO is 3351 issued multiple times during the same SMTP connection, different 3352 answers may be returned by the server. More significantly, 5yz 3353 responses to the MAIL command MUST NOT be cached. 3355 When a mail message is to be delivered to multiple recipients, and 3356 the SMTP server to which a copy of the message is to be sent is the 3357 same for multiple recipients, then only one copy of the message 3358 SHOULD be transmitted. That is, the SMTP client SHOULD use the 3359 command sequence: MAIL, RCPT, RCPT, ..., RCPT, DATA instead of the 3360 sequence: MAIL, RCPT, DATA, ..., MAIL, RCPT, DATA. However, if there 3361 are very many addresses, a limit on the number of RCPT commands per 3362 MAIL command MAY be imposed. This efficiency feature SHOULD be 3363 implemented. 3365 Similarly, to achieve timely delivery, the SMTP client MAY support 3366 multiple concurrent outgoing mail transactions. However, some limit 3367 may be appropriate to protect the host from devoting all its 3368 resources to mail. 3370 4.5.4.2. Receiving Strategy 3372 The SMTP server SHOULD attempt to keep a pending listen on the SMTP 3373 port (specified by IANA as port 25) at all times. This requires the 3374 support of multiple incoming TCP connections for SMTP. Some limit 3375 MAY be imposed, but servers that cannot handle more than one SMTP 3376 transaction at a time are not in conformance with the intent of this 3377 specification. 3379 As discussed above, when the SMTP server receives mail from a 3380 particular host address, it could activate its own SMTP queuing 3381 mechanisms to retry any mail pending for that host address. 3383 4.5.5. Messages with a Null Reverse-Path 3385 There are several types of notification messages that are required by 3386 existing and proposed Standards to be sent with a null reverse-path, 3387 namely non-delivery notifications as discussed in Section 3.6.2 and 3388 Section 3.6.3, other kinds of Delivery Status Notifications (DSNs, 3389 RFC 3461 [34]), and Message Disposition Notifications (MDNs, RFC 8098 3390 [37]). All of these kinds of messages are notifications about a 3391 previous message, and they are sent to the reverse-path of the 3392 previous mail message. (If the delivery of such a notification 3393 message fails, that usually indicates a problem with the mail system 3394 of the host to which the notification message is addressed. For this 3395 reason, at some hosts the MTA is set up to forward such failed 3396 notification messages to someone who is able to fix problems with the 3397 mail system, e.g., via the postmaster alias.) 3399 All other types of messages (i.e., any message which is not required 3400 by a Standards-Track RFC to have a null reverse-path) SHOULD be sent 3401 with a valid, non-null reverse-path. 3403 Implementers of automated email processors should be careful to make 3404 sure that the various kinds of messages with a null reverse-path are 3405 handled correctly. In particular, such systems SHOULD NOT reply to 3406 messages with a null reverse-path, and they SHOULD NOT add a non-null 3407 reverse-path, or change a null reverse-path to a non-null one, to 3408 such messages when forwarding. 3410 5. Address Resolution and Mail Handling 3412 5.1. Locating the Target Host 3414 Once an SMTP client lexically identifies a domain to which mail will 3415 be delivered for processing (as described in Sections 2.3.5 and 3.6), 3416 a DNS lookup MUST be performed to resolve the domain name (RFC 1035 3417 [4]). The names are expected to be fully-qualified domain names 3418 (FQDNs): mechanisms for inferring FQDNs from partial names or local 3419 aliases are outside of this specification. Due to a history of 3420 problems, SMTP servers used for initial submission of messages SHOULD 3421 NOT make such inferences (Message Submission Servers [42] have 3422 somewhat more flexibility) and intermediate (relay) SMTP servers MUST 3423 NOT make them. 3425 The lookup first attempts to locate an MX record associated with the 3426 name. If a CNAME record is found, the resulting name is processed as 3427 if it were the initial name. If a non-existent domain error is 3428 returned, this situation MUST be reported as an error. If a 3429 temporary error is returned, the message MUST be queued and retried 3430 later (see Section 4.5.4.1). If an empty list of MXs is returned, 3431 the address is treated as if it was associated with an implicit MX 3432 RR, with a preference of 0, pointing to that host. If MX records are 3433 present, but none of them are usable, or the implicit MX is unusable, 3434 this situation MUST be reported as an error. 3436 If one or more MX RRs are found for a given name, SMTP systems MUST 3437 NOT utilize any address RRs associated with that name unless they are 3438 located using the MX RRs; the "implicit MX" rule above applies only 3439 if there are no MX records present. If MX records are present, but 3440 none of them are usable, this situation MUST be reported as an error. 3442 When a domain name associated with an MX RR is looked up and the 3443 associated data field obtained, the data field of that response MUST 3444 contain a domain name that conforms to the specifications of 3445 Section 2.3.5. 3446 [[5321bis Editor's Note: Depending on how the "null MX" discussion 3447 unfolds, some additional text may be in order here (20140718)]] 3448 That domain name, when queried, MUST return at least one address 3449 record (e.g., A or AAAA RR) that gives the IP address of the SMTP 3450 server to which the message should be directed. Any other response, 3451 specifically including a value that will return a CNAME record when 3452 queried, lies outside the scope of this Standard. The prohibition on 3453 labels in the data that resolve to CNAMEs is discussed in more detail 3454 in RFC 2181, Section 10.3 [28]. 3456 When the lookup succeeds, the mapping can result in a list of 3457 alternative delivery addresses rather than a single address, because 3458 of multiple MX records, multihoming, or both. To provide reliable 3459 mail transmission, the SMTP client MUST be able to try (and retry) 3460 each of the relevant addresses in this list in order, until a 3461 delivery attempt succeeds. However, there MAY also be a configurable 3462 limit on the number of alternate addresses that can be tried. In any 3463 case, the SMTP client SHOULD try at least two addresses. 3465 Two types of information are used to rank the host addresses: 3466 multiple MX records, and multihomed hosts. 3468 MX records contain a preference indication that MUST be used in 3469 sorting if more than one such record appears (see below). Lower 3470 numbers are more preferred than higher ones. If there are multiple 3471 destinations with the same preference and there is no clear reason to 3472 favor one (e.g., by recognition of an easily reached address), then 3473 the sender-SMTP MUST randomize them to spread the load across 3474 multiple mail exchangers for a specific organization. 3476 The destination host (perhaps taken from the preferred MX record) may 3477 be multihomed, in which case the domain name resolver will return a 3478 list of alternative IP addresses. It is the responsibility of the 3479 domain name resolver interface to have ordered this list by 3480 decreasing preference if necessary, and the SMTP sender MUST try them 3481 in the order presented. 3483 Although the capability to try multiple alternative addresses is 3484 required, specific installations may want to limit or disable the use 3485 of alternative addresses. The question of whether a sender should 3486 attempt retries using the different addresses of a multihomed host 3487 has been controversial. The main argument for using the multiple 3488 addresses is that it maximizes the probability of timely delivery, 3489 and indeed sometimes the probability of any delivery; the counter- 3490 argument is that it may result in unnecessary resource use. Note 3491 that resource use is also strongly determined by the sending strategy 3492 discussed in Section 4.5.4.1. 3494 If an SMTP server receives a message with a destination for which it 3495 is a designated Mail eXchanger, it MAY relay the message (potentially 3496 after having rewritten the MAIL FROM and/or RCPT TO addresses), make 3497 final delivery of the message, or hand it off using some mechanism 3498 outside the SMTP-provided transport environment. Of course, neither 3499 of the latter require that the list of MX records be examined 3500 further. 3502 If it determines that it should relay the message without rewriting 3503 the address, it MUST sort the MX records to determine candidates for 3504 delivery. The records are first ordered by preference, with the 3505 lowest-numbered records being most preferred. The relay host MUST 3506 then inspect the list for any of the names or addresses by which it 3507 might be known in mail transactions. If a matching record is found, 3508 all records at that preference level and higher-numbered ones MUST be 3509 discarded from consideration. If there are no records left at that 3510 point, it is an error condition, and the message MUST be returned as 3511 undeliverable. If records do remain, they SHOULD be tried, best 3512 preference first, as described above. 3514 5.2. IPv6 and MX Records 3516 In the contemporary Internet, SMTP clients and servers may be hosted 3517 on IPv4 systems, IPv6 systems, or dual-stack systems that are 3518 compatible with either version of the Internet Protocol. The host 3519 domains to which MX records point may, consequently, contain "A RR"s 3520 (IPv4), "AAAA RR"s (IPv6), or any combination of them. While RFC 3521 3974 [39] discusses some operational experience in mixed 3522 environments, it was not comprehensive enough to justify 3523 standardization, and some of its recommendations appear to be 3524 inconsistent with this specification. The appropriate actions to be 3525 taken either will depend on local circumstances, such as performance 3526 of the relevant networks and any conversions that might be necessary, 3527 or will be obvious (e.g., an IPv6-only client need not attempt to 3528 look up A RRs or attempt to reach IPv4-only servers). Designers of 3529 SMTP implementations that might run in IPv6 or dual-stack 3530 environments should study the procedures above, especially the 3531 comments about multihomed hosts, and, preferably, provide mechanisms 3532 to facilitate operational tuning and mail interoperability between 3533 IPv4 and IPv6 systems while considering local circumstances. 3535 6. Problem Detection and Handling 3537 6.1. Reliable Delivery and Replies by Email 3539 When the receiver-SMTP accepts a piece of mail (by sending a "250 OK" 3540 message in response to DATA), it is accepting responsibility for 3541 delivering or relaying the message. It must take this responsibility 3542 seriously. It MUST NOT lose the message for frivolous reasons, such 3543 as because the host later crashes or because of a predictable 3544 resource shortage. Some reasons that are not considered frivolous 3545 are discussed in the next subsection and in Section 7.8. 3547 If there is a delivery failure after acceptance of a message, the 3548 receiver-SMTP MUST formulate and mail a notification message. This 3549 notification MUST be sent using a null ("<>") reverse-path in the 3550 envelope. The recipient of this notification MUST be the address 3551 from the envelope return path (or the Return-Path: line). However, 3552 if this address is null ("<>"), the receiver-SMTP MUST NOT send a 3553 notification. Obviously, nothing in this section can or should 3554 prohibit local decisions (i.e., as part of the same system 3555 environment as the receiver-SMTP) to log or otherwise transmit 3556 information about null address events locally if that is desired. If 3557 the address is an explicit source route, it MUST be stripped down to 3558 its final hop. 3560 For example, suppose that an error notification must be sent for a 3561 message that arrived with: 3563 MAIL FROM:<@a,@b:user@d> 3565 The notification message MUST be sent using: 3567 RCPT TO: 3569 Some delivery failures after the message is accepted by SMTP will be 3570 unavoidable. For example, it may be impossible for the receiving 3571 SMTP server to validate all the delivery addresses in RCPT command(s) 3572 due to a "soft" domain system error, because the target is a mailing 3573 list (see earlier discussion of RCPT), or because the server is 3574 acting as a relay and has no immediate access to the delivering 3575 system. 3577 To avoid receiving duplicate messages as the result of timeouts, a 3578 receiver-SMTP MUST seek to minimize the time required to respond to 3579 the final . end of data indicator. See RFC 1047 [17] for 3580 a discussion of this problem. 3582 6.2. Unwanted, Unsolicited, and "Attack" Messages 3584 Utility and predictability of the Internet mail system requires that 3585 messages that can be delivered should be delivered, regardless of any 3586 syntax or other faults associated with those messages and regardless 3587 of their content. If they cannot be delivered, and cannot be 3588 rejected by the SMTP server during the SMTP transaction, they should 3589 be "bounced" (returned with non-delivery notification messages) as 3590 described above. In today's world, in which many SMTP server 3591 operators have discovered that the quantity of undesirable bulk email 3592 vastly exceeds the quantity of desired mail and in which accepting a 3593 message may trigger additional undesirable traffic by providing 3594 verification of the address, those principles may not be practical. 3596 As discussed in Section 7.8 and Section 7.9 below, dropping mail 3597 without notification of the sender is permitted in practice. 3598 However, it is extremely dangerous and violates a long tradition and 3599 community expectations that mail is either delivered or returned. If 3600 silent message-dropping is misused, it could easily undermine 3601 confidence in the reliability of the Internet's mail systems. So 3602 silent dropping of messages should be considered only in those cases 3603 where there is very high confidence that the messages are seriously 3604 fraudulent or otherwise inappropriate. 3606 To stretch the principle of delivery if possible even further, it may 3607 be a rational policy to not deliver mail that has an invalid return 3608 address, although the history of the network is that users are 3609 typically better served by delivering any message that can be 3610 delivered. Reliably determining that a return address is invalid can 3611 be a difficult and time-consuming process, especially if the putative 3612 sending system is not directly accessible or does not fully and 3613 accurately support VRFY and, even if a "drop messages with invalid 3614 return addresses" policy is adopted, it SHOULD be applied only when 3615 there is near-certainty that the return addresses are, in fact, 3616 invalid. 3618 Conversely, if a message is rejected because it is found to contain 3619 hostile content (a decision that is outside the scope of an SMTP 3620 server as defined in this document), rejection ("bounce") messages 3621 SHOULD NOT be sent unless the receiving site is confident that those 3622 messages will be usefully delivered. The preference and default in 3623 these cases is to avoid sending non-delivery messages when the 3624 incoming message is determined to contain hostile content. 3626 6.3. Loop Detection 3628 Simple counting of the number of "Received:" header fields in a 3629 message has proven to be an effective, although rarely optimal, 3630 method of detecting loops in mail systems. SMTP servers using this 3631 technique SHOULD use a large rejection threshold, normally at least 3632 100 Received entries. Whatever mechanisms are used, servers MUST 3633 contain provisions for detecting and stopping trivial loops. 3635 6.4. Compensating for Irregularities 3637 Unfortunately, variations, creative interpretations, and outright 3638 violations of Internet mail protocols do occur; some would suggest 3639 that they occur quite frequently. The debate as to whether a well- 3640 behaved SMTP receiver or relay should reject a malformed message, 3641 attempt to pass it on unchanged, or attempt to repair it to increase 3642 the odds of successful delivery (or subsequent reply) began almost 3643 with the dawn of structured network mail and shows no signs of 3644 abating. Advocates of rejection claim that attempted repairs are 3645 rarely completely adequate and that rejection of bad messages is the 3646 only way to get the offending software repaired. Advocates of 3647 "repair" or "deliver no matter what" argue that users prefer that 3648 mail go through it if at all possible and that there are significant 3649 market pressures in that direction. In practice, these market 3650 pressures may be more important to particular vendors than strict 3651 conformance to the standards, regardless of the preference of the 3652 actual developers. 3654 The problems associated with ill-formed messages were exacerbated by 3655 the introduction of the split-UA mail reading protocols (Post Office 3656 Protocol (POP) version 2 [14], Post Office Protocol (POP) version 3 3657 [23], IMAP version 2 [19], and PCMAIL [18]). These protocols 3658 encouraged the use of SMTP as a posting (message submission) 3659 protocol, and SMTP servers as relay systems for these client hosts 3660 (which are often only intermittently connected to the Internet). 3661 Historically, many of those client machines lacked some of the 3662 mechanisms and information assumed by SMTP (and indeed, by the mail 3663 format protocol, RFC 822 [13]). Some could not keep adequate track 3664 of time; others had no concept of time zones; still others could not 3665 identify their own names or addresses; and, of course, none could 3666 satisfy the assumptions that underlay RFC 822's conception of 3667 authenticated addresses. 3669 In response to these weak SMTP clients, many SMTP systems now 3670 complete messages that are delivered to them in incomplete or 3671 incorrect form. This strategy is generally considered appropriate 3672 when the server can identify or authenticate the client, and there 3673 are prior agreements between them. By contrast, there is at best 3674 great concern about fixes applied by a relay or delivery SMTP server 3675 that has little or no knowledge of the user or client machine. Many 3676 of these issues are addressed by using a separate protocol, such as 3677 that defined in RFC 6409 [42], for message submission, rather than 3678 using originating SMTP servers for that purpose. 3680 The following changes to a message being processed MAY be applied 3681 when necessary by an originating SMTP server, or one used as the 3682 target of SMTP as an initial posting (message submission) protocol: 3684 o Addition of a message-id field when none appears 3686 o Addition of a date, time, or time zone when none appears 3688 o Correction of addresses to proper FQDN format 3690 The less information the server has about the client, the less likely 3691 these changes are to be correct and the more caution and conservatism 3692 should be applied when considering whether or not to perform fixes 3693 and how. These changes MUST NOT be applied by an SMTP server that 3694 provides an intermediate relay function. 3696 In all cases, properly operating clients supplying correct 3697 information are preferred to corrections by the SMTP server. In all 3698 cases, documentation SHOULD be provided in trace header fields and/or 3699 header field comments for actions performed by the servers. 3701 7. Security Considerations 3702 7.1. Mail Security and Spoofing 3704 SMTP mail is inherently insecure in that it is feasible for even 3705 fairly casual users to negotiate directly with receiving and relaying 3706 SMTP servers and create messages that will trick a naive recipient 3707 into believing that they came from somewhere else. Constructing such 3708 a message so that the "spoofed" behavior cannot be detected by an 3709 expert is somewhat more difficult, but not sufficiently so as to be a 3710 deterrent to someone who is determined and knowledgeable. 3711 Consequently, as knowledge of Internet mail increases, so does the 3712 knowledge that SMTP mail inherently cannot be authenticated, or 3713 integrity checks provided, at the transport level. Real mail 3714 security lies only in end-to-end methods involving the message 3715 bodies, such as those that use digital signatures (see RFC 1847 [21] 3716 and, e.g., Pretty Good Privacy (PGP) in RFC 4880 [45] or Secure/ 3717 Multipurpose Internet Mail Extensions (S/MIME) in RFC 8551 [38]). 3719 Various protocol extensions and configuration options that provide 3720 authentication at the transport level (e.g., from an SMTP client to 3721 an SMTP server) improve somewhat on the traditional situation 3722 described above. However, in general, they only authenticate one 3723 server to another rather than a chain of relays and servers, much 3724 less authenticating users or user machines. Consequently, unless 3725 they are accompanied by careful handoffs of responsibility in a 3726 carefully designed trust environment, they remain inherently weaker 3727 than end-to-end mechanisms that use digitally signed messages rather 3728 than depending on the integrity of the transport system. 3730 Efforts to make it more difficult for users to set envelope return 3731 path and header "From" fields to point to valid addresses other than 3732 their own are largely misguided: they frustrate legitimate 3733 applications in which mail is sent by one user on behalf of another, 3734 in which error (or normal) replies should be directed to a special 3735 address, or in which a single message is sent to multiple recipients 3736 on different hosts. (Systems that provide convenient ways for users 3737 to alter these header fields on a per-message basis should attempt to 3738 establish a primary and permanent mailbox address for the user so 3739 that Sender header fields within the message data can be generated 3740 sensibly.) 3742 This specification does not further address the authentication issues 3743 associated with SMTP other than to advocate that useful functionality 3744 not be disabled in the hope of providing some small margin of 3745 protection against a user who is trying to fake mail. 3747 7.2. "Blind" Copies 3749 Addresses that do not appear in the message header section may appear 3750 in the RCPT commands to an SMTP server for a number of reasons. The 3751 two most common involve the use of a mailing address as a "list 3752 exploder" (a single address that resolves into multiple addresses) 3753 and the appearance of "blind copies". Especially when more than one 3754 RCPT command is present, and in order to avoid defeating some of the 3755 purpose of these mechanisms, SMTP clients and servers SHOULD NOT copy 3756 the full set of RCPT command arguments into the header section, 3757 either as part of trace header fields or as informational or private- 3758 extension header fields. [[CREF20: [rfc5321bis] [[Note in draft - 3759 Suggestion from 20070124 that got lost: delete "especially" and "the 3760 full set of" -- copying the first one can be as harmful as copying 3761 all of them, at least without verifying that the addresses do appear 3762 in the headers. See G.7.9 and ticket #15.]] Since this rule is often 3763 violated in practice, and cannot be enforced, sending SMTP systems 3764 that are aware of "bcc" use MAY find it helpful to send each blind 3765 copy as a separate message transaction containing only a single RCPT 3766 command. 3768 There is no inherent relationship between either "reverse" (from the 3769 MAIL command) or "forward" (RCPT) addresses in the SMTP transaction 3770 ("envelope") and the addresses in the header section. Receiving 3771 systems SHOULD NOT attempt to deduce such relationships and use them 3772 to alter the header section of the message for delivery. The popular 3773 "Apparently-to" header field is a violation of this principle as well 3774 as a common source of unintended information disclosure and SHOULD 3775 NOT be used. 3777 7.3. VRFY, EXPN, and Security 3779 As discussed in Section 3.5, individual sites may want to disable 3780 either or both of VRFY or EXPN for security reasons (see below). As 3781 a corollary to the above, implementations that permit this MUST NOT 3782 appear to have verified addresses that are not, in fact, verified. 3783 If a site disables these commands for security reasons, the SMTP 3784 server MUST return a 252 response, rather than a code that could be 3785 confused with successful or unsuccessful verification. 3787 Returning a 250 reply code with the address listed in the VRFY 3788 command after having checked it only for syntax violates this rule. 3789 Of course, an implementation that "supports" VRFY by always returning 3790 550 whether or not the address is valid is equally not in 3791 conformance. 3793 On the public Internet, the contents of mailing lists have become 3794 popular as an address information source for so-called "spammers." 3795 The use of EXPN to "harvest" addresses has increased as list 3796 administrators have installed protections against inappropriate uses 3797 of the lists themselves. However, VRFY and EXPN are still useful for 3798 authenticated users and within an administrative domain. For 3799 example, VRFY and EXPN are useful for performing internal audits of 3800 how email gets routed to check and to make sure no one is 3801 automatically forwarding sensitive mail outside the organization. 3802 Sites implementing SMTP authentication may choose to make VRFY and 3803 EXPN available only to authenticated requestors. Implementations 3804 SHOULD still provide support for EXPN, but sites SHOULD carefully 3805 evaluate the tradeoffs. 3807 Whether disabling VRFY provides any real marginal security depends on 3808 a series of other conditions. In many cases, RCPT commands can be 3809 used to obtain the same information about address validity. On the 3810 other hand, especially in situations where determination of address 3811 validity for RCPT commands is deferred until after the DATA command 3812 is received, RCPT may return no information at all, while VRFY is 3813 expected to make a serious attempt to determine validity before 3814 generating a response code (see discussion above). 3816 7.4. Mail Rerouting Based on the 251 and 551 Response Codes 3818 Before a client uses the 251 or 551 reply codes from a RCPT command 3819 to automatically update its future behavior (e.g., updating the 3820 user's address book), it should be certain of the server's 3821 authenticity. If it does not, it may be subject to a man in the 3822 middle attack. 3824 7.5. Information Disclosure in Announcements 3826 There has been an ongoing debate about the tradeoffs between the 3827 debugging advantages of announcing server type and version (and, 3828 sometimes, even server domain name) in the greeting response or in 3829 response to the HELP command and the disadvantages of exposing 3830 information that might be useful in a potential hostile attack. The 3831 utility of the debugging information is beyond doubt. Those who 3832 argue for making it available point out that it is far better to 3833 actually secure an SMTP server rather than hope that trying to 3834 conceal known vulnerabilities by hiding the server's precise identity 3835 will provide more protection. Sites are encouraged to evaluate the 3836 tradeoff with that issue in mind; implementations SHOULD minimally 3837 provide for making type and version information available in some way 3838 to other network hosts. 3840 7.6. Information Disclosure in Trace Fields 3842 In some circumstances, such as when mail originates from within a LAN 3843 whose hosts are not directly on the public Internet, trace 3844 ("Received") header fields produced in conformance with this 3845 specification may disclose host names and similar information that 3846 would not normally be available. This ordinarily does not pose a 3847 problem, but sites with special concerns about name disclosure should 3848 be aware of it. Also, the optional FOR clause should be supplied 3849 with caution or not at all when multiple recipients are involved lest 3850 it inadvertently disclose the identities of "blind copy" recipients 3851 to others. 3853 7.7. Information Disclosure in Message Forwarding 3855 As discussed in Section 3.4, use of the 251 or 551 reply codes to 3856 identify the replacement address associated with a mailbox may 3857 inadvertently disclose sensitive information. Sites that are 3858 concerned about those issues should ensure that they select and 3859 configure servers appropriately. 3861 7.8. Local Operational Requirement and Resistance to Attacks 3863 In recent years, there has been an increase of attacks on SMTP 3864 servers, either in conjunction with attempts to discover addresses 3865 for sending unsolicited messages or simply to make the servers 3866 inaccessible to others (i.e., as an application-level denial of 3867 service attack). There may also be important local circumstances 3868 that justify departures from some of the limits specified in this 3869 documents especially ones involving maximums or minimums. While the 3870 means of doing so are beyond the scope of this Standard, rational 3871 operational behavior requires that servers be permitted to detect 3872 such attacks and take action to defend themselves. For example, if a 3873 server determines that a large number of RCPT commands are being 3874 sent, most or all with invalid addresses, as part of such an attack, 3875 it would be reasonable for the server to close the connection after 3876 generating an appropriate number of 5yz (normally 550) replies. 3878 7.9. Scope of Operation of SMTP Servers 3880 It is a well-established principle that an SMTP server may refuse to 3881 accept mail for any operational or technical reason that makes sense 3882 to the site providing the server. However, cooperation among sites 3883 and installations makes the Internet possible. If sites take 3884 excessive advantage of the right to reject traffic, the ubiquity of 3885 email availability (one of the strengths of the Internet) will be 3886 threatened; considerable care should be taken and balance maintained 3887 if a site decides to be selective about the traffic it will accept 3888 and process. 3890 In recent years, use of the relay function through arbitrary sites 3891 has been used as part of hostile efforts to hide the actual origins 3892 of mail. Some sites have decided to limit the use of the relay 3893 function to known or identifiable sources, and implementations SHOULD 3894 provide the capability to perform this type of filtering. When mail 3895 is rejected for these or other policy reasons, a 550 code SHOULD be 3896 used in response to EHLO (or HELO), MAIL, or RCPT as appropriate. 3898 8. IANA Considerations 3900 IANA maintains three registries in support of this specification, all 3901 of which were created for RFC 2821 or earlier. This document expands 3902 the third one as specified below. The registry references listed are 3903 as of the time of publication; IANA does not guarantee the locations 3904 associated with the URLs. The registries are as follows: 3906 o The first, "Simple Mail Transfer Protocol (SMTP) Service 3907 Extensions" [50], consists of SMTP service extensions with the 3908 associated keywords, and, as needed, parameters and verbs. As 3909 specified in Section 2.2.2, no entry may be made in this registry 3910 that starts in an "X". Entries may be made only for service 3911 extensions (and associated keywords, parameters, or verbs) that 3912 are defined in Standards-Track or Experimental RFCs specifically 3913 approved by the IESG for this purpose. 3915 o The second registry, "Address Literal Tags" [51], consists of 3916 "tags" that identify forms of domain literals other than those for 3917 IPv4 addresses (specified in RFC 821 and in this document). The 3918 initial entry in that registry is for IPv6 addresses (specified in 3919 this document). Additional literal types require standardization 3920 before being used; none are anticipated at this time. 3922 o The third, "Mail Transmission Types" [50], established by RFC 821 3923 and renewed by this specification, is a registry of link and 3924 protocol identifiers to be used with the "via" and "with" 3925 subclauses of the time stamp ("Received:" header field) described 3926 in Section 4.4. Link and protocol identifiers in addition to 3927 those specified in this document may be registered only by 3928 standardization or by way of an RFC-documented, IESG-approved, 3929 Experimental protocol extension. This name space is for 3930 identification and not limited in size: the IESG is encouraged to 3931 approve on the basis of clear documentation and a distinct method 3932 rather than preferences about the properties of the method itself. 3934 An additional subsection has been added to the "VIA link types" 3935 and "WITH protocol types" subsections of this registry to contain 3936 registrations of "Additional-registered-clauses" as described 3937 above. The registry will contain clause names, a description, a 3938 summary of the syntax of the associated String, and a reference. 3939 As new clauses are defined, they may, in principle, specify 3940 creation of their own registries if the Strings consist of 3941 reserved terms or keywords rather than less restricted strings. 3942 As with link and protocol identifiers, additional clauses may be 3943 registered only by standardization or by way of an RFC-documented, 3944 IESG-approved, Experimental protocol extension. The additional 3945 clause name space is for identification and is not limited in 3946 size: the IESG is encouraged to approve on the basis of clear 3947 documentation, actual use or strong signs that the clause will be 3948 used, and a distinct requirement rather than preferences about the 3949 properties of the clause itself. 3951 In addition, if additional trace header fields (i.e., in addition to 3952 Return-path and Received) are ever created, those trace fields MUST 3953 be added to the IANA registry established by BCP 90 (RFC 3864) [9] 3954 for use with RFC 5322 [12]. 3956 9. Acknowledgments 3958 Many people contributed to the development of RFCs 2821 and 5321. 3959 Those documents should be consulted for those acknowledgments. 3961 Neither this document nor RFCs 2821 or 5321 would have been possible 3962 without the many contribution and insights of the late Jon Postel. 3963 Those contributions of course include the original specification of 3964 SMTP in RFC 821. A considerable quantity of text from RFC 821 still 3965 appears in this document as do several of Jon's original examples 3966 that have been updated only as needed to reflect other changes in the 3967 specification. 3969 The following filed errata against RFC 5321 that were not rejected at 3970 the time of submission: Jasen Betts, Adrien de Croy Guillaume Fortin- 3971 Debigare Roberto Javier Godoy, David Romerstein, Dominic Sayers, 3972 Rodrigo Speller, Alessandro Vesely, and Brett Watson. Some of those 3973 individuals made additional suggestions after the EMAILCORE WG was 3974 initiated. In addition to the above, several of whom continued to 3975 make other suggestions, specific suggestions that led to corrections 3976 and improvements in early versions of the current specification were 3977 received from Dave Crocker, Ned Freed, Arnt Gulbrandsen, Tony Hansen, 3978 Barry Leiba, Ivar Lumi, Pete Resnick, Hector Santos, Paul Smith and 3979 others. 3981 chetti contributed an analysis that clarified the ABNF productions 3982 that implicitly reference other documents. 3984 The EMAILCORE Working Group was chartered in September 2020 with 3985 Alexey Melnikov and Seth Blank as co-chairs. Todd Herr replaced Seth 3986 Blank early in 2021. Without their leadership and technical 3987 contributions, this document would never have been completed. 3989 10. References 3991 10.1. Normative References 3993 [1] Bradner, S., "Key words for use in RFCs to Indicate 3994 Requirement Levels", BCP 14, RFC 2119, 3995 DOI 10.17487/RFC2119, March 1997, 3996 . 3998 [2] American National Standards Institute (formerly United 3999 States of America Standards Institute), "USA Code for 4000 Information Interchange", ANSI X3.4-1968, 1968. 4002 ANSI X3.4-1968 has been replaced by newer versions with 4003 slight modifications, but the 1968 version remains 4004 definitive for the Internet. 4006 [3] Postel, J., "Simple Mail Transfer Protocol", STD 10, 4007 RFC 821, DOI 10.17487/RFC0821, August 1982, 4008 . 4010 [4] Mockapetris, P., "Domain names - implementation and 4011 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 4012 November 1987, . 4014 [5] Braden, R., Ed., "Requirements for Internet Hosts - 4015 Application and Support", STD 3, RFC 1123, 4016 DOI 10.17487/RFC1123, October 1989, 4017 . 4019 [6] Klensin, J., Freed, N., and K. Moore, "SMTP Service 4020 Extension for Message Size Declaration", STD 10, RFC 1870, 4021 DOI 10.17487/RFC1870, November 1995, 4022 . 4024 [7] Vaudreuil, G., "Enhanced Mail System Status Codes", 4025 RFC 3463, DOI 10.17487/RFC3463, January 2003, 4026 . 4028 [8] Newman, C., "ESMTP and LMTP Transmission Types 4029 Registration", RFC 3848, DOI 10.17487/RFC3848, July 2004, 4030 . 4032 [9] Klyne, G., Nottingham, M., and J. Mogul, "Registration 4033 Procedures for Message Header Fields", BCP 90, RFC 3864, 4034 DOI 10.17487/RFC3864, September 2004, 4035 . 4037 [10] Hinden, R. and S. Deering, "IP Version 6 Addressing 4038 Architecture", RFC 4291, DOI 10.17487/RFC4291, February 4039 2006, . 4041 [11] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 4042 Specifications: ABNF", STD 68, RFC 5234, 4043 DOI 10.17487/RFC5234, January 2008, 4044 . 4046 [12] Resnick, P., "Internet Message Format", RFC 5322, 4047 September 2008. 4049 10.2. Informative References 4051 [13] Crocker, D., "STANDARD FOR THE FORMAT OF ARPA INTERNET 4052 TEXT MESSAGES", STD 11, RFC 822, DOI 10.17487/RFC0822, 4053 August 1982, . 4055 [14] Butler, M., Postel, J., Chase, D., Goldberger, J., and J. 4056 Reynolds, "Post Office Protocol: Version 2", RFC 937, 4057 DOI 10.17487/RFC0937, February 1985, 4058 . 4060 [15] Postel, J. and J. Reynolds, "File Transfer Protocol", 4061 STD 9, RFC 959, DOI 10.17487/RFC0959, October 1985, 4062 . 4064 [16] Partridge, C., "Mail routing and the domain system", 4065 STD 10, RFC 974, DOI 10.17487/RFC0974, January 1986, 4066 . 4068 [17] Partridge, C., "Duplicate messages and SMTP", RFC 1047, 4069 DOI 10.17487/RFC1047, February 1988, 4070 . 4072 [18] Lambert, M., "PCMAIL: A distributed mail system for 4073 personal computers", RFC 1056, DOI 10.17487/RFC1056, June 4074 1988, . 4076 [19] Crispin, M., "Interactive Mail Access Protocol: Version 4077 2", RFC 1176, DOI 10.17487/RFC1176, August 1990, 4078 . 4080 [20] Durand, A. and F. Dupont, "SMTP 521 Reply Code", RFC 1846, 4081 DOI 10.17487/RFC1846, September 1995, 4082 . 4084 [21] Galvin, J., Murphy, S., Crocker, S., and N. Freed, 4085 "Security Multiparts for MIME: Multipart/Signed and 4086 Multipart/Encrypted", RFC 1847, DOI 10.17487/RFC1847, 4087 October 1995, . 4089 [22] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D. 4090 Crocker, "SMTP Service Extensions", STD 10, RFC 1869, 4091 DOI 10.17487/RFC1869, November 1995, 4092 . 4094 [23] Myers, J. and M. Rose, "Post Office Protocol - Version 3", 4095 STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996, 4096 . 4098 [24] De Winter, J., "SMTP Service Extension for Remote Message 4099 Queue Starting", RFC 1985, DOI 10.17487/RFC1985, August 4100 1996, . 4102 [25] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 4103 Extensions (MIME) Part One: Format of Internet Message 4104 Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996, 4105 . 4107 [26] Moore, K., "MIME (Multipurpose Internet Mail Extensions) 4108 Part Three: Message Header Extensions for Non-ASCII Text", 4109 RFC 2047, DOI 10.17487/RFC2047, November 1996, 4110 . 4112 [27] Kille, S., "MIXER (Mime Internet X.400 Enhanced Relay): 4113 Mapping between X.400 and RFC 822/MIME", RFC 2156, 4114 DOI 10.17487/RFC2156, January 1998, 4115 . 4117 [28] Elz, R. and R. Bush, "Clarifications to the DNS 4118 Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997, 4119 . 4121 [29] Freed, N. and K. Moore, "MIME Parameter Value and Encoded 4122 Word Extensions: Character Sets, Languages, and 4123 Continuations", RFC 2231, DOI 10.17487/RFC2231, November 4124 1997, . 4126 [30] Klensin, J., Ed., "Simple Mail Transfer Protocol", 4127 RFC 2821, DOI 10.17487/RFC2821, April 2001, 4128 . 4130 [31] Freed, N., "SMTP Service Extension for Command 4131 Pipelining", STD 60, RFC 2920, DOI 10.17487/RFC2920, 4132 September 2000, . 4134 [32] Freed, N., "Behavior of and Requirements for Internet 4135 Firewalls", RFC 2979, DOI 10.17487/RFC2979, October 2000, 4136 . 4138 [33] Vaudreuil, G., "SMTP Service Extensions for Transmission 4139 of Large and Binary MIME Messages", RFC 3030, 4140 DOI 10.17487/RFC3030, December 2000, 4141 . 4143 [34] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service 4144 Extension for Delivery Status Notifications (DSNs)", 4145 RFC 3461, DOI 10.17487/RFC3461, January 2003, 4146 . 4148 [35] Moore, K. and G. Vaudreuil, "An Extensible Message Format 4149 for Delivery Status Notifications", RFC 3464, 4150 DOI 10.17487/RFC3464, January 2003, 4151 . 4153 [36] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4154 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, 4155 . 4157 [37] Hansen, T., Ed. and A. Melnikov, Ed., "Message Disposition 4158 Notification", STD 85, RFC 8098, DOI 10.17487/RFC8098, 4159 February 2017, . 4161 [38] Schaad, J., Ramsdell, B., and S. Turner, "Secure/ 4162 Multipurpose Internet Mail Extensions (S/MIME) Version 4.0 4163 Message Specification", RFC 8551, DOI 10.17487/RFC8551, 4164 April 2019, . 4166 [39] Nakamura, M. and J. Hagino, "SMTP Operational Experience 4167 in Mixed IPv4/v6 Environments", RFC 3974, 4168 DOI 10.17487/RFC3974, January 2005, 4169 . 4171 [40] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 4172 Resource Identifier (URI): Generic Syntax", STD 66, 4173 RFC 3986, DOI 10.17487/RFC3986, January 2005, 4174 . 4176 [41] Kitterman, S., "Sender Policy Framework (SPF) for 4177 Authorizing Use of Domains in Email, Version 1", RFC 7208, 4178 DOI 10.17487/RFC7208, April 2014, 4179 . 4181 [42] Gellens, R. and J. Klensin, "Message Submission for Mail", 4182 STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011, 4183 . 4185 [43] Fenton, J., "Analysis of Threats Motivating DomainKeys 4186 Identified Mail (DKIM)", RFC 4686, DOI 10.17487/RFC4686, 4187 September 2006, . 4189 [44] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed., 4190 "DomainKeys Identified Mail (DKIM) Signatures", STD 76, 4191 RFC 6376, DOI 10.17487/RFC6376, September 2011, 4192 . 4194 [45] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. 4195 Thayer, "OpenPGP Message Format", RFC 4880, 4196 DOI 10.17487/RFC4880, November 2007, 4197 . 4199 [46] Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced 4200 Mail System Status Codes", BCP 138, RFC 5248, 4201 DOI 10.17487/RFC5248, June 2008, 4202 . 4204 [47] Klensin, J., Freed, N., Rose, M., and D. Crocker, Ed., 4205 "SMTP Service Extension for 8-bit MIME Transport", STD 71, 4206 RFC 6152, DOI 10.17487/RFC6152, March 2011, 4207 . 4209 [48] Klensin, J., "SMTP 521 and 556 Reply Codes", RFC 7504, 4210 DOI 10.17487/RFC7504, June 2015, 4211 . 4213 [49] Levine, J. and M. Delany, "A "Null MX" No Service Resource 4214 Record for Domains That Accept No Mail", RFC 7505, 4215 DOI 10.17487/RFC7505, June 2015, 4216 . 4218 [50] Internet Assigned Number Authority (IANA), "IANA Mail 4219 Parameters", 2007, 4220 . 4222 [51] Internet Assigned Number Authority (IANA), "Address 4223 Literal Tags", 2007, 4224 . 4226 [52] RFC Editor, "RFC Errata - RFC 5321", 2019, 4227 . 4229 Captured 2019-11-19 4231 [53] IANA, "SMTP Service Extensions", 2021, 4232 . 4235 Notes in draft: RFC Editor: Please adjust date field to 4236 reflect whatever you want for a registry that is updated 4237 periodically. IANA: Please determine if the above URL is 4238 a sufficiently stable reference and adjust as appropriate 4239 if it is not. 4241 Appendix A. TCP Transport Service 4243 The TCP connection supports the transmission of 8-bit bytes. The 4244 SMTP data is 7-bit ASCII characters. Each character is transmitted 4245 as an 8-bit byte with the high-order bit cleared to zero. Service 4246 extensions may modify this rule to permit transmission of full 8-bit 4247 data bytes as part of the message body, or, if specifically designed 4248 to do so, in SMTP commands or responses. 4250 Appendix B. Generating SMTP Commands from RFC 822 Header Fields 4252 Some systems use an RFC 822 header section (only) in a mail 4253 submission protocol, or otherwise generate SMTP commands from RFC 822 4254 header fields when such a message is handed to an MTA from a UA. 4255 While the MTA-UA protocol is a private matter, not covered by any 4256 Internet Standard, there are problems with this approach. For 4257 example, there have been repeated problems with proper handling of 4258 "bcc" copies and redistribution lists when information that 4259 conceptually belongs to the mail envelope is not separated early in 4260 processing from header field information (and kept separate). 4262 It is recommended that the UA provide its initial ("submission 4263 client") MTA with an envelope separate from the message itself. 4264 However, if the envelope is not supplied, SMTP commands SHOULD be 4265 generated as follows: 4267 1. Each recipient address from a TO, CC, or BCC header field SHOULD 4268 be copied to a RCPT command (generating multiple message copies 4269 if that is required for queuing or delivery). This includes any 4270 addresses listed in a RFC 822 "group". Any BCC header fields 4271 SHOULD then be removed from the header section. Once this 4272 process is completed, the remaining header fields SHOULD be 4273 checked to verify that at least one TO, CC, or BCC header field 4274 remains. If none do, then a BCC header field with no additional 4275 information SHOULD be inserted as specified in [12]. 4277 2. The return address in the MAIL command SHOULD, if possible, be 4278 derived from the system's identity for the submitting (local) 4279 user, and the "From:" header field otherwise. If there is a 4280 system identity available, it SHOULD also be copied to the Sender 4281 header field if it is different from the address in the From 4282 header field. (Any Sender header field that was already there 4283 SHOULD be removed.) Systems may provide a way for submitters to 4284 override the envelope return address, but may want to restrict 4285 its use to privileged users. This will not prevent mail forgery, 4286 but may lessen its incidence; see Section 7.1. 4288 When an MTA is being used in this way, it bears responsibility for 4289 ensuring that the message being transmitted is valid. The mechanisms 4290 for checking that validity, and for handling (or returning) messages 4291 that are not valid at the time of arrival, are part of the MUA-MTA 4292 interface and not covered by this specification. 4294 A submission protocol based on Standard RFC 822 information alone 4295 MUST NOT be used to gateway a message from a foreign (non-SMTP) mail 4296 system into an SMTP environment. Additional information to construct 4297 an envelope must come from some source in the other environment, 4298 whether supplemental header fields or the foreign system's envelope. 4300 Attempts to gateway messages using only their header "To" and "Cc" 4301 fields have repeatedly caused mail loops and other behavior adverse 4302 to the proper functioning of the Internet mail environment. These 4303 problems have been especially common when the message originates from 4304 an Internet mailing list and is distributed into the foreign 4305 environment using envelope information. When these messages are then 4306 processed by a header-section-only remailer, loops back to the 4307 Internet environment (and the mailing list) are almost inevitable. 4309 Appendix C. Source Routes 4311 Historically, the was a reverse source routing list of 4312 hosts and a source mailbox. The first host in the was 4313 historically the host sending the MAIL command; today, source routes 4314 SHOULD NOT appear in the reverse-path. Similarly, the 4315 may be a source routing lists of hosts and a destination mailbox. 4316 However, in general, the SHOULD contain only a mailbox 4317 and domain name, relying on the domain name system to supply routing 4318 information if required. The use of source routes is deprecated (see 4319 Appendix F.2); while servers MUST be prepared to receive and handle 4320 them as discussed in Section 3.3 and Appendix F.2, clients SHOULD NOT 4321 transmit them and this section is included in the current 4322 specification only to provide context. It has been modified somewhat 4323 from the material in RFC 821 to prevent server actions that might 4324 confuse clients or subsequent servers that do not expect a full 4325 source route implementation. 4327 Historically, for relay purposes, the forward-path may have been a 4328 source route of the form "@ONE,@TWO:JOE@THREE", where ONE, TWO, and 4329 THREE MUST be fully-qualified domain names. This form was used to 4330 emphasize the distinction between an address and a route. The 4331 mailbox (here, JOE@THREE) is an absolute address, and the route is 4332 information about how to get there. The two concepts should not be 4333 confused. 4335 If source routes are used contrary to requirements and 4336 recommendations elsewhere in this specification, RFC 821 and the text 4337 below should be consulted for the mechanisms for constructing and 4338 updating the forward-path. A server that is reached by means of a 4339 source route (e.g., its domain name appears first in the list in the 4340 forward-path) MUST remove its domain name from any forward-paths in 4341 which that domain name appears before forwarding the message and MAY 4342 remove all other source routing information. The reverse-path SHOULD 4343 NOT be updated by servers conforming to this specification. 4345 Notice that the forward-path and reverse-path appear in the SMTP 4346 commands and replies, but not necessarily in the message. That is, 4347 there is no need for these paths and especially this syntax to appear 4348 in the "To:" , "From:", "CC:", etc. fields of the message header 4349 section. Conversely, SMTP servers MUST NOT derive final message 4350 routing information from message header fields. 4352 When the list of hosts is present despite the recommendations and 4353 requirements above, it is a "reverse" source route and indicates that 4354 the mail was relayed through each host on the list (the first host in 4355 the list was the most recent relay). This list is used as a source 4356 route to return non-delivery notices to the sender. If, contrary to 4357 the recommendations here, a relay host adds itself to the beginning 4358 of the list, it MUST use its name as known in the transport 4359 environment to which it is relaying the mail rather than that of the 4360 transport environment from which the mail came (if they are 4361 different). Note that a situation could easily arise in which some 4362 relay hosts add their names to the reverse source route and others do 4363 not, generating discontinuities in the routing list. This is another 4364 reason why servers needing to return a message SHOULD ignore the 4365 source route entirely and simply use the domain as specified in the 4366 Mailbox. 4368 Appendix D. Scenarios 4370 This section presents complete scenarios of several types of SMTP 4371 sessions. In the examples, "C:" indicates what is said by the SMTP 4372 client, and "S:" indicates what is said by the SMTP server. 4374 D.1. A Typical SMTP Transaction Scenario 4376 This SMTP example shows mail sent by Smith at host bar.com, and to 4377 Jones, Green, and Brown at host foo.com. Here we assume that host 4378 bar.com contacts host foo.com directly. The mail is accepted for 4379 Jones and Brown. Green does not have a mailbox at host foo.com. 4381 S: 220 foo.com Simple Mail Transfer Service Ready 4382 C: EHLO bar.com 4383 S: 250-foo.com greets bar.com 4384 S: 250-8BITMIME 4385 S: 250-SIZE 4386 S: 250-DSN 4387 S: 250 HELP 4388 C: MAIL FROM: 4389 S: 250 OK 4390 C: RCPT TO: 4391 S: 250 OK 4392 C: RCPT TO: 4393 S: 550 No such user here 4394 C: RCPT TO: 4395 S: 250 OK 4396 C: DATA 4397 S: 354 Start mail input; end with . 4398 C: Blah blah blah... 4399 C: ...etc. etc. etc. 4400 C: . 4401 S: 250 OK 4402 C: QUIT 4403 S: 221 foo.com Service closing transmission channel 4405 D.2. Aborted SMTP Transaction Scenario 4407 S: 220 foo.com Simple Mail Transfer Service Ready 4408 C: EHLO bar.com 4409 S: 250-foo.com greets bar.com 4410 S: 250-8BITMIME 4411 S: 250-SIZE 4412 S: 250-DSN 4413 S: 250 HELP 4414 C: MAIL FROM: 4415 S: 250 OK 4416 C: RCPT TO: 4417 S: 250 OK 4418 C: RCPT TO: 4419 S: 550 No such user here 4420 C: RSET 4421 S: 250 OK 4422 C: QUIT 4423 S: 221 foo.com Service closing transmission channel 4425 D.3. Relayed Mail Scenario 4427 Step 1 -- Source Host to Relay Host 4429 The source host performs a DNS lookup on XYZ.COM (the destination 4430 address) and finds DNS MX records specifying xyz.com as the best 4431 preference and foo.com as a lower preference. It attempts to open a 4432 connection to xyz.com and fails. It then opens a connection to 4433 foo.com, with the following dialogue: 4435 S: 220 foo.com Simple Mail Transfer Service Ready 4436 C: EHLO bar.com 4437 S: 250-foo.com greets bar.com 4438 S: 250-8BITMIME 4439 S: 250-SIZE 4440 S: 250-DSN 4441 S: 250 HELP 4442 C: MAIL FROM: 4443 S: 250 OK 4444 C: RCPT TO: 4445 S: 250 OK 4446 C: DATA 4447 S: 354 Start mail input; end with . 4448 C: Date: Thu, 21 May 1998 05:33:29 -0700 4449 C: From: John Q. Public 4450 C: Subject: The Next Meeting of the Board 4451 C: To: Jones@xyz.com 4452 C: 4453 C: Bill: 4454 C: The next meeting of the board of directors will be 4455 C: on Tuesday. 4456 C: John. 4457 C: . 4458 S: 250 OK 4459 C: QUIT 4460 S: 221 foo.com Service closing transmission channel 4462 Step 2 -- Relay Host to Destination Host 4464 foo.com, having received the message, now does a DNS lookup on 4465 xyz.com. It finds the same set of MX records, but cannot use the one 4466 that points to itself (or to any other host as a worse preference). 4467 It tries to open a connection to xyz.com itself and succeeds. Then 4468 we have: 4470 S: 220 xyz.com Simple Mail Transfer Service Ready 4471 C: EHLO foo.com 4472 S: 250 xyz.com is on the air 4473 C: MAIL FROM: 4474 S: 250 OK 4475 C: RCPT TO: 4476 S: 250 OK 4477 C: DATA 4478 S: 354 Start mail input; end with . 4479 C: Received: from bar.com by foo.com ; Thu, 21 May 1998 4480 C: 05:33:29 -0700 4481 C: Date: Thu, 21 May 1998 05:33:29 -0700 4482 C: From: John Q. Public 4483 C: Subject: The Next Meeting of the Board 4484 C: To: Jones@xyz.com 4485 C: 4486 C: Bill: 4487 C: The next meeting of the board of directors will be 4488 C: on Tuesday. 4489 C: John. 4490 C: . 4491 S: 250 OK 4492 C: QUIT 4493 S: 221 xyz.com Service closing transmission channel 4495 D.4. Verifying and Sending Scenario 4497 S: 220 foo.com Simple Mail Transfer Service Ready 4498 C: EHLO bar.com 4499 S: 250-foo.com greets bar.com 4500 S: 250-8BITMIME 4501 S: 250-SIZE 4502 S: 250-DSN 4503 S: 250-VRFY 4504 S: 250 HELP 4505 C: VRFY Crispin 4506 S: 250 Mark Crispin 4507 C: MAIL FROM: 4508 S: 250 OK 4509 C: RCPT TO: 4510 S: 250 OK 4511 C: DATA 4512 S: 354 Start mail input; end with . 4513 C: Blah blah blah... 4514 C: ...etc. etc. etc. 4515 C: . 4516 S: 250 OK 4517 C: QUIT 4518 S: 221 foo.com Service closing transmission channel 4520 Appendix E. Other Gateway Issues 4522 In general, gateways between the Internet and other mail systems 4523 SHOULD attempt to preserve any layering semantics across the 4524 boundaries between the two mail systems involved. Gateway- 4525 translation approaches that attempt to take shortcuts by mapping 4526 (such as mapping envelope information from one system to the message 4527 header section or body of another) have generally proven to be 4528 inadequate in important ways. Systems translating between 4529 environments that do not support both envelopes and a header section 4530 and Internet mail must be written with the understanding that some 4531 information loss is almost inevitable. 4533 Appendix F. Deprecated Features of RFC 821 4535 A few features of RFC 821 have proven to be problematic and SHOULD 4536 NOT be used in Internet mail. Some of these features were deprecated 4537 in RFC 2821 in 2001; source routing and two-digit years in dates were 4538 deprecated by RFC 1123 in 1989. Of the domain literal forms, RFC 4539 1123 required support only for the dotted decimal form. With the 4540 possible exception of old, hardware-embedded, applications, there is 4541 no longer any excuse for these features to appear on the contemporary 4542 Internet. 4544 F.1. TURN 4546 This command, described in RFC 821, raises important security issues 4547 since, in the absence of strong authentication of the host requesting 4548 that the client and server switch roles, it can easily be used to 4549 divert mail from its correct destination. Its use is deprecated; 4550 SMTP systems SHOULD NOT use it unless the server can authenticate the 4551 client. 4553 F.2. Source Routing 4555 RFC 821 utilized the concept of explicit source routing to get mail 4556 from one host to another via a series of relays. The requirement to 4557 utilize source routes in regular mail traffic was eliminated by the 4558 introduction of the domain name system "MX" record and the last 4559 significant justification for them was eliminated by the 4560 introduction, in RFC 1123, of a clear requirement that addresses 4561 following an "@" must all be fully-qualified domain names. 4562 Consequently, the only remaining justifications for the use of source 4563 routes are support for very old SMTP clients or MUAs and in mail 4564 system debugging. They can, however, still be useful in the latter 4565 circumstance and for routing mail around serious, but temporary, 4566 problems such as problems with the relevant DNS records. 4568 SMTP servers MUST continue to accept source route syntax as specified 4569 in the main body of this document and in RFC 1123. They MAY, if 4570 necessary, ignore the routes and utilize only the target domain in 4571 the address. If they do utilize the source route, the message MUST 4572 be sent to the first domain shown in the address. In particular, a 4573 server MUST NOT guess at shortcuts within the source route. 4575 Clients SHOULD NOT utilize explicit source routing except under 4576 unusual circumstances, such as debugging or potentially relaying 4577 around firewall or mail system configuration errors. 4579 F.3. HELO 4581 As discussed in Sections 3.1 and 4.1.1, EHLO SHOULD be used rather 4582 than HELO when the server will accept the former. Servers MUST 4583 continue to accept and process HELO in order to support older 4584 clients. 4586 F.4. #-literals 4588 RFC 821 provided for specifying an Internet address as a decimal 4589 integer host number prefixed by a pound sign, "#". In practice, that 4590 form has been obsolete since the introduction of TCP/IP. It is 4591 deprecated and MUST NOT be used. 4593 F.5. Dates and Years 4595 When dates are inserted into messages by SMTP clients or servers 4596 (e.g., in trace header fields), four-digit years MUST BE used. Two- 4597 digit years are deprecated; three-digit years were never permitted in 4598 the Internet mail system. 4600 F.6. Sending versus Mailing 4602 In addition to specifying a mechanism for delivering messages to 4603 user's mailboxes, RFC 821 provided additional, optional, commands to 4604 deliver messages directly to the user's terminal screen. These 4605 commands (SEND, SAML, SOML) were rarely implemented, and changes in 4606 workstation technology and the introduction of other protocols may 4607 have rendered them obsolete even where they are implemented. 4609 [[5321bis Editor's Note: does this need a stronger reference to 821, 4610 2821, and/or 5321? Also, is anything else needed given the removal 4611 of these commands and comments about, e.g., their opening mail 4612 transaction sessions, from the mail body of the text?]] 4613 Clients SHOULD NOT provide SEND, SAML, or SOML as services. Servers 4614 MAY implement them. If they are implemented by servers, the 4615 implementation model specified in RFC 821 MUST be used and the 4616 command names MUST be published in the response to the EHLO command. 4618 Appendix G. Other Outstanding Issues 4620 [[RFC Editor: Please remove this section before publication.]] 4622 In December 2019, an issue was raised on the ietf-smtp@ietf.org list 4623 that led to a broad discussion of ways in which existing practice had 4624 diverged from the specifications and recommendations of RFC 5321 in 4625 the more than eleven years since it was published (some of those 4626 issues probably affect the boundary between RFC 5321 and 5322 and 4627 hence the latter as well). In most cases, those divergences call for 4628 revision of the Technical Specification to match the practice, 4629 clarification of the specification text in other ways, or a more 4630 comprehensive explanation of why the practices recommended by the 4631 specification should really be followed. 4633 Those discussions raised two other issues, which were that 4635 o The publication of the Submission Server specification of RFC 6409 4636 in November 2011 may not have been fully reflected in RFC 5321 4637 (despite the even earlier publication of RFC 4409) and 4639 o There may be inconsistencies between the July 2009 Internet Mail 4640 Architecture description of RFC 5598 and the model described in 4641 RFC 5321. The issue called out in Appendix G.3 below may be an 4642 example of one of those inconsistencies. 4644 Those discrepancies should be identified and discussed and decisions 4645 made to fix them (and where) or to ignore them and let them continue. 4647 There has also been discussion on the mailing list, perhaps amounting 4648 to very rough consensus, that any revision of RFC 5321 and/or 5322 4649 should be accompanied by a separate Applicability Statement document 4650 that would make recommendations about applicability or best practices 4651 in particular areas rather than trying to get everything into the two 4652 technical specifications. This appendix does not attempt to identify 4653 which issues should get which treatment. 4655 This work is now (starting in the last half of 2020) being considered 4656 in the EMAILCORE WG. This appendix will act as a temporary record of 4657 issues that should be discussed and decided upon before a revised 4658 SMTP specification (or a related Applicability Statement) is 4659 published, issues that have not been reflected in errata (see 4660 Appendix H.1 below for those covered by errata). 4662 Ticket numbers listed below reference the list in 4663 https://trac.ietf.org/trac/emailcore/report/1 . 4665 G.1. IP Address literals 4667 The specification is unclear about whether IP address literals, 4668 particularly IP address literals used as arguments to the EHLO 4669 command, are required to be accepted or whether they are allowed to 4670 be rejected as part of the general "operational necessity" exception. 4671 Some have suggested that rejection of them is so common as an anti- 4672 spam measure that the use of such literals should be deprecated 4673 entirely in the specification, others that the are still useful and 4674 used and/or that, whatever is said about IP address literals within 4675 an SMTP session (e.g., in MAIL or RCPT commands), they should 4676 continue to be allowed (and required) in EHLO. 4677 Ticket #1. 4679 G.2. Repeated Use of EHLO 4681 While the specification says that an SMTP client's sending EHLO again 4682 after it has been issued (starting an SMTP session and treats it as 4683 if RSET had been sent (closing the session) followed by EHLO, there 4684 are apparently applications, at least some of them involving setting 4685 up of secure connections, in which the second EHLO is required and 4686 does not imply RSET. Does the specification need to be adjusted to 4687 reflect or call out those cases? 4689 After extended discussion in October 2020, it appears that the 4690 easiest fix to these problems is to clarify the conditions for 4691 termination of a mail transaction in Section 3.3 and to clearly 4692 specify the effect of a second (or subsequent) EHLO command in 4693 Section 4.1.4. 4694 See also Appendix G.7.4. 4695 Ticket #2. Both changes have been made in draft-ietf-emailcore- 4696 rfc5321bis-01. 4698 G.3. Meaning of "MTA" and Related Terminology 4700 A terminology issue has come up about what the term "MTA" actually 4701 refers to, a question that became at least slightly more complicated 4702 when we formalized RFC 6409 Submission Servers. Does the document 4703 need to be adjusted to be more clear about this topic? Note that the 4704 answer may interact with the question asked in Section 2 above. 4705 Possibly along the same lines, RFC 2821 changed the RFC 821 4706 terminology from "sender-SMTP" and "receiver-SMTP" to "SMTP client" 4707 and "SMTP server" respectively. As things have evolved, it is 4708 possible that newer terminology is a source of confusion and that the 4709 terminology should be changed back, something that also needs 4710 discussion. 4711 Ticket #3. 4713 G.4. Originator, or Originating System, Authentication 4715 Should RFC 5321bis address authentication and related issues or 4716 should Section 3.9 or other text be reshaped (in addition to or 4717 instead of the comment on that section) to lay a better foundation 4718 for such work, either in the context of mailing lists or more 4719 generally? 4720 This may interact with Erratum 4055 and Ticket #30 below. 4722 G.5. Remove or deprecate the work-around from code 552 to 452 4724 The suggestion in Section 4.5.3.1.10 may have outlived its usefulness 4725 and/or be inconsistent with current practice. Should it be removed 4726 and/or explicitly deprecated? 4727 Ticket #5. 4729 SHOULD requirement removed. 4731 G.6. Clarify where the protocol stands with respect to submission and 4732 TLS issues 4734 1. submission on port 587 4736 2. submission on port 465 4738 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 [[CREF21: See correspondence on this ticket 2021-07-06 through 4780 2021-07-09.]] 4782 G.7.5. Issues with mailing lists, aliases, and forwarding 4784 CREF comment in Section 3.9. May also want to note forwarding as an 4785 email address portability issue. Note that, if changes are made in 4786 this area, they should be kept consistent with the description and 4787 discussion of the 251 and 551 in Section 4.2 and Section 3.5 as well 4788 as Section 3.4 to avoid introducing inconsistencies. In addition, 4789 there are some terminology issues about the use of the term "lists", 4790 identified in erratum 1820, that should be reviewed after any more 4791 substantive changes are made to the relevant sections. 4792 Ticket #12 and Ticket #34. 4794 G.7.6. Requirements for domain name and/or IP address in EHLO 4796 CREF comment in Section 4.1.4 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? 4870 G.7.16. Mail Transaction Discussion 4872 Does the discussion of mail transactions need more work (see CREF in 4873 Section 3.3.)? 4875 G.7.17. Hop-by-hop Authentication and/or Encryption 4877 Should this document discuss hop-by-hop authentication or, for that 4878 matter, encryption? (See CREF in Section 2.) 4880 G.7.18. More Text About 554 Given 521, etc. 4882 Does reply code 554 need additional or different explanation in the 4883 light of the addition of the new 521 code and/or the new (in 5321bis 4884 Section 4.2.4.2? (See CREF in Section 4.2.3.) 4886 G.7.19. Minimum Lengths and Quantities 4888 Are the minimum lengths and quantities specified in Section 4.5.3 4889 still appropriate or do they need adjusting? (See CREF at the 4890 beginning of that section.) Also note potential interaction with the 4891 proposed LIMITS SMTP extension (draft-freed-smtp-limits) which may 4892 make this question OBE. 4894 G.8. Enhanced Reply Codes and DSNs 4896 Enhanced Mail System Status Codes (RFC 3463) [7] were added to SMTP 4897 before RFC 5321 was published and are now, together with a 4898 corresponding registry [46], widely deployed and in extensive use in 4899 the network. Similar, the structure and extensions options for 4900 Delivery Status Notifications [35] is implemented, deployed, and in 4901 wide use. Is it time to fold all or part of those mature 4902 specifications into the SMTP spec or at least to mention and 4903 normatively reference them? And, as an aside, do those specs need 4904 work or, if they are kept separate, is it time to move them to 4905 Internet Standard? 4907 At least one of the current references to RFC 3463 indicates that it 4908 SHOULD be used. That presumably makes the reference normative 4909 because one needs that specification to know what the present 4910 document requires. It has been moved in the -03 version of this 4911 draft, but, unless it is move to Internet Standard, it will require 4912 downref treatment. 4914 G.9. Revisiting Quoted Strings 4916 Recent discussions both in and out of the IETF have highlighted 4917 instances of non-compliance with the specification of a Local-part 4918 consisting of a Quoted-string, whether any content of QcontentSMTP 4919 that actually requires special treatment consists of qtextSMTP, 4920 quoted-pairSMTP, or both. Section 4.1.2 (of RFC 5321, repeated 4921 above) ends with a few paragraphs of warnings (essentially a partial 4922 applicability statement), the first of which cautions against 4923 cleverness with either Quoted-string or case sensitivity as a threat 4924 to interoperability. 4926 The Quoted-string portion of that discussion has apparently been 4927 widely not read or ignored. Do we need to do something else? If we 4928 do an Applicability Statement, would it be useful to either reference 4929 the discussion in this document from there or to move the discussion 4930 there and reference it (normatively?) from here? 4932 There has been a separate discussion of empty quoted strings in 4933 addresses, i.e., whether the production should be 4934 required to included at least one non-whitespace character. It is 4935 separate from this issue but would be further impacted or distorted 4936 from the considerations identified in this Section. 4937 Ticket #21. May also interact with Ticket #35. 4939 G.10. Internationalization 4941 RFC 5321 came long before work on internationalization of email 4942 addresses and headers (other than by use of encoded words in MINE) 4943 and specifically before the work of the EAI WG leading to the 4944 SMTPUTF8 specifications, specifically RFCs 6530ff. The second 4945 explanatory paragraph at the end of Section 4.1.2 ("Systems MUST NOT 4946 define mailboxes ...") is an extremely strong prohibition against the 4947 use of non-ASCII characters in SMTP commands and the requirements 4948 about message content in Section 2.3.1 an equally strong one for 4949 content. Would it be appropriate to add something like "in the 4950 absence of relevant extensions" there? Also, given [mis]behavior 4951 seen in the wild, does that paragraph (or an A/S) need an explicit 4952 caution about SMTP servers or clients assuming they can apply the 4953 popular web convention of using %NN sequences as a way to encode non- 4954 ASCII characters ( in RFC 3986) and assuming some later 4955 system will interpret it as they expect? Would it be appropriate to 4956 add an Internationalization Considerations section to the body of 4957 this document if only for the purpose of pointing people elsewhere? 4959 More broadly, while the EAI WG's extensions for non-ASCII headers and 4960 addresses are explicitly out of scope for the EMAILCORE WG (at least 4961 for 5321bis (and 5322bis), those documents make assumptions and 4962 interpretations of the core documents. Are there areas in which 4963 5321bis could and should be clarified to lay a more solid foundation 4964 for the EAI/SMTPUTF8 work and, if so, what are they? 4966 G.11. SMTP Clients, Servers, Senders, and Receivers 4968 RFC 821 used the terms "SMTP-sender" and "SMTP-receiver". In RFC 4969 2821 (and hence in 5321), we switched that to "client" and "server" 4970 (See the discussion in Section 1.2). In part because a relay is a 4971 server and then a client (in some recent practice, even interleaving 4972 the two functions by opening the connection to the next host in line 4973 and sending commands before the incoming transaction is complete), 4974 RFC 5321 continues to use the original terminology in some places. 4975 Should we revisit that usage, possibly even returning to consistent 4976 use of the original terminology? 4978 G.12. Extension Keywords Starting in 'X-' 4980 Section 2.2.2 contains a discussion of SMTP keywords starting in "X". 4981 Given general experience with such things and RFC 6648, is there any 4982 reason to not deprecate that practice entirely and remove that text? 4983 If we do so, should Section 4.1.5 be dropped or rewritten to make 4984 clear this is an obsolete practice? 4985 Ticket #42. 4987 G.13. Deprecating HELO 4989 RFC 5321 (and 2821 before it) very carefully circle around the status 4990 of HELO, even recommending its use as a fallback when EHLO is sent 4991 and a "command not recognized" response is received. We are just a 4992 few months short of 20 years; is it time to deprecate the thing and 4993 clean out some or all of that text? And, given a recent (4Q2020) 4994 discussion on the EMAILCORE list, should EHLO be explicitly bound to 4995 SMTP over TCP with the older transports allowed only with HELO? 4996 While those questions may seem independent, separating them is fairly 4997 hard given the way the text is now constructed. 4999 Resolved 2021-01-19: No change 5000 Ticket #43. 5002 G.14. The FOR Clause in Trace Fields: Semantics, Security 5003 Considerations, and Other Issues 5005 The FOR clause in time-stamp ("Received:") fields is seriously under- 5006 defined. It is optional, the syntax is clear, but its semantics and 5007 use, while perhaps obvious from content and the application of common 5008 sense, have never been defined ("never" going back to 821). Do we 5009 want to better define it? Is there any chance that a definition 5010 would invalid existing, conforming and sensible, implementations? If 5011 we do want to define semantics, draft text and advice as to where it 5012 should go are invited. 5014 (Paragraph added 2021-08-18) 5015 In particular, recentdiscussions point strongly to the need for a 5016 statement to the effect that the value of the for clause must contain 5017 one of the addresses that caused the message to be routed to the 5018 recipient of this message copy (thanks Ned), that no mare than one 5019 address can appear, and that showing one address when there are 5020 multiple RCPT commands may be a security and/or privacy issue (thanks 5021 Ned and Viktor and see ). More detailed or specific 5023 guidance, including case analysis, are probably material for the A/s, 5024 but that is obviously up to the WG. 5026 Note the existing discussions in Section 7.2 and Section 7.6 as they 5027 may need adjustment, or at least cross-references, especially if FOR 5028 is more precisely defined. 5030 There is probably an error in Section 7.6. Its last sentence implies 5031 a possible interaction between messages with multiple recipients and 5032 the FOR clause of trace fields. However, because the syntax of the 5033 FOR clause only allows one Mailbox (or Path), it isn't clear if that 5034 statement is meaningful. Should it be revised to discuss other 5035 situations in which including FOR might not be desirable from a 5036 security or privacy standpoint? (See above -- this paragraph 5037 deliberately not changed in -04). 5038 Ticket #55 5040 G.15. Resistance to Attacks and Operational Necessity 5042 Section 7.8 is often cited as allowing an exception to the rules of 5043 the specification for reasons of operational necessity, not just 5044 attack resistance. I (JcK) believe the broader interpretation was 5045 intended by YAM (the section was new in RFC 5321). Recommendation: 5046 change the title to explicitly include "Local Operational 5047 Requirements" and add text to indicate that attack resistance is not 5048 the only possible source of such requirements. 5050 Appendix H. RFC 5321 Errata Summary and Tentative Change Log 5052 [[RFC Editor: Please remove this section before publication.]] 5054 H.1. RFC 5321 Errata Summary 5056 This document addresses the following errata filed against RFC 5321 5057 since its publication in October 2008 [52]. As with the previous 5058 appendix, ticket numbers included below reference 5059 https://trac.ietf.org/trac/emailcore/report/1 . [[CREF22: [[Note in 5060 Draft: Items with comments below have not yet been resolved as 5061 errata. As of the end of November 2020, none of them have been 5062 checked and verified by the emailcore WG.]]]]. 5064 1683 ABNF error. Section 4.4 5065 Ticket #23. 5067 4198 Description error. Section 4.2. 5068 RESOLVED, ticket #24, 2020-12-14. 5070 2578 Syntax description error. Section 4.1.2 5072 1543 Wrong code in description Section 3.8 5073 Ticket #26 5075 4315 ABNF - IPv6 Section 4.1.3. [[CREF23: [5321bis]The IPv6 syntax 5076 has been adjusted since 5321 was published (the erratum mentions 5077 RFC 5952, but RFC 6874 and draft-carpenter-6man-rfc6874bis should 5078 also be considered). See the rewritten form and the comment in 5079 the section cited in the previous sentence, at least for the RFC 5080 5952 issues. The editor awaits instructions. See 5081 https://www.rfc-editor.org/errata/eid4315]] 5082 Ticket #27. 5084 5414 ABNF for Quoted-string Section 4.1.2 5085 Ticket #22. 5087 1851 Location of text on unexpected close Section 4.1.1.5. Text 5088 moved per email 2020-12-31. 5089 Ticket #28. 5091 3447 Use of normative language (e.g., more "MUST"s), possible 5092 confusion in some sections Section 4.4. [[CREF24: [5321bis]As 5093 Barry notes in his verifier comments on the erratum (see 5094 https://www.rfc-editor.org/errata/eid3447), the comments and 5095 suggestions here raise a number of interesting (and difficult) 5096 issues. One of the issues is that the core of RFCs 5321 (and 5097 2821) is text carried over from Jon Postel's RFC 821, a document 5098 that was not only written in a different style than the IETF uses 5099 today but that was written at a time when no one had dreamt of RFC 5100 2119 or even the IETF itself. It appears to me that trying to 5101 patch that style might easily result in a document that is harder 5102 to read as well as being error prone. If we want to get the 5103 document entirely into contemporary style, we really should bite 5104 the bullet and do a complete rewrite. To respond to a different 5105 point in Barry's discussion, I think an explicit statement that 5106 5321/5322 and their predecessors differ in places and why would be 5107 helpful. Text, and suggestions about where to put it, are 5108 solicited. A list of differences might be a good idea too, but 5109 getting it right might be more work than there is available energy 5110 to do correctly. ]] 5112 5711 Missing leading spaces in example Appendix D.3. [[CREF25: 5113 [5321bis]Well, this is interesting because the XML is correct and 5114 the spaces are there, embedded in artwork. So either the XML2RFC 5115 processor at the time took those leading spaces out or the RFC 5116 Editor improved on the document and the change was not caught in 5117 AUTH48, perhaps because rfcdiff ignores white space. We just need 5118 to watch for future iterations. ]] 5119 As of 2021-03-15, both the txt and html-ized versions of draft- 5120 ietf-emailcore-rfc5321bis-02 were showing identical output for 5121 both parts of the example, so the problem appears to be OBE at 5122 worst. 5123 Ticket #29 (closed 2021-03-16) 5125 4055 Erratum claims the the description of SPF and DKIM is wrong. 5126 It is not clear what 5321bis should really say about them, but the 5127 current text probably needs work (or dropping, which is what the 5128 proposed erratum suggests). 5129 Text changed; ticket should probably be closed after WG reviews 5130 -04. 5131 Ticket #30. 5133 [[CREF26: [5321bis]Note that rejected errata have _not_ been reviewed 5134 to see if they contain anything useful that should be discussed again 5135 with the possibility of rethinking and changing text. Volunteers 5136 sought.]] 5138 H.2. Changes from RFC 5321 (published October 2008) to the initial 5139 (-00) version of this draft 5141 o Acknowledgments section (Section 9) trimmed back for new document. 5143 o Introductory paragraph to Appendix F extended to make it clear 5144 that these features were deprecated a long time ago and really 5145 should not be in use any more. 5147 o Adjusted some language to clarify that source routes really, 5148 really, should not be used or depended upon. 5150 o IPv6 address syntax replaced by a copy of the IPv6 URI syntax and 5151 a note added. 5153 o Production index added as a first step in tying all productions to 5154 their sources. As part of the effort to make the document more 5155 easily navigable, table of contents entries have been created for 5156 the individual command descriptions. 5158 o Clarified the relationship between the SMTP "letters, digits, and 5159 hyphens" and DNS "preferred name syntax" (Section 2.3.5). 5161 o Revised the reply code sections to add new 521 and 556 codes, 5162 clarify relationships, and be explicit about the requirement for 5163 clients to rely on first digits rather than the sequences in 5164 Section 4.3.2. 5166 o In conjunction with the above, explicitly obsolete RFCs 1846 and 5167 7504 (but that might not be right -- see email 2021-10-03. 5169 o Incorporated a correction reflecting Errata ID 2578. 5171 o Some small editorial changes made to eliminate redundant 5172 statements that were very close together. Other, equally small, 5173 editorial changes have been made to improve grammar or clarity. 5175 o A few questions, marked "[[5321bis Editor's Note:", or "[[Note in 5176 Draft" have been added for the group to resolve. Other questions, 5177 especially those in the errata summary, are simply included in 5178 narrative comments in CREFs. 5180 o Checked and rationalized "response" (to a command) and "reply 5181 code" terminology. One can talk about a "999 response" but only a 5182 "999 reply code". There is no such thing as a "response code". 5184 o Added note about length limit on mailbox names ("email 5185 addresses"). 5187 o Added an "errata summary" subsection to this change log/ 5188 comparison to 5321 in this Appendix. The entire Appendix will, of 5189 course, disappear at the time of RFC publication unless someone 5190 wants to make a strong case for retaining it. 5192 o Rationalized CREFs to 2821, 5321, 5321bis etc.; added note to 5193 readers below the Abstract. 5195 o Temporarily added a "Note on Reading This Working Draft" after the 5196 Abstract. 5198 H.3. Changes Among Versions of Rfc5321bis 5200 H.3.1. Changes from draft-klensin-rfc5321bis-00 (posted 2012-12-02) to 5201 -01 5203 Substantively, these two versions differ only by suppression of the 5204 CREF and other discussion associated with the evolution from RFC 2821 5205 to RFC 5321. That change includes an update to the document's Note 5206 to Readers, the date, the file name, and the addition of this change 5207 log subsection. 5209 H.3.2. Changes from draft-klensin-rfc5321bis-01 (20191203) to -02 5211 o Minor clarifications to improve text, e.g., addition of NOOP to 5212 the list of non-mail transaction examples in Section 4.1.4. 5214 o Added topics exposed in the ietf-smtp list and the IETF list 5215 "dogfood" discussion during December 2019 and an index listing of 5216 substantive issues identified only in CREFs in the prior draft as 5217 a new Appendix G.. 5219 H.3.3. Changes from draft-klensin-rfc5321bis-02 (2019-12-27) to -03 5221 o Added more text to Appendix G.7.1 to specifically call out the 5222 session-opening policy issues surrounding these codes. 5224 o Added discussion of "1yz" reinstatement in Appendix G.7.11. 5226 o Added discussion of timeouts in Appendix G.7.12. 5228 o Added subsection on Enhanced Status Codes and DSNs to the 5229 outstanding issues list Appendix G.8. 5231 o Replaced reference to RFC 1652 (8BITMIME) with the Internet 5232 Standard version, RFC 6152. 5234 o With help from cketti, clarified the ABNF productions whose 5235 terminals appear in other documents. 5237 o Added discussions of Quoted-string, Internationalization, and 5238 client-server versus sender-receiver terminology to Appendix G. 5240 o Added note to the Abstract. 5242 H.3.4. Changes from draft-klensin-rfc5321bis-03 (2020-07-02) to draft- 5243 ietf-emailcore-rfc5321bis-00 5245 o Added a paragraph about non-null quoted strings to Appendix G.9. 5247 o Added an explicit pointer to email insecurity and TLS to 5248 Appendix G.6. Inspired by Ben Kaduk's comment on the WG Charter, 5249 2020-09-09. 5251 o Converted document from individual to emailcore WG effort. 5253 H.3.5. Changes from draft-ietf-emailcore-rfc5321bis-00 (2020-10-06) to 5254 -01 5256 o Editorial: Corrected "blackslash" to "backslash" 5258 o Rewrote the introduction to Appendix G slightly to reflect the 5259 creation of the EMAILCORE WG. 5261 o Applied fixes for repeated use of EHLO. See Appendix G.2. 5263 o Added two new questions, one about "X" extensions (Appendix G.12) 5264 and one about the status of HELO (Appendix G.13). 5266 o Removed mention of SEND, SAML, SOML from the main body of the text 5267 (Ticket #20). 5269 o Added a warning about side effects to Appendix G.7.5. 5271 o Added ticket numbers to descriptions of issues and changes, 5272 adjusted some text so relationships would be more clear, and added 5273 subsections to the Appendix G and H lists to pick up on tickets 5274 that were not easily identified in those sections of with the 5275 text. 5277 o Made several additions to the Index, including one to deal with 5278 SEND et al., as above. 5280 H.3.6. Changes from draft-ietf-emailcore-rfc5321bis-01 (2020-12-25) to 5281 -02 5283 o Corrected discussion mailing list to point to emailcore@ietf.org 5284 in the introductory note. 5286 o Added new subsection(s) to Appendix G to reflect newly discovered 5287 issues. 5289 o Changed "as discussed in" references in Section 4.5.5 per ticket 5290 #45. 5292 o Corrected a misleading use of the term "mailbox" in Section 3.3. 5294 o Changed descriptions of use of first digit in replies per ticket 5295 #13. See Appendix G.7.7. 5297 o Moved paragraph per ticket #28, erratum 1851. 5299 o Added more clarifying cross-references, clarified some CREFs, and 5300 cleaned out some of those that no longer seemed relevant. 5302 o Removed "updates 1123" is unnecessary and obsolete. 5304 o Updated several references. 5306 H.3.7. Changes from draft-ietf-emailcore-rfc5321bis-02 (2021-02-21) to 5307 -03 5309 o Editorial: Fixed some instances of constructions like "RCPT TO 5310 command". The name of the command is RCPT. Sloppy editing in 5311 2008. 5313 o Added text and cross-references to clarify the role of 452 and 552 5314 in "too many recipients" situations. 5316 o Added Appendix G.15 to discuss changes to better reflect 5317 "operational necessity" issue. 5319 o Added detail for erratum 5711, ticket #29. 5321 o Added new subsections of Appendix G.7 to keep some previously- 5322 unnoted CREF notes from getting lost. Also removed some CREFs 5323 that were notes on changes made before the WG was created or 5324 appeared to no longer have value and trimmed or rewrote some of 5325 the remaining ones. 5327 o More discussion of Ticket #13, See Appendix G.7.7. 5329 o Identified Ticket #41 as closed. See Appendix Appendix G.7.3; 5330 notes removed from Section 2.3.5. 5332 o "SHOULD" requirement for interpreting 552 "too many recipients" 5333 removed from Section 4.5.3.1.10, explanation added, and text 5334 cleaned up. Also removed the parenthetical historical notes on 5335 the return code definitions in Section 4.2. See Appendix G.5. 5336 (Ticket #5) 5338 o Modified Appendix G.8 to add a note about the normative status of 5339 RFC 3463 and moved that reference. 5341 o Several clarifications to initiation and termination of mail 5342 transactions in Section 4.1.4. 5344 o Several additional minor editorial improvements. 5346 o Note for drafts -03 and -04 only: Notes were posted to the list on 5347 2021-07-09 about tickets #7, #10, #14 (closed), #19, #20, $30, and 5348 #42. Even though some comments about them appeared in the 5349 subsequent day or so, there appears to have been insufficient time 5350 for discussions to stabilize sufficiently for changes to be 5351 included in this version of the I-D. 5353 H.3.8. Changes from draft-ietf-emailcore-rfc5321bis-03 (2021-07-10) to 5354 -04 5356 o Clarified that the "period" in . is really the ASCII 5357 one in Section 3.3. 5358 [[CREF27: Editor's note: change treated as Editorial without a 5359 ticket. If there are objections, speak up.]] 5361 o Several other small editorial corrections. 5363 o Added several notes about the possible need to add text to reflect 5364 the presence of MSAs and to clarify whether MUAs send messages 5365 directly to MTAs or whether, in that case, the MUAs are just 5366 incorporating MSA functions. 5368 o Added new text to Appendix G.14 reflecting discussions of the 5369 Received...FOR issue. 5371 o Adjusted discussion of erratum 4315 (Ticket #27) to reflect more 5372 recent IPv6 syntax developments. 5374 o Adjusted discussion of the various "mail not accepted" codes, 5375 rewrote Section 4.2.4.2, annotated and inserted cross-references 5376 in relevant response code descriptions and (tentatively) 5377 identified this document as obsoleting RFC 7505. Editor's guess, 5378 reinforced by a brief conversation with John Levine (lead author 5379 of 7505), is that we should incorporate text as needed and 5380 obsolete it. The changes include replacing the reference to the 5381 "nullMX" I-D with RFC 7505, which I am appalled that neither I nor 5382 anyone else noticed earlier. Cf. Appendix G.7.1, Section 4.2.4.2, 5383 and Ticket #6. 5385 Index 5387 A 5388 Argument Syntax 5389 A-d-l 43 5390 Additional-Registered-Clauses 64 5391 address-literal 43 5392 Addtl-Link 64 5393 Addtl-Protocol 64 5394 ALPHA 42 5395 Argument 43 5396 At-domain 43 5397 atext 43 5398 Atom 44 5399 By-domain 63 5400 CFWS 43 5401 CRLF 42 5402 dcontent 45 5403 DIGIT 42 5404 Domain 43 5405 Dot-string 44 5406 esmtp-keyword 43 5407 esmtp-param 43 5408 esmtp-value 43 5409 Extended-Domain 63 5410 For 64 5411 Forward-Path 43 5412 From-domain 63 5413 FWS 43 5414 General-address-literal 45 5415 Greeting 49 5416 h16 46 5417 HEXDIG 42 5418 ID 64 5419 IPv4-address-literal 45 5420 IPv6-addr 46 5421 IPv6-address-literal 45 5422 Keyword 43 5423 Ldh-str 43 5424 Let-dig 43 5425 Link 64 5426 Local-part 44 5427 ls32 46 5428 Mail-parameters 43 5429 Mailbox 43 5430 Opt-info 63 5431 Path 43 5432 Protocol 64 5433 QcontentSMTP 44 5434 qtextSMTP 44 5435 quoted-pairSMTP 44 5436 Quoted-string 44 5437 Rcpt-parameters 43 5438 Reply-code 49 5439 Reply-line 49 5440 Return-path-line 63 5441 Reverse-Path 43 5442 Snum 46 5443 SP 42 5444 Stamp 63 5445 Standardized-tag 45 5446 String 44 5447 sub-domain 43 5448 TCP-info 63 5449 textstring 49 5450 Time-stamp-line 63 5451 Via 63 5452 With 63 5454 C 5455 Command Syntax 5456 data 40 5457 ehlo 20, 35 5458 expn 41 5459 helo 35 5460 help 41 5461 mail 37 5462 noop 41 5463 quit 42 5464 rcpt 38 5465 rset 40 5466 send, saml, soml 104 5467 vrfy 40 5469 Author's Address 5471 John C. Klensin 5472 1770 Massachusetts Ave, Suite 322 5473 Cambridge, MA 02140 5474 USA 5476 EMail: john-ietf@jck.com