idnits 2.17.00 (12 Aug 2021) /tmp/idnits37988/draft-klensin-rfc5321bis-01.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 RFC1846, but the abstract doesn't seem to mention this, which it should. -- The draft header indicates that this document updates RFC1123, 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 == Using lowercase 'not' together with uppercase 'MUST', 'SHALL', 'SHOULD', or 'RECOMMENDED' is not an accepted usage according to RFC 2119. Please use uppercase 'NOT' together with RFC 2119 keywords (if that is what you mean). Found 'SHOULD not' in this paragraph: When an SMTP server returns a permanent error status (5yz) code after the DATA command is completed with ., it MUST NOT make any subsequent attempt to deliver the message. As with temporary error status codes, the SMTP client retains responsibility for the message, but SHOULD not again attempt delivery to the same server without user review of the message and response and appropriate intervention. -- The document date (December 3, 2019) is 899 days in the past. Is this intentional? 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 4428 -- Possible downref: Non-RFC (?) normative reference: ref. '2' ** Obsolete normative reference: RFC 821 (ref. '8') (Obsoleted by RFC 2821) -- Obsolete informational reference (is this intentional?): RFC 822 (ref. '16') (Obsoleted by RFC 2822) -- Obsolete informational reference (is this intentional?): RFC 974 (ref. '19') (Obsoleted by RFC 2821) -- Obsolete informational reference (is this intentional?): RFC 1652 (ref. '23') (Obsoleted by RFC 6152) -- Obsolete informational reference (is this intentional?): RFC 1869 (ref. '26') (Obsoleted by RFC 2821) -- Obsolete informational reference (is this intentional?): RFC 2821 (ref. '34') (Obsoleted by RFC 5321) -- Obsolete informational reference (is this intentional?): RFC 3501 (ref. '39') (Obsoleted by RFC 9051) -- Obsolete informational reference (is this intentional?): RFC 3798 (ref. '40') (Obsoleted by RFC 8098) -- Obsolete informational reference (is this intentional?): RFC 3851 (ref. '41') (Obsoleted by RFC 5751) -- Obsolete informational reference (is this intentional?): RFC 4408 (ref. '42') (Obsoleted by RFC 7208) -- Obsolete informational reference (is this intentional?): RFC 4409 (ref. '43') (Obsoleted by RFC 6409) -- Obsolete informational reference (is this intentional?): RFC 4871 (ref. '45') (Obsoleted by RFC 6376) Summary: 2 errors (**), 0 flaws (~~), 3 warnings (==), 18 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group J. Klensin 3 Internet-Draft December 3, 2019 4 Obsoletes: 5321, 1846, 7504 (if 5 approved) 6 Updates: 1123 (if approved) 7 Intended status: Standards Track 8 Expires: June 5, 2020 10 Simple Mail Transfer Protocol 11 draft-klensin-rfc5321bis-01 13 Abstract 15 This document is a specification of the basic protocol for Internet 16 electronic mail transport. It consolidates, updates, and clarifies 17 several previous documents, making all or parts of most of them 18 obsolete. It covers the SMTP extension mechanisms and best practices 19 for the contemporary Internet, but does not provide details about 20 particular extensions. Although SMTP was designed as a mail 21 transport and delivery protocol, this specification also contains 22 information that is important to its use as a "mail submission" 23 protocol for "split-UA" (User Agent) mail reading systems and mobile 24 environments. 26 Note on Reading This Working Draft 28 This working draft is extensively annotated with information about 29 changes made over the decade since RFC 5321 appeared, especially when 30 those changes might be controversial or should get careful review. 31 Anything marked in CREF comments with "[5321bis]" is current. In 32 general, unless those are marked with "[[Note in Draft", in the 33 contents of an "Editor's note", or are in the "Errata Summary" 34 appendix (Appendix G.1, they are just notes on changes that have 35 already been made and where those changes originated. Comments 36 identified as "2821ter" arose after the Last Call on what became 37 RFC5321, sometimes before AUTH48 on that document or a bit later. 38 Those, of course, should still be reviewed. Surviving comments about 39 rfc5321bis-00 followed by a letter indicate intermediate working 40 versions of this draft and can be ignored unless the origin of 41 changes is important. As one can tell from the dates (when they are 42 given), this document has been periodically updated over a very long 43 period of time. 45 Status of This Memo 47 This Internet-Draft is submitted in full conformance with the 48 provisions of BCP 78 and BCP 79. 50 Internet-Drafts are working documents of the Internet Engineering 51 Task Force (IETF). Note that other groups may also distribute 52 working documents as Internet-Drafts. The list of current Internet- 53 Drafts is at https://datatracker.ietf.org/drafts/current/. 55 Internet-Drafts are draft documents valid for a maximum of six months 56 and may be updated, replaced, or obsoleted by other documents at any 57 time. It is inappropriate to use Internet-Drafts as reference 58 material or to cite them other than as "work in progress." 60 This Internet-Draft will expire on June 5, 2020. 62 Copyright Notice 64 Copyright (c) 2019 IETF Trust and the persons identified as the 65 document authors. All rights reserved. 67 This document is subject to BCP 78 and the IETF Trust's Legal 68 Provisions Relating to IETF Documents 69 (https://trustee.ietf.org/license-info) in effect on the date of 70 publication of this document. Please review these documents 71 carefully, as they describe your rights and restrictions with respect 72 to this document. Code Components extracted from this document must 73 include Simplified BSD License text as described in Section 4.e of 74 the Trust Legal Provisions and are provided without warranty as 75 described in the Simplified BSD License. 77 Table of Contents 79 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5 80 1.1. Transport of Electronic Mail . . . . . . . . . . . . . . 5 81 1.2. History and Context for This Document . . . . . . . . . . 5 82 1.3. Document Conventions . . . . . . . . . . . . . . . . . . 7 83 2. The SMTP Model . . . . . . . . . . . . . . . . . . . . . . . 7 84 2.1. Basic Structure . . . . . . . . . . . . . . . . . . . . . 7 85 2.2. The Extension Model . . . . . . . . . . . . . . . . . . . 9 86 2.2.1. Background . . . . . . . . . . . . . . . . . . . . . 9 87 2.2.2. Definition and Registration of Extensions . . . . . . 10 88 2.2.3. Special Issues with Extensions . . . . . . . . . . . 11 89 2.3. SMTP Terminology . . . . . . . . . . . . . . . . . . . . 12 90 2.3.1. Mail Objects . . . . . . . . . . . . . . . . . . . . 12 91 2.3.2. Senders and Receivers . . . . . . . . . . . . . . . . 12 92 2.3.3. Mail Agents and Message Stores . . . . . . . . . . . 13 93 2.3.4. Host . . . . . . . . . . . . . . . . . . . . . . . . 13 94 2.3.5. Domain Names . . . . . . . . . . . . . . . . . . . . 13 95 2.3.6. Buffer and State Table . . . . . . . . . . . . . . . 14 96 2.3.7. Commands and Replies . . . . . . . . . . . . . . . . 14 97 2.3.8. Lines . . . . . . . . . . . . . . . . . . . . . . . . 15 98 2.3.9. Message Content and Mail Data . . . . . . . . . . . . 15 99 2.3.10. Originator, Delivery, Relay, and Gateway Systems . . 15 100 2.3.11. Mailbox and Address . . . . . . . . . . . . . . . . . 16 101 2.4. General Syntax Principles and Transaction Model . . . . . 16 102 3. The SMTP Procedures: An Overview . . . . . . . . . . . . . . 18 103 3.1. Session Initiation . . . . . . . . . . . . . . . . . . . 18 104 3.2. Client Initiation . . . . . . . . . . . . . . . . . . . . 19 105 3.3. Mail Transactions . . . . . . . . . . . . . . . . . . . . 19 106 3.4. Forwarding for Address Correction or Updating . . . . . . 22 107 3.5. Commands for Debugging Addresses . . . . . . . . . . . . 23 108 3.5.1. Overview . . . . . . . . . . . . . . . . . . . . . . 23 109 3.5.2. VRFY Normal Response . . . . . . . . . . . . . . . . 25 110 3.5.3. Meaning of VRFY or EXPN Success Response . . . . . . 26 111 3.5.4. Semantics and Applications of EXPN . . . . . . . . . 26 112 3.6. Relaying and Mail Routing . . . . . . . . . . . . . . . . 26 113 3.6.1. Source Routes and Relaying . . . . . . . . . . . . . 26 114 3.6.2. Mail eXchange Records and Relaying . . . . . . . . . 27 115 3.6.3. Message Submission Servers as Relays . . . . . . . . 28 116 3.7. Mail Gatewaying . . . . . . . . . . . . . . . . . . . . . 29 117 3.7.1. Header Fields in Gatewaying . . . . . . . . . . . . . 29 118 3.7.2. Received Lines in Gatewaying . . . . . . . . . . . . 29 119 3.7.3. Addresses in Gatewaying . . . . . . . . . . . . . . . 30 120 3.7.4. Other Header Fields in Gatewaying . . . . . . . . . . 30 121 3.7.5. Envelopes in Gatewaying . . . . . . . . . . . . . . . 30 122 3.8. Terminating Sessions and Connections . . . . . . . . . . 30 123 3.9. Mailing Lists and Aliases . . . . . . . . . . . . . . . . 31 124 3.9.1. Alias . . . . . . . . . . . . . . . . . . . . . . . . 32 125 3.9.2. List . . . . . . . . . . . . . . . . . . . . . . . . 32 126 4. The SMTP Specifications . . . . . . . . . . . . . . . . . . . 32 127 4.1. SMTP Commands . . . . . . . . . . . . . . . . . . . . . . 32 128 4.1.1. Command Semantics and Syntax . . . . . . . . . . . . 32 129 4.1.2. Command Argument Syntax . . . . . . . . . . . . . . . 41 130 4.1.3. Address Literals . . . . . . . . . . . . . . . . . . 43 131 4.1.4. Order of Commands . . . . . . . . . . . . . . . . . . 45 132 4.1.5. Private-Use Commands . . . . . . . . . . . . . . . . 46 133 4.2. SMTP Replies . . . . . . . . . . . . . . . . . . . . . . 47 134 4.2.1. Reply Code Severities and Theory . . . . . . . . . . 48 135 4.2.2. Reply Codes by Function Groups . . . . . . . . . . . 51 136 4.2.3. Reply Codes in Numeric Order . . . . . . . . . . . . 52 137 4.2.4. Some specific code situations and relationships . . . 54 138 4.3. Sequencing of Commands and Replies . . . . . . . . . . . 55 139 4.3.1. Sequencing Overview . . . . . . . . . . . . . . . . . 55 140 4.3.2. Command-Reply Sequences . . . . . . . . . . . . . . . 56 142 4.4. Trace Information . . . . . . . . . . . . . . . . . . . . 58 143 4.5. Additional Implementation Issues . . . . . . . . . . . . 62 144 4.5.1. Minimum Implementation . . . . . . . . . . . . . . . 62 145 4.5.2. Transparency . . . . . . . . . . . . . . . . . . . . 63 146 4.5.3. Sizes and Timeouts . . . . . . . . . . . . . . . . . 64 147 4.5.4. Retry Strategies . . . . . . . . . . . . . . . . . . 68 148 4.5.5. Messages with a Null Reverse-Path . . . . . . . . . . 70 149 5. Address Resolution and Mail Handling . . . . . . . . . . . . 70 150 5.1. Locating the Target Host . . . . . . . . . . . . . . . . 71 151 5.2. IPv6 and MX Records . . . . . . . . . . . . . . . . . . . 73 152 6. Problem Detection and Handling . . . . . . . . . . . . . . . 73 153 6.1. Reliable Delivery and Replies by Email . . . . . . . . . 73 154 6.2. Unwanted, Unsolicited, and "Attack" Messages . . . . . . 74 155 6.3. Loop Detection . . . . . . . . . . . . . . . . . . . . . 75 156 6.4. Compensating for Irregularities . . . . . . . . . . . . . 75 157 7. Security Considerations . . . . . . . . . . . . . . . . . . . 77 158 7.1. Mail Security and Spoofing . . . . . . . . . . . . . . . 77 159 7.2. "Blind" Copies . . . . . . . . . . . . . . . . . . . . . 78 160 7.3. VRFY, EXPN, and Security . . . . . . . . . . . . . . . . 78 161 7.4. Mail Rerouting Based on the 251 and 551 Response 162 Codes . . . . . . . . . . . . . . . . . . . . . . . . . . 79 163 7.5. Information Disclosure in Announcements . . . . . . . . . 79 164 7.6. Information Disclosure in Trace Fields . . . . . . . . . 80 165 7.7. Information Disclosure in Message Forwarding . . . . . . 80 166 7.8. Resistance to Attacks . . . . . . . . . . . . . . . . . . 80 167 7.9. Scope of Operation of SMTP Servers . . . . . . . . . . . 80 168 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 81 169 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 82 170 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 82 171 10.1. Normative References . . . . . . . . . . . . . . . . . . 82 172 10.2. Informative References . . . . . . . . . . . . . . . . . 84 173 Appendix A. TCP Transport Service . . . . . . . . . . . . . . . 88 174 Appendix B. Generating SMTP Commands from RFC 822 Header Fields 88 175 Appendix C. Source Routes . . . . . . . . . . . . . . . . . . . 89 176 Appendix D. Scenarios . . . . . . . . . . . . . . . . . . . . . 90 177 D.1. A Typical SMTP Transaction Scenario . . . . . . . . . . . 90 178 D.2. Aborted SMTP Transaction Scenario . . . . . . . . . . . . 91 179 D.3. Relayed Mail Scenario . . . . . . . . . . . . . . . . . . 92 180 D.4. Verifying and Sending Scenario . . . . . . . . . . . . . 93 181 Appendix E. Other Gateway Issues . . . . . . . . . . . . . . . . 94 182 Appendix F. Deprecated Features of RFC 821 . . . . . . . . . . . 94 183 F.1. TURN . . . . . . . . . . . . . . . . . . . . . . . . . . 94 184 F.2. Source Routing . . . . . . . . . . . . . . . . . . . . . 94 185 F.3. HELO . . . . . . . . . . . . . . . . . . . . . . . . . . 95 186 F.4. #-literals . . . . . . . . . . . . . . . . . . . . . . . 95 187 F.5. Dates and Years . . . . . . . . . . . . . . . . . . . . . 95 188 F.6. Sending versus Mailing . . . . . . . . . . . . . . . . . 95 189 Appendix G. Change log for RFC 5321bis . . . . . . . . . . . . . 96 190 G.1. RFC 5321 Errata Summary . . . . . . . . . . . . . . . . . 96 191 G.2. Changes from RFC 5321 (published October 2008) to the 192 initial (-00) version of this draft . . . . . . . . . . . 97 193 G.3. Changes Among Versions of Rfc5321Bis . . . . . . . . . . 98 194 G.3.1. Changes from draft-klensin-rfc5321bis-00 (posted 195 2012-12-02) to -01 . . . . . . . . . . . . . . . . . 98 196 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 197 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 100 199 1. Introduction 201 1.1. Transport of Electronic Mail 203 The objective of the Simple Mail Transfer Protocol (SMTP) is to 204 transfer mail reliably and efficiently. 206 SMTP is independent of the particular transmission subsystem and 207 requires only a reliable ordered data stream channel. While this 208 document specifically discusses transport over TCP, other transports 209 are possible. Appendices to RFC 821 [8] describe some of them. 211 An important feature of SMTP is its capability to transport mail 212 across multiple networks, usually referred to as "SMTP mail relaying" 213 (see Section 3.6). A network consists of the mutually-TCP-accessible 214 hosts on the public Internet, the mutually-TCP-accessible hosts on a 215 firewall-isolated TCP/IP Intranet, or hosts in some other LAN or WAN 216 environment utilizing a non-TCP transport-level protocol. Using 217 SMTP, a process can transfer mail to another process on the same 218 network or to some other network via a relay or gateway process 219 accessible to both networks. 221 In this way, a mail message may pass through a number of intermediate 222 relay or gateway hosts on its path from sender to ultimate recipient. 223 The Mail eXchanger mechanisms of the domain name system (RFC 1035 224 [7], RFC 974 [19], and Section 5 of this document) are used to 225 identify the appropriate next-hop destination for a message being 226 transported. 228 1.2. History and Context for This Document 230 This document is a specification of the basic protocol for the 231 Internet electronic mail transport. It consolidates, updates and 232 clarifies, but does not add new or change existing functionality of 233 the following: 235 o the original SMTP (Simple Mail Transfer Protocol) specification of 236 RFC 821 [8], 238 o domain name system requirements and implications for mail 239 transport from RFC 1035 [7] and RFC 974 [19], 241 o the clarifications and applicability statements in RFC 1123 [3], 243 o the new error codes added by RFC 1846 [24] and later by RFC 7504> 244 [48], obsoleting both of those documents, and 246 o material drawn from the SMTP Extension mechanisms in RFC 1869 247 [26]. 249 o Editorial and clarification changes to RFC 2821 [34] to bring that 250 specification to Draft Standard. 252 It obsoletes RFC 821, RFC 974, RFC 1869, and RFC 2821 and updates RFC 253 1123 (replacing the mail transport materials of RFC 1123). However, 254 RFC 821 specifies some features that were not in significant use in 255 the Internet by the mid-1990s and (in appendices) some additional 256 transport models. Those sections are omitted here in the interest of 257 clarity and brevity; readers needing them should refer to RFC 821. 259 It also includes some additional material from RFC 1123 that required 260 amplification. This material has been identified in multiple ways, 261 mostly by tracking flaming on various lists and newsgroups and 262 problems of unusual readings or interpretations that have appeared as 263 the SMTP extensions have been deployed. Where this specification 264 moves beyond consolidation and actually differs from earlier 265 documents, it supersedes them technically as well as textually. 267 Although SMTP was designed as a mail transport and delivery protocol, 268 this specification also contains information that is important to its 269 use as a "mail submission" protocol, as recommended for Post Office 270 Protocol (POP) (RFC 937 [17], RFC 1939 [27]) and IMAP (RFC 3501 271 [39]). In general, the separate mail submission protocol specified 272 in RFC 4409 [43] is now preferred to direct use of SMTP; more 273 discussion of that subject appears in that document. 275 Section 2.3 provides definitions of terms specific to this document. 276 Except when the historical terminology is necessary for clarity, this 277 document uses the current 'client' and 'server' terminology to 278 identify the sending and receiving SMTP processes, respectively. 280 A companion document, RFC 5322 [11], discusses message header 281 sections and bodies and specifies formats and structures for them. 283 1.3. Document Conventions 285 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 286 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 287 document are to be interpreted as described in RFC 2119 [1]. As each 288 of these terms was intentionally and carefully chosen to improve the 289 interoperability of email, each use of these terms is to be treated 290 as a conformance requirement. 292 Because this document has a long history and to avoid the risk of 293 various errors and of confusing readers and documents that point to 294 this one, most examples and the domain names they contain are 295 preserved from RFC 2821. Readers are cautioned that these are 296 illustrative examples that should not actually be used in either code 297 or configuration files. 299 2. The SMTP Model 301 [[CREF1: [5321bis] [[Editor's Note: There have been extensive and 302 repeated discussions on the SMTP and IETF lists about whether this 303 document should say something about hop-by-hop (MTA-to-MTA) SMTP 304 authentication and, if so, what?? Note that end to end message 305 authentication is almost certainly out of scope for SMTP.]]]] 307 2.1. Basic Structure 309 The SMTP design can be pictured as: 311 +----------+ +----------+ 312 +------+ | | | | 313 | User |<-->| | SMTP | | 314 +------+ | Client- |Commands/Replies| Server- | 315 +------+ | SMTP |<-------------->| SMTP | +------+ 316 | File |<-->| | and Mail | |<-->| File | 317 |System| | | | | |System| 318 +------+ +----------+ +----------+ +------+ 319 SMTP client SMTP server 321 When an SMTP client has a message to transmit, it establishes a two- 322 way transmission channel to an SMTP server. The responsibility of an 323 SMTP client is to transfer mail messages to one or more SMTP servers, 324 or report its failure to do so. 326 The means by which a mail message is presented to an SMTP client, and 327 how that client determines the identifier(s) ("names") of the 328 domain(s) to which mail messages are to be transferred, are local 329 matters. They are not addressed by this document. In some cases, 330 the designated domain(s), or those determined by an SMTP client, will 331 identify the final destination(s) of the mail message. In other 332 cases, common with SMTP clients associated with implementations of 333 the POP (RFC 937 [17], RFC 1939 [27]) or IMAP (RFC 3501 [39]) 334 protocols, or when the SMTP client is inside an isolated transport 335 service environment, the domain determined will identify an 336 intermediate destination through which all mail messages are to be 337 relayed. SMTP clients that transfer all traffic regardless of the 338 target domains associated with the individual messages, or that do 339 not maintain queues for retrying message transmissions that initially 340 cannot be completed, may otherwise conform to this specification but 341 are not considered fully-capable. Fully-capable SMTP 342 implementations, including the relays used by these less capable 343 ones, and their destinations, are expected to support all of the 344 queuing, retrying, and alternate address functions discussed in this 345 specification. In many situations and configurations, the less- 346 capable clients discussed above SHOULD be using the message 347 submission protocol (RFC 4409 [43]) rather than SMTP. 349 The means by which an SMTP client, once it has determined a target 350 domain, determines the identity of an SMTP server to which a copy of 351 a message is to be transferred, and then performs that transfer, are 352 covered by this document. To effect a mail transfer to an SMTP 353 server, an SMTP client establishes a two-way transmission channel to 354 that SMTP server. An SMTP client determines the address of an 355 appropriate host running an SMTP server by resolving a destination 356 domain name to either an intermediate Mail eXchanger host or a final 357 target host. 359 An SMTP server may be either the ultimate destination or an 360 intermediate "relay" (that is, it may assume the role of an SMTP 361 client after receiving the message) or "gateway" (that is, it may 362 transport the message further using some protocol other than SMTP). 363 SMTP commands are generated by the SMTP client and sent to the SMTP 364 server. SMTP replies are sent from the SMTP server to the SMTP 365 client in response to the commands. 367 In other words, message transfer can occur in a single connection 368 between the original SMTP-sender and the final SMTP-recipient, or can 369 occur in a series of hops through intermediary systems. In either 370 case, once the server has issued a success response at the end of the 371 mail data, a formal handoff of responsibility for the message occurs: 372 the protocol requires that a server MUST accept responsibility for 373 either delivering the message or properly reporting the failure to do 374 so (see Sections 6.1, 6.2, and 7.8, below). 376 Once the transmission channel is established and initial handshaking 377 is completed, the SMTP client normally initiates a mail transaction. 378 Such a transaction consists of a series of commands to specify the 379 originator and destination of the mail and transmission of the 380 message content (including any lines in the header section or other 381 structure) itself. When the same message is sent to multiple 382 recipients, this protocol encourages the transmission of only one 383 copy of the data for all recipients at the same destination (or 384 intermediate relay) host. 386 The server responds to each command with a reply; replies may 387 indicate that the command was accepted, that additional commands are 388 expected, or that a temporary or permanent error condition exists. 389 Commands specifying the sender or recipients may include server- 390 permitted SMTP service extension requests, as discussed in 391 Section 2.2. The dialog is purposely lock-step, one-at-a-time, 392 although this can be modified by mutually agreed upon extension 393 requests such as command pipelining (RFC 2920 [35]). 395 Once a given mail message has been transmitted, the client may either 396 request that the connection be shut down or may initiate other mail 397 transactions. In addition, an SMTP client may use a connection to an 398 SMTP server for ancillary services such as verification of email 399 addresses or retrieval of mailing list subscriber addresses. 401 As suggested above, this protocol provides mechanisms for the 402 transmission of mail. Historically, this transmission normally 403 occurred directly from the sending user's host to the receiving 404 user's host when the two hosts are connected to the same transport 405 service. When they are not connected to the same transport service, 406 transmission occurs via one or more relay SMTP servers. A very 407 common case in the Internet today involves submission of the original 408 message to an intermediate, "message submission" server, which is 409 similar to a relay but has some additional properties; such servers 410 are discussed in Section 2.3.10 and at some length in RFC 4409 [43]. 411 An intermediate host that acts as either an SMTP relay or as a 412 gateway into some other transmission environment is usually selected 413 through the use of the domain name service (DNS) Mail eXchanger 414 mechanism. Explicit "source" routing (see Section 5 and Appendix C 415 and Appendix F.2) SHOULD NOT be used. [[CREF2: [5321bis] JcK 416 20090123 - redundant sentence removed.]] 418 2.2. The Extension Model 420 2.2.1. Background 422 In an effort that started in 1990, approximately a decade after RFC 423 821 was completed, the protocol was modified with a "service 424 extensions" model that permits the client and server to agree to 425 utilize shared functionality beyond the original SMTP requirements. 426 The SMTP extension mechanism defines a means whereby an extended SMTP 427 client and server may recognize each other, and the server can inform 428 the client as to the service extensions that it supports. 430 Contemporary SMTP implementations MUST support the basic extension 431 mechanisms. For instance, servers MUST support the EHLO command even 432 if they do not implement any specific extensions and clients SHOULD 433 preferentially utilize EHLO rather than HELO. (However, for 434 compatibility with older conforming implementations, SMTP clients and 435 servers MUST support the original HELO mechanisms as a fallback.) 436 Unless the different characteristics of HELO must be identified for 437 interoperability purposes, this document discusses only EHLO. 439 SMTP is widely deployed and high-quality implementations have proven 440 to be very robust. However, the Internet community now considers 441 some services to be important that were not anticipated when the 442 protocol was first designed. If support for those services is to be 443 added, it must be done in a way that permits older implementations to 444 continue working acceptably. The extension framework consists of: 446 o The SMTP command EHLO, superseding the earlier HELO, 448 o a registry of SMTP service extensions, 450 o additional parameters to the SMTP MAIL and RCPT commands, and 452 o optional replacements for commands defined in this protocol, such 453 as for DATA in non-ASCII transmissions (RFC 3030 [37]). 455 SMTP's strength comes primarily from its simplicity. Experience with 456 many protocols has shown that protocols with few options tend towards 457 ubiquity, whereas protocols with many options tend towards obscurity. 459 Each and every extension, regardless of its benefits, must be 460 carefully scrutinized with respect to its implementation, deployment, 461 and interoperability costs. In many cases, the cost of extending the 462 SMTP service will likely outweigh the benefit. 464 2.2.2. Definition and Registration of Extensions 466 The IANA maintains a registry of SMTP service extensions. A 467 corresponding EHLO keyword value is associated with each extension. 468 Each service extension registered with the IANA must be defined in a 469 formal Standards-Track or IESG-approved Experimental protocol 470 document. The definition must include: 472 o the textual name of the SMTP service extension; 474 o the EHLO keyword value associated with the extension; 475 o the syntax and possible values of parameters associated with the 476 EHLO keyword value; 478 o any additional SMTP verbs associated with the extension 479 (additional verbs will usually be, but are not required to be, the 480 same as the EHLO keyword value); 482 o any new parameters the extension associates with the MAIL or RCPT 483 verbs; 485 o a description of how support for the extension affects the 486 behavior of a server and client SMTP; and 488 o the increment by which the extension is increasing the maximum 489 length of the commands MAIL and/or RCPT, over that specified in 490 this Standard. 492 In addition, any EHLO keyword value starting with an upper or lower 493 case "X" refers to a local SMTP service extension used exclusively 494 through bilateral agreement. Keywords beginning with "X" MUST NOT be 495 used in a registered service extension. Conversely, keyword values 496 presented in the EHLO response that do not begin with "X" MUST 497 correspond to a Standard, Standards-Track, or IESG-approved 498 Experimental SMTP service extension registered with IANA. A 499 conforming server MUST NOT offer non-"X"-prefixed keyword values that 500 are not described in a registered extension. 502 Additional verbs and parameter names are bound by the same rules as 503 EHLO keywords; specifically, verbs beginning with "X" are local 504 extensions that may not be registered or standardized. Conversely, 505 verbs not beginning with "X" must always be registered. 507 2.2.3. Special Issues with Extensions 509 Extensions that change fairly basic properties of SMTP operation are 510 permitted. The text in other sections of this document must be 511 understood in that context. In particular, extensions can change the 512 minimum limits specified in Section 4.5.3, can change the ASCII 513 character set requirement as mentioned above, or can introduce some 514 optional modes of message handling. 516 In particular, if an extension implies that the delivery path 517 normally supports special features of that extension, and an 518 intermediate SMTP system finds a next hop that does not support the 519 required extension, it MAY choose, based on the specific extension 520 and circumstances, to requeue the message and try later and/or try an 521 alternate MX host. If this strategy is employed, the timeout to fall 522 back to an unextended format (if one is available) SHOULD be less 523 than the normal timeout for bouncing as undeliverable (e.g., if 524 normal timeout is three days, the requeue timeout before attempting 525 to transmit the mail without the extension might be one day). 527 2.3. SMTP Terminology 529 2.3.1. Mail Objects 531 SMTP transports a mail object. A mail object contains an envelope 532 and content. 534 The SMTP envelope is sent as a series of SMTP protocol units 535 (described in Section 3). It consists of an originator address (to 536 which error reports should be directed), one or more recipient 537 addresses, and optional protocol extension material. Historically, 538 variations on the reverse-path (originator) address specification 539 command (MAIL) could be used to specify alternate delivery modes, 540 such as immediate display; those variations have now been deprecated 541 (see Appendix F and Appendix F.6). 543 The SMTP content is sent in the SMTP DATA protocol unit and has two 544 parts: the header section and the body. If the content conforms to 545 other contemporary standards, the header section consists of a 546 collection of header fields, each consisting of a header name, a 547 colon, and data, structured as in the message format specification 548 (RFC 5322 [11]); the body, if structured, is defined according to 549 MIME (RFC 2045 [29]). The content is textual in nature, expressed 550 using the US-ASCII repertoire [2]. Although SMTP extensions (such as 551 "8BITMIME", RFC 1652 [23]) may relax this restriction for the content 552 body, the content header fields are always encoded using the US-ASCII 553 repertoire. Two MIME extensions (RFC 2047 [30] and RFC 2231 [33]) 554 define an algorithm for representing header values outside the US- 555 ASCII repertoire, while still encoding them using the US-ASCII 556 repertoire. 558 2.3.2. Senders and Receivers 560 In RFC 821, the two hosts participating in an SMTP transaction were 561 described as the "SMTP-sender" and "SMTP-receiver". This document 562 has been changed to reflect current industry terminology and hence 563 refers to them as the "SMTP client" (or sometimes just "the client") 564 and "SMTP server" (or just "the server"), respectively. Since a 565 given host may act both as server and client in a relay situation, 566 "receiver" and "sender" terminology is still used where needed for 567 clarity. 569 2.3.3. Mail Agents and Message Stores 571 Additional mail system terminology became common after RFC 821 was 572 published and, where convenient, is used in this specification. In 573 particular, SMTP servers and clients provide a mail transport service 574 and therefore act as "Mail Transfer Agents" (MTAs). "Mail User 575 Agents" (MUAs or UAs) are normally thought of as the sources and 576 targets of mail. At the source, an MUA might collect mail to be 577 transmitted from a user and hand it off to an MTA; the final 578 ("delivery") MTA would be thought of as handing the mail off to an 579 MUA (or at least transferring responsibility to it, e.g., by 580 depositing the message in a "message store"). However, while these 581 terms are used with at least the appearance of great precision in 582 other environments, the implied boundaries between MUAs and MTAs 583 often do not accurately match common, and conforming, practices with 584 Internet mail. Hence, the reader should be cautious about inferring 585 the strong relationships and responsibilities that might be implied 586 if these terms were used elsewhere. 588 2.3.4. Host 590 For the purposes of this specification, a host is a computer system 591 attached to the Internet (or, in some cases, to a private TCP/IP 592 network) and supporting the SMTP protocol. Hosts are known by names 593 (see the next section); they SHOULD NOT be identified by numerical 594 addresses, i.e., by address literals as described in Section 4.1.2. 596 2.3.5. Domain Names 598 A domain name (or often just a "domain") consists of one or more 599 components, separated by dots if more than one appears. In the case 600 of a top-level domain used by itself in an email address, a single 601 string is used without any dots. This makes the requirement, 602 described in more detail below, that only fully-qualified domain 603 names appear in SMTP transactions on the public Internet, 604 particularly important where top-level domains are involved. These 605 components ("labels" in DNS terminology, RFC 1035 [7]) are restricted 606 for SMTP purposes to consist of a sequence of letters, digits, and 607 hyphens drawn from the ASCII character set [2] and conforming to what 608 RFC 1035 Section 2.3.1 calls the "preferred name syntax". Domain 609 names are used as names of hosts and of other entities in the domain 610 name hierarchy. For example, a domain may refer to an alias (label 611 of a CNAME RR) or the label of Mail eXchanger records to be used to 612 deliver mail instead of representing a host name. See RFC 1035 [7] 613 and Section 5 of this specification. 615 The domain name, as described in this document and in RFC 1035 [7], 616 is the entire, fully-qualified name (often referred to as an "FQDN"). 618 A domain name that is not in FQDN form is no more than a local alias. 619 Local aliases MUST NOT appear in any SMTP transaction. 621 Only resolvable, fully-qualified domain names (FQDNs) are permitted 622 when domain names are used in SMTP. 623 [[CREF3: [[5321bis Editor's Note: does "in the public DNS" or 624 equivalent need to be added to "resolvable"???]]]] 625 In other words, names that can be resolved to MX RRs or address 626 (i.e., A or AAAA) RRs (as discussed in Section 5) are permitted, as 627 are CNAME RRs whose targets can be resolved, in turn, to MX or 628 address RRs. 629 [[CREF4: [[5321bis Editor's Note: it is not clear whether "In other 630 words" really meant "for example" or it is was intended that the only 631 labels permitted are those that own records in one of the above RR 632 types]]]] 633 [[CREF5: [[5321bis Editor's Note: More generally, does this section 634 need work to clarify the relationship to private domain names 635 (discussed on SMTP list starting 2013-03-26)]]]] 636 Local nicknames or unqualified names MUST NOT be used. There are two 637 exceptions to the rule requiring FQDNs: 639 o The domain name given in the EHLO command MUST be either a primary 640 host name (a domain name that resolves to an address RR) or, if 641 the host has no name, an address literal, as described in 642 Section 4.1.3 and discussed further in the EHLO discussion of 643 Section 4.1.4. 645 o The reserved mailbox name "postmaster" may be used in a RCPT 646 command without domain qualification (see Section 4.1.1.3) and 647 MUST be accepted if so used. 649 2.3.6. Buffer and State Table 651 SMTP sessions are stateful, with both parties carefully maintaining a 652 common view of the current state. In this document, we model this 653 state by a virtual "buffer" and a "state table" on the server that 654 may be used by the client to, for example, "clear the buffer" or 655 "reset the state table", causing the information in the buffer to be 656 discarded and the state to be returned to some previous state. 658 2.3.7. Commands and Replies 660 SMTP commands and, unless altered by a service extension, message 661 data, are transmitted from the sender to the receiver via the 662 transmission channel in "lines". 664 An SMTP reply is an acknowledgment (positive or negative) sent in 665 "lines" from receiver to sender via the transmission channel in 666 response to a command. The general form of a reply is a numeric 667 completion code (indicating failure or success) usually followed by a 668 text string. The codes are for use by programs and the text is 669 usually intended for human users. RFC 3463 [38], specifies further 670 structuring of the reply strings, including the use of supplemental 671 and more specific completion codes (see also RFC 5248 [46]). 673 2.3.8. Lines 675 Lines consist of zero or more data characters terminated by the 676 sequence ASCII character "CR" (hex value 0D) followed immediately by 677 ASCII character "LF" (hex value 0A). This termination sequence is 678 denoted as in this document. Conforming implementations MUST 679 NOT recognize or generate any other character or character sequence 680 as a line terminator. Limits MAY be imposed on line lengths by 681 servers (see Section 4). 683 In addition, the appearance of "bare" "CR" or "LF" characters in text 684 (i.e., either without the other) has a long history of causing 685 problems in mail implementations and applications that use the mail 686 system as a tool. SMTP client implementations MUST NOT transmit 687 these characters except when they are intended as line terminators 688 and then MUST, as indicated above, transmit them only as a 689 sequence. 691 2.3.9. Message Content and Mail Data 693 The terms "message content" and "mail data" are used interchangeably 694 in this document to describe the material transmitted after the DATA 695 command is accepted and before the end of data indication is 696 transmitted. Message content includes the message header section and 697 the possibly structured message body. The MIME specification (RFC 698 2045 [29]) provides the standard mechanisms for structured message 699 bodies. 701 2.3.10. Originator, Delivery, Relay, and Gateway Systems 703 This specification makes a distinction among four types of SMTP 704 systems, based on the role those systems play in transmitting 705 electronic mail. An "originating" system (sometimes called an SMTP 706 originator) introduces mail into the Internet or, more generally, 707 into a transport service environment. A "delivery" SMTP system is 708 one that receives mail from a transport service environment and 709 passes it to a mail user agent or deposits it in a message store that 710 a mail user agent is expected to subsequently access. A "relay" SMTP 711 system (usually referred to just as a "relay") receives mail from an 712 SMTP client and transmits it, without modification to the message 713 data other than adding trace information, to another SMTP server for 714 further relaying or for delivery. 716 A "gateway" SMTP system (usually referred to just as a "gateway") 717 receives mail from a client system in one transport environment and 718 transmits it to a server system in another transport environment. 719 Differences in protocols or message semantics between the transport 720 environments on either side of a gateway may require that the gateway 721 system perform transformations to the message that are not permitted 722 to SMTP relay systems. For the purposes of this specification, 723 firewalls that rewrite addresses should be considered as gateways, 724 even if SMTP is used on both sides of them (see RFC 2979 [36]). 725 [[CREF6: [5321bis] [[Note in draft/Placeholder: There has been a 726 request to expand this section, possibly into a more extensive model 727 of Internet mail. Comments from others solicited. In particular, 728 does RFC 5598 make that suggestion OBE?]] ]] 730 2.3.11. Mailbox and Address 732 As used in this specification, an "address" is a character string 733 that identifies a user to whom mail will be sent or a location into 734 which mail will be deposited. The term "mailbox" refers to that 735 depository. The two terms are typically used interchangeably unless 736 the distinction between the location in which mail is placed (the 737 mailbox) and a reference to it (the address) is important. An 738 address normally consists of user and domain specifications. The 739 standard mailbox naming convention is defined to be "local- 740 part@domain"; contemporary usage permits a much broader set of 741 applications than simple "user names". Consequently, and due to a 742 long history of problems when intermediate hosts have attempted to 743 optimize transport by modifying them, the local-part MUST be 744 interpreted and assigned semantics only by the host specified in the 745 domain part of the address. 747 2.4. General Syntax Principles and Transaction Model 749 SMTP commands and replies have a rigid syntax. All commands begin 750 with a command verb. All replies begin with a three digit numeric 751 code. In some commands and replies, arguments are required following 752 the verb or reply code. Some commands do not accept arguments (after 753 the verb), and some reply codes are followed, sometimes optionally, 754 by free form text. In both cases, where text appears, it is 755 separated from the verb or reply code by a space character. Complete 756 definitions of commands and replies appear in Section 4. 758 Verbs and argument values (e.g., "TO:" or "to:" in the RCPT command 759 and extension name keywords) are not case sensitive, with the sole 760 exception in this specification of a mailbox local-part (SMTP 761 Extensions may explicitly specify case-sensitive elements). That is, 762 a command verb, an argument value other than a mailbox local-part, 763 and free form text MAY be encoded in upper case, lower case, or any 764 mixture of upper and lower case with no impact on its meaning. The 765 local-part of a mailbox MUST BE treated as case sensitive. 766 Therefore, SMTP implementations MUST take care to preserve the case 767 of mailbox local-parts. In particular, for some hosts, the user 768 "smith" is different from the user "Smith". However, exploiting the 769 case sensitivity of mailbox local-parts impedes interoperability and 770 is discouraged. Mailbox domains follow normal DNS rules and are 771 hence not case sensitive. 773 A few SMTP servers, in violation of this specification (and RFC 821) 774 require that command verbs be encoded by clients in upper case. 775 Implementations MAY wish to employ this encoding to accommodate those 776 servers. 778 The argument clause consists of a variable-length character string 779 ending with the end of the line, i.e., with the character sequence 780 . The receiver will take no action until this sequence is 781 received. 783 The syntax for each command is shown with the discussion of that 784 command. Common elements and parameters are shown in Section 4.1.2. 786 Commands and replies are composed of characters from the ASCII 787 character set [2]. When the transport service provides an 8-bit byte 788 (octet) transmission channel, each 7-bit character is transmitted, 789 right justified, in an octet with the high-order bit cleared to zero. 790 More specifically, the unextended SMTP service provides 7-bit 791 transport only. An originating SMTP client that has not successfully 792 negotiated an appropriate extension with a particular server (see the 793 next paragraph) MUST NOT transmit messages with information in the 794 high-order bit of octets. If such messages are transmitted in 795 violation of this rule, receiving SMTP servers MAY clear the high- 796 order bit or reject the message as invalid. In general, a relay SMTP 797 SHOULD assume that the message content it has received is valid and, 798 assuming that the envelope permits doing so, relay it without 799 inspecting that content. Of course, if the content is mislabeled and 800 the data path cannot accept the actual content, this may result in 801 the ultimate delivery of a severely garbled message to the recipient. 802 Delivery SMTP systems MAY reject such messages, or return them as 803 undeliverable, rather than deliver them. In the absence of a server- 804 offered extension explicitly permitting it, a sending SMTP system is 805 not permitted to send envelope commands in any character set other 806 than US-ASCII. Receiving systems SHOULD reject such commands, 807 normally using "500 syntax error - invalid character" replies. 809 8-bit message content transmission MAY be requested of the server by 810 a client using extended SMTP facilities, notably the "8BITMIME" 811 extension, RFC 1652 [23]. 8BITMIME SHOULD be supported by SMTP 812 servers. However, it MUST NOT be construed as authorization to 813 transmit unrestricted 8-bit material, nor does 8BITMIME authorize 814 transmission of any envelope material in other than ASCII. 8BITMIME 815 MUST NOT be requested by senders for material with the high bit on 816 that is not in MIME format with an appropriate content-transfer 817 encoding; servers MAY reject such messages. 819 The metalinguistic notation used in this document corresponds to the 820 "Augmented BNF" used in other Internet mail system documents. The 821 reader who is not familiar with that syntax should consult the ABNF 822 specification in RFC 5234 [5]. Metalanguage terms used in running 823 text are surrounded by pointed brackets (e.g., ) for clarity. 824 The reader is cautioned that the grammar expressed in the 825 metalanguage is not comprehensive. There are many instances in which 826 provisions in the text constrain or otherwise modify the syntax or 827 semantics implied by the grammar. 829 3. The SMTP Procedures: An Overview 831 This section contains descriptions of the procedures used in SMTP: 832 session initiation, mail transaction, forwarding mail, verifying 833 mailbox names and expanding mailing lists, and opening and closing 834 exchanges. Comments on relaying, a note on mail domains, and a 835 discussion of changing roles are included at the end of this section. 836 Several complete scenarios are presented in Appendix D. 838 3.1. Session Initiation 840 An SMTP session is initiated when a client opens a connection to a 841 server and the server responds with an opening message. 843 SMTP server implementations MAY include identification of their 844 software and version information in the connection greeting reply 845 after the 220 code, a practice that permits more efficient isolation 846 and repair of any problems. Implementations MAY make provision for 847 SMTP servers to disable the software and version announcement where 848 it causes security concerns. While some systems also identify their 849 contact point for mail problems, this is not a substitute for 850 maintaining the required "postmaster" address (see Section 4). 852 The SMTP protocol allows a server to formally reject a mail session 853 while still allowing the initial connection as follows: a 554 854 response MAY be given in the initial connection opening message 855 instead of the 220. A server taking this approach MUST still wait 856 for the client to send a QUIT (see Section 4.1.1.10) before closing 857 the connection and SHOULD respond to any intervening commands with 858 "503 bad sequence of commands". Since an attempt to make an SMTP 859 connection to such a system is probably in error, a server returning 860 a 554 response on connection opening SHOULD provide enough 861 information in the reply text to facilitate debugging of the sending 862 system. 864 3.2. Client Initiation 866 Once the server has sent the greeting (welcoming) message and the 867 client has received it, the client normally sends the EHLO command to 868 the server, indicating the client's identity. In addition to opening 869 the session, use of EHLO indicates that the client is able to process 870 service extensions and requests that the server provide a list of the 871 extensions it supports. Older SMTP systems that are unable to 872 support service extensions, and contemporary clients that do not 873 require service extensions in the mail session being initiated, MAY 874 use HELO instead of EHLO. Servers MUST NOT return the extended EHLO- 875 style response to a HELO command. For a particular connection 876 attempt, if the server returns a "command not recognized" response to 877 EHLO, the client SHOULD be able to fall back and send HELO. 879 In the EHLO command, the host sending the command identifies itself; 880 the command may be interpreted as saying "Hello, I am " (and, 881 in the case of EHLO, "and I support service extension requests"). 883 3.3. Mail Transactions 885 There are three steps to SMTP mail transactions. The transaction 886 starts with a MAIL command that gives the sender identification. (In 887 general, the MAIL command may be sent only when no mail transaction 888 is in progress; see Section 4.1.4.) A series of one or more RCPT 889 commands follows, giving the receiver information. Then, a DATA 890 command initiates transfer of the mail data and is terminated by the 891 "end of mail" data indicator, which also confirms the transaction. 893 The first step in the procedure is the MAIL command. 895 MAIL FROM: [SP ] 897 This command tells the SMTP-receiver that a new mail transaction is 898 starting and to reset all its state tables and buffers, including any 899 recipients or mail data. The portion of the first or 900 only argument contains the source mailbox (between "<" and ">" 901 brackets), which can be used to report errors (see Section 4.2 for a 902 discussion of error reporting). If accepted, the SMTP server returns 903 a "250 OK" reply. If the mailbox specification is not acceptable for 904 some reason, the server MUST return a reply indicating whether the 905 failure is permanent (i.e., will occur again if the client tries to 906 send the same address again) or temporary (i.e., the address might be 907 accepted if the client tries again later). Despite the apparent 908 scope of this requirement, there are circumstances in which the 909 acceptability of the reverse-path may not be determined until one or 910 more forward-paths (in RCPT commands) can be examined. In those 911 cases, the server MAY reasonably accept the reverse-path (with a 250 912 reply) and then report problems after the forward-paths are received 913 and examined. Normally, failures produce 550 or 553 replies. 915 Historically, the was permitted to contain more than 916 just a mailbox; however, contemporary systems SHOULD NOT use source 917 routing (see Appendix C). 919 The optional are associated with negotiated SMTP 920 service extensions (see Section 2.2). 922 The second step in the procedure is the RCPT command. This step of 923 the procedure can be repeated any number of times. 925 RCPT TO: [ SP ] 927 The first or only argument to this command includes a forward-path 928 (normally a mailbox and domain, always surrounded by "<" and ">" 929 brackets) identifying one recipient. If accepted, the SMTP server 930 returns a "250 OK" reply and stores the forward-path. If the 931 recipient is known not to be a deliverable address, the SMTP server 932 returns a 550 reply, typically with a string such as "no such user - 933 " and the mailbox name (other circumstances and reply codes are 934 possible). 936 The can contain more than just a mailbox. 937 Historically, the was permitted to contain a source 938 routing list of hosts and the destination mailbox; however, 939 contemporary SMTP clients SHOULD NOT utilize source routes (see 940 Appendix C). Servers MUST be prepared to encounter a list of source 941 routes in the forward-path, but they SHOULD ignore the routes or MAY 942 decline to support the relaying they imply. Similarly, servers MAY 943 decline to accept mail that is destined for other hosts or systems. 944 These restrictions make a server useless as a relay for clients that 945 do not support full SMTP functionality. Consequently, restricted- 946 capability clients MUST NOT assume that any SMTP server on the 947 Internet can be used as their mail processing (relaying) site. If a 948 RCPT command appears without a previous MAIL command, the server MUST 949 return a 503 "Bad sequence of commands" response. The optional 950 are associated with negotiated SMTP service 951 extensions (see Section 2.2). [[CREF7: [5321bis] JcK Note for 952 2821ter (5321bis): this section would be improved by being more 953 specific about where mail transactions begin and end and then talking 954 about "transaction state" here, rather than specific prior commands. 955 --JcK]] 957 Since it has been a common source of errors, it is worth noting that 958 spaces are not permitted on either side of the colon following FROM 959 in the MAIL command or TO in the RCPT command. The syntax is exactly 960 as given above. 962 The third step in the procedure is the DATA command (or some 963 alternative specified in a service extension). 965 DATA 967 If accepted, the SMTP server returns a 354 Intermediate reply and 968 considers all succeeding lines up to but not including the end of 969 mail data indicator to be the message text. When the end of text is 970 successfully received and stored, the SMTP-receiver sends a "250 OK" 971 reply. 973 Since the mail data is sent on the transmission channel, the end of 974 mail data must be indicated so that the command and reply dialog can 975 be resumed. SMTP indicates the end of the mail data by sending a 976 line containing only a "." (period or full stop). A transparency 977 procedure is used to prevent this from interfering with the user's 978 text (see Section 4.5.2). 980 The end of mail data indicator also confirms the mail transaction and 981 tells the SMTP server to now process the stored recipients and mail 982 data. If accepted, the SMTP server returns a "250 OK" reply. The 983 DATA command can fail at only two points in the protocol exchange: 985 If there was no MAIL, or no RCPT, command, or all such commands were 986 rejected, the server MAY return a "command out of sequence" (503) or 987 "no valid recipients" (554) reply in response to the DATA command. 988 If one of those replies (or any other 5yz reply) is received, the 989 client MUST NOT send the message data; more generally, message data 990 MUST NOT be sent unless a 354 reply is received. 992 If the verb is initially accepted and the 354 reply issued, the DATA 993 command should fail only if the mail transaction was incomplete (for 994 example, no recipients), if resources were unavailable (including, of 995 course, the server unexpectedly becoming unavailable), or if the 996 server determines that the message should be rejected for policy or 997 other reasons. 999 However, in practice, some servers do not perform recipient 1000 verification until after the message text is received. These servers 1001 SHOULD treat a failure for one or more recipients as a "subsequent 1002 failure" and return a mail message as discussed in Section 6 and, in 1003 particular, in Section 6.1. Using a "550 mailbox not found" (or 1004 equivalent) reply code after the data are accepted makes it difficult 1005 or impossible for the client to determine which recipients failed. 1007 When the RFC 822 format ([16], [11]) is being used, the mail data 1008 include the header fields such as those named Date, Subject, To, Cc, 1009 and From. Server SMTP systems SHOULD NOT reject messages based on 1010 perceived defects in the RFC 822 or MIME (RFC 2045 [29]) message 1011 header section or message body. In particular, they MUST NOT reject 1012 messages in which the numbers of Resent-header fields do not match or 1013 Resent-to appears without Resent-from and/or Resent-date. 1015 Mail transaction commands MUST be used in the order discussed above. 1017 3.4. Forwarding for Address Correction or Updating 1019 Forwarding support is most often required to consolidate and simplify 1020 addresses within, or relative to, some enterprise and less frequently 1021 to establish addresses to link a person's prior address with a 1022 current one. Silent forwarding of messages (without server 1023 notification to the sender), for security or non-disclosure purposes, 1024 is common in the contemporary Internet. 1026 In both the enterprise and the "new address" cases, information 1027 hiding (and sometimes security) considerations argue against exposure 1028 of the "final" address through the SMTP protocol as a side effect of 1029 the forwarding activity. This may be especially important when the 1030 final address may not even be reachable by the sender. Consequently, 1031 the "forwarding" mechanisms described in Section 3.2 of RFC 821, and 1032 especially the 251 (corrected destination) and 551 reply codes from 1033 RCPT must be evaluated carefully by implementers and, when they are 1034 available, by those configuring systems (see also Section 7.4). 1036 In particular: 1038 o Servers MAY forward messages when they are aware of an address 1039 change. When they do so, they MAY either provide address-updating 1040 information with a 251 code, or may forward "silently" and return 1041 a 250 code. However, if a 251 code is used, they MUST NOT assume 1042 that the client will actually update address information or even 1043 return that information to the user. 1045 Alternately, 1047 o Servers MAY reject messages or return them as non-deliverable when 1048 they cannot be delivered precisely as addressed. When they do so, 1049 they MAY either provide address-updating information with a 551 1050 code, or may reject the message as undeliverable with a 550 code 1051 and no address-specific information. However, if a 551 code is 1052 used, they MUST NOT assume that the client will actually update 1053 address information or even return that information to the user. 1055 SMTP server implementations that support the 251 and/or 551 reply 1056 codes SHOULD provide configuration mechanisms so that sites that 1057 conclude that they would undesirably disclose information can disable 1058 or restrict their use. 1060 3.5. Commands for Debugging Addresses 1062 3.5.1. Overview 1064 SMTP provides commands to verify a user name or obtain the content of 1065 a mailing list. This is done with the VRFY and EXPN commands, which 1066 have character string arguments. Implementations SHOULD support VRFY 1067 and EXPN (however, see Section 3.5.2 and Section 7.3). 1069 For the VRFY command, the string is a user name or a user name and 1070 domain (see below). If a normal (i.e., 250) response is returned, 1071 the response MAY include the full name of the user and MUST include 1072 the mailbox of the user. It MUST be in either of the following 1073 forms: 1075 User Name 1076 local-part@domain 1078 When a name that is the argument to VRFY could identify more than one 1079 mailbox, the server MAY either note the ambiguity or identify the 1080 alternatives. In other words, any of the following are legitimate 1081 responses to VRFY: 1083 553 User ambiguous 1085 or 1087 553- Ambiguous; Possibilities are 1088 553-Joe Smith 1089 553-Harry Smith 1090 553 Melvin Smith 1092 or 1094 553-Ambiguous; Possibilities 1095 553- 1096 553- 1097 553 1099 Under normal circumstances, a client receiving a 553 reply would be 1100 expected to expose the result to the user. Use of exactly the forms 1101 given, and the "user ambiguous" or "ambiguous" keywords, possibly 1102 supplemented by extended reply codes, such as those described in RFC 1103 3463 [38], will facilitate automated translation into other languages 1104 as needed. Of course, a client that was highly automated or that was 1105 operating in another language than English might choose to try to 1106 translate the response to return some other indication to the user 1107 than the literal text of the reply, or to take some automated action 1108 such as consulting a directory service for additional information 1109 before reporting to the user. 1111 For the EXPN command, the string identifies a mailing list, and the 1112 successful (i.e., 250) multiline response MAY include the full name 1113 of the users and MUST give the mailboxes on the mailing list. 1115 In some hosts, the distinction between a mailing list and an alias 1116 for a single mailbox is a bit fuzzy, since a common data structure 1117 may hold both types of entries, and it is possible to have mailing 1118 lists containing only one mailbox. If a request is made to apply 1119 VRFY to a mailing list, a positive response MAY be given if a message 1120 so addressed would be delivered to everyone on the list, otherwise an 1121 error SHOULD be reported (e.g., "550 That is a mailing list, not a 1122 user" or "252 Unable to verify members of mailing list"). If a 1123 request is made to expand a user name, the server MAY return a 1124 positive response consisting of a list containing one name, or an 1125 error MAY be reported (e.g., "550 That is a user name, not a mailing 1126 list"). 1128 In the case of a successful multiline reply (normal for EXPN), 1129 exactly one mailbox is to be specified on each line of the reply. 1130 The case of an ambiguous request is discussed above. 1132 "User name" is a fuzzy term and has been used deliberately. An 1133 implementation of the VRFY or EXPN commands MUST include at least 1134 recognition of local mailboxes as "user names". However, since 1135 current Internet practice often results in a single host handling 1136 mail for multiple domains, hosts, especially hosts that provide this 1137 functionality, SHOULD accept the "local-part@domain" form as a "user 1138 name"; hosts MAY also choose to recognize other strings as "user 1139 names". 1141 The case of expanding a mailbox list requires a multiline reply, such 1142 as: 1144 C: EXPN Example-People 1145 S: 250-Jon Postel 1146 S: 250-Fred Fonebone 1147 S: 250 Sam Q. Smith 1149 or 1151 C: EXPN Executive-Washroom-List 1152 S: 550 Access Denied to You. 1154 The character string arguments of the VRFY and EXPN commands cannot 1155 be further restricted due to the variety of implementations of the 1156 user name and mailbox list concepts. On some systems, it may be 1157 appropriate for the argument of the EXPN command to be a file name 1158 for a file containing a mailing list, but again there are a variety 1159 of file naming conventions in the Internet. Similarly, historical 1160 variations in what is returned by these commands are such that the 1161 response SHOULD be interpreted very carefully, if at all, and SHOULD 1162 generally only be used for diagnostic purposes. 1164 3.5.2. VRFY Normal Response 1166 When normal (2yz or 551) responses are returned from a VRFY or EXPN 1167 request, the reply MUST include the name using a "" construction, where "domain" is a fully-qualified 1169 domain name. In circumstances exceptional enough to justify 1170 violating the intent of this specification, free-form text MAY be 1171 returned. In order to facilitate parsing by both computers and 1172 people, addresses SHOULD appear in pointed brackets. When addresses, 1173 rather than free-form debugging information, are returned, EXPN and 1174 VRFY MUST return only valid domain addresses that are usable in SMTP 1175 RCPT commands. Consequently, if an address implies delivery to a 1176 program or other system, the mailbox name used to reach that target 1177 MUST be given. Paths (explicit source routes) MUST NOT be returned 1178 by VRFY or EXPN. 1180 Server implementations SHOULD support both VRFY and EXPN. For 1181 security reasons, implementations MAY provide local installations a 1182 way to disable either or both of these commands through configuration 1183 options or the equivalent (see Section 7.3). When these commands are 1184 supported, they are not required to work across relays when relaying 1185 is supported. Since they were both optional in RFC 821, but VRFY was 1186 made mandatory in RFC 1123 [3], if EXPN is supported, it MUST be 1187 listed as a service extension in an EHLO response. VRFY MAY be 1188 listed as a convenience but, since support for it is required, SMTP 1189 clients are not required to check for its presence on the extension 1190 list before using it. 1192 3.5.3. Meaning of VRFY or EXPN Success Response 1194 A server MUST NOT return a 250 code in response to a VRFY or EXPN 1195 command unless it has actually verified the address. In particular, 1196 a server MUST NOT return 250 if all it has done is to verify that the 1197 syntax given is valid. In that case, 502 (Command not implemented) 1198 or 500 (Syntax error, command unrecognized) SHOULD be returned. As 1199 stated elsewhere, implementation (in the sense of actually validating 1200 addresses and returning information) of VRFY and EXPN are strongly 1201 recommended. Hence, implementations that return 500 or 502 for VRFY 1202 are not in full compliance with this specification. 1204 There may be circumstances where an address appears to be valid but 1205 cannot reasonably be verified in real time, particularly when a 1206 server is acting as a mail exchanger for another server or domain. 1207 "Apparent validity", in this case, would normally involve at least 1208 syntax checking and might involve verification that any domains 1209 specified were ones to which the host expected to be able to relay 1210 mail. In these situations, reply code 252 SHOULD be returned. These 1211 cases parallel the discussion of RCPT verification in Section 2.1. 1212 Similarly, the discussion in Section 3.4 applies to the use of reply 1213 codes 251 and 551 with VRFY (and EXPN) to indicate addresses that are 1214 recognized but that would be forwarded or rejected were mail received 1215 for them. Implementations generally SHOULD be more aggressive about 1216 address verification in the case of VRFY than in the case of RCPT, 1217 even if it takes a little longer to do so. 1219 3.5.4. Semantics and Applications of EXPN 1221 EXPN is often very useful in debugging and understanding problems 1222 with mailing lists and multiple-target-address aliases. Some systems 1223 have attempted to use source expansion of mailing lists as a means of 1224 eliminating duplicates. The propagation of aliasing systems with 1225 mail on the Internet for hosts (typically with MX and CNAME DNS 1226 records), for mailboxes (various types of local host aliases), and in 1227 various proxying arrangements has made it nearly impossible for these 1228 strategies to work consistently, and mail systems SHOULD NOT attempt 1229 them. 1231 3.6. Relaying and Mail Routing 1233 3.6.1. Source Routes and Relaying 1235 In general, the availability of Mail eXchanger records in the domain 1236 name system (RFC 1035 [7], RFC 974 [19]) makes the use of explicit 1237 source routes in the Internet mail system unnecessary. Many 1238 historical problems with the interpretation of explicit source routes 1239 have made their use undesirable. SMTP clients SHOULD NOT generate 1240 explicit source routes except under unusual circumstances. SMTP 1241 servers MAY decline to act as mail relays or to accept addresses that 1242 specify source routes. When route information is encountered, SMTP 1243 servers MAY ignore the route information and simply send to the final 1244 destination specified as the last element in the route and SHOULD do 1245 so. There has been an invalid practice of using names that do not 1246 appear in the DNS as destination names, with the senders counting on 1247 the intermediate hosts specified in source routing to resolve any 1248 problems. If source routes are stripped, this practice will cause 1249 failures. This is one of several reasons why SMTP clients MUST NOT 1250 generate invalid source routes or depend on serial resolution of 1251 names in such routes. [[CREF8: [5321bis] Jck 20091023: "of names..." 1252 added for clarity"]] 1254 When source routes are not used, the process described in RFC 821 for 1255 constructing a reverse-path from the forward-path is not applicable 1256 and the reverse-path at the time of delivery will simply be the 1257 address that appeared in the MAIL command. 1259 3.6.2. Mail eXchange Records and Relaying 1261 A relay SMTP server is usually the target of a DNS MX record that 1262 designates it, rather than the final delivery system. The relay 1263 server may accept or reject the task of relaying the mail in the same 1264 way it accepts or rejects mail for a local user. If it accepts the 1265 task, it then becomes an SMTP client, establishes a transmission 1266 channel to the next SMTP server specified in the DNS (according to 1267 the rules in Section 5), and sends it the mail. If it declines to 1268 relay mail to a particular address for policy reasons, a 550 response 1269 SHOULD be returned. 1271 This specification does not deal with the verification of return 1272 paths for use in delivery notifications. Recent work, such as that 1273 on SPF [42] and DKIM [44] [45], has been done to provide ways to 1274 ascertain that an address is valid or belongs to the person who 1275 actually sent the message. 1276 [[5321bis Editor's Note: Proposed erratum (4055) suggests that DKIM 1277 and SPF have nothing to do with this and that everything after the 1278 first sentence should be dropped. An alternative would be to tune 1279 the texts. ???]] 1280 A server MAY attempt to verify the return path before using its 1281 address for delivery notifications, but methods of doing so are not 1282 defined here nor is any particular method recommended at this time. 1284 3.6.3. Message Submission Servers as Relays 1286 Many mail-sending clients exist, especially in conjunction with 1287 facilities that receive mail via POP3 or IMAP, that have limited 1288 capability to support some of the requirements of this specification, 1289 such as the ability to queue messages for subsequent delivery 1290 attempts. For these clients, it is common practice to make private 1291 arrangements to send all messages to a single server for processing 1292 and subsequent distribution. SMTP, as specified here, is not ideally 1293 suited for this role. A standardized mail submission protocol has 1294 been developed that is gradually superseding practices based on SMTP 1295 (see RFC 4409 [43]). In any event, because these arrangements are 1296 private and fall outside the scope of this specification, they are 1297 not described here. 1299 It is important to note that MX records can point to SMTP servers 1300 that act as gateways into other environments, not just SMTP relays 1301 and final delivery systems; see Sections 3.7 and 5. 1303 If an SMTP server has accepted the task of relaying the mail and 1304 later finds that the destination is incorrect or that the mail cannot 1305 be delivered for some other reason, then it MUST construct an 1306 "undeliverable mail" notification message and send it to the 1307 originator of the undeliverable mail (as indicated by the reverse- 1308 path). Formats specified for non-delivery reports by other standards 1309 (see, for example, RFC 3461 [12] and RFC 3464 [13]) SHOULD be used if 1310 possible. 1312 This notification message must be from the SMTP server at the relay 1313 host or the host that first determines that delivery cannot be 1314 accomplished. Of course, SMTP servers MUST NOT send notification 1315 messages about problems transporting notification messages. One way 1316 to prevent loops in error reporting is to specify a null reverse-path 1317 in the MAIL command of a notification message. When such a message 1318 is transmitted, the reverse-path MUST be set to null (see 1319 Section 4.5.5 for additional discussion). A MAIL command with a null 1320 reverse-path appears as follows: 1322 MAIL FROM:<> 1324 As discussed in Section 6.4, a relay SMTP has no need to inspect or 1325 act upon the header section or body of the message data and MUST NOT 1326 do so except to add its own "Received:" header field (Section 4.4) 1327 and, optionally, to attempt to detect looping in the mail system (see 1328 Section 6.3). Of course, this prohibition also applies to any 1329 modifications of these header fields or text (see also Section 7.9). 1331 3.7. Mail Gatewaying 1333 While the relay function discussed above operates within the Internet 1334 SMTP transport service environment, MX records or various forms of 1335 explicit routing may require that an intermediate SMTP server perform 1336 a translation function between one transport service and another. As 1337 discussed in Section 2.3.10, when such a system is at the boundary 1338 between two transport service environments, we refer to it as a 1339 "gateway" or "gateway SMTP". 1341 Gatewaying mail between different mail environments, such as 1342 different mail formats and protocols, is complex and does not easily 1343 yield to standardization. However, some general requirements may be 1344 given for a gateway between the Internet and another mail 1345 environment. 1347 3.7.1. Header Fields in Gatewaying 1349 Header fields MAY be rewritten when necessary as messages are 1350 gatewayed across mail environment boundaries. This may involve 1351 inspecting the message body or interpreting the local-part of the 1352 destination address in spite of the prohibitions in Section 6.4. 1354 Other mail systems gatewayed to the Internet often use a subset of 1355 the RFC 822 header section or provide similar functionality with a 1356 different syntax, but some of these mail systems do not have an 1357 equivalent to the SMTP envelope. Therefore, when a message leaves 1358 the Internet environment, it may be necessary to fold the SMTP 1359 envelope information into the message header section. A possible 1360 solution would be to create new header fields to carry the envelope 1361 information (e.g., "X-SMTP-MAIL:" and "X-SMTP-RCPT:"); however, this 1362 would require changes in mail programs in foreign environments and 1363 might risk disclosure of private information (see Section 7.2). 1365 3.7.2. Received Lines in Gatewaying 1367 When forwarding a message into or out of the Internet environment, a 1368 gateway MUST prepend a Received: line, but it MUST NOT alter in any 1369 way a Received: line that is already in the header section. 1371 "Received:" header fields of messages originating from other 1372 environments may not conform exactly to this specification. However, 1373 the most important use of Received: lines is for debugging mail 1374 faults, and this debugging can be severely hampered by well-meaning 1375 gateways that try to "fix" a Received: line. As another consequence 1376 of trace header fields arising in non-SMTP environments, receiving 1377 systems MUST NOT reject mail based on the format of a trace header 1378 field and SHOULD be extremely robust in the light of unexpected 1379 information or formats in those header fields. 1381 The gateway SHOULD indicate the environment and protocol in the "via" 1382 clauses of Received header field(s) that it supplies. 1384 3.7.3. Addresses in Gatewaying 1386 From the Internet side, the gateway SHOULD accept all valid address 1387 formats in SMTP commands and in the RFC 822 header section, and all 1388 valid RFC 822 messages. Addresses and header fields generated by 1389 gateways MUST conform to applicable standards (including this one and 1390 RFC 5322 [11]). Gateways are, of course, subject to the same rules 1391 for handling source routes as those described for other SMTP systems 1392 in Section 3.3. 1394 3.7.4. Other Header Fields in Gatewaying 1396 The gateway MUST ensure that all header fields of a message that it 1397 forwards into the Internet mail environment meet the requirements for 1398 Internet mail. In particular, all addresses in "From:", "To:", 1399 "Cc:", etc., header fields MUST be transformed (if necessary) to 1400 satisfy the standard header syntax of RFC 5322 [11], MUST reference 1401 only fully-qualified domain names, and MUST be effective and useful 1402 for sending replies. The translation algorithm used to convert mail 1403 from the Internet protocols to another environment's protocol SHOULD 1404 ensure that error messages from the foreign mail environment are 1405 delivered to the reverse-path from the SMTP envelope, not to an 1406 address in the "From:", "Sender:", or similar header fields of the 1407 message. 1409 3.7.5. Envelopes in Gatewaying 1411 Similarly, when forwarding a message from another environment into 1412 the Internet, the gateway SHOULD set the envelope return path in 1413 accordance with an error message return address, if supplied by the 1414 foreign environment. If the foreign environment has no equivalent 1415 concept, the gateway must select and use a best approximation, with 1416 the message originator's address as the default of last resort. 1418 3.8. Terminating Sessions and Connections 1420 An SMTP connection is terminated when the client sends a QUIT 1421 command. The server responds with a positive reply code, after which 1422 it closes the connection. 1424 An SMTP server MUST NOT intentionally close the connection under 1425 normal operational circumstances (see Section 7.8) except: 1427 o After receiving a QUIT command and responding with a 221 reply. 1429 o After detecting the need to shut down the SMTP service and 1430 returning a 421 reply code. This reply code can be issued after 1431 the server receives any command or, if necessary, asynchronously 1432 from command receipt (on the assumption that the client will 1433 receive it after the next command is issued). 1435 o After a timeout, as specified in Section 4.5.3.2, occurs waiting 1436 for the client to send a command or data. 1438 In particular, a server that closes connections in response to 1439 commands that are not understood is in violation of this 1440 specification. Servers are expected to be tolerant of unknown 1441 commands, issuing a 500 reply and awaiting further instructions from 1442 the client. 1444 An SMTP server that is forcibly shut down via external means SHOULD 1445 attempt to send a line containing a 421 reply code to the SMTP client 1446 before exiting. The SMTP client will normally read the 421 reply 1447 code after sending its next command. 1449 SMTP clients that experience a connection close, reset, or other 1450 communications failure due to circumstances not under their control 1451 (in violation of the intent of this specification but sometimes 1452 unavoidable) SHOULD, to maintain the robustness of the mail system, 1453 treat the mail transaction as if a 421 response had been received and 1454 act accordingly. 1456 3.9. Mailing Lists and Aliases 1458 [[CREF9: [5321bis] If "alias and list models" are explained 1459 elsewhere, cross reference". Also note that this section appears to 1460 prohibit an exploder from adding List-* headers. That needs to be 1461 finessed.]] 1462 An SMTP-capable host SHOULD support both the alias and the list 1463 models of address expansion for multiple delivery. When a message is 1464 delivered or forwarded to each address of an expanded list form, the 1465 return address in the envelope ("MAIL FROM:") MUST be changed to be 1466 the address of a person or other entity who administers the list. 1467 However, in this case, the message header section (RFC 5322 [11]) 1468 MUST be left unchanged; in particular, the "From" field of the header 1469 section is unaffected. 1471 An important mail facility is a mechanism for multi-destination 1472 delivery of a single message, by transforming (or "expanding" or 1473 "exploding") a pseudo-mailbox address into a list of destination 1474 mailbox addresses. When a message is sent to such a pseudo-mailbox 1475 (sometimes called an "exploder"), copies are forwarded or 1476 redistributed to each mailbox in the expanded list. Servers SHOULD 1477 simply utilize the addresses on the list; application of heuristics 1478 or other matching rules to eliminate some addresses, such as that of 1479 the originator, is strongly discouraged. We classify such a pseudo- 1480 mailbox as an "alias" or a "list", depending upon the expansion 1481 rules. 1483 3.9.1. Alias 1485 To expand an alias, the recipient mailer simply replaces the pseudo- 1486 mailbox address in the envelope with each of the expanded addresses 1487 in turn; the rest of the envelope and the message body are left 1488 unchanged. The message is then delivered or forwarded to each 1489 expanded address. 1491 3.9.2. List 1493 A mailing list may be said to operate by "redistribution" rather than 1494 by "forwarding". To expand a list, the recipient mailer replaces the 1495 pseudo-mailbox address in the envelope with each of the expanded 1496 addresses in turn. The return (backward-pointing) address in the 1497 envelope is changed so that all error messages generated by the final 1498 deliveries will be returned to a list administrator, not to the 1499 message originator, who generally has no control over the contents of 1500 the list and will typically find error messages annoying. Note that 1501 the key difference between handling aliases (Section 3.9.1) and 1502 forwarding (this subsection) is the change to the backward-pointing 1503 address in this case. When a list constrains its processing to the 1504 very limited set of modifications and actions described here, it is 1505 attempting to emulate an MTA; such lists can be treated as a 1506 continuation in email transit. 1508 There exist mailing lists that perform additional, sometimes 1509 extensive, modifications to a message and its envelope. Such mailing 1510 lists need to be viewed as full MUAs, which accept a delivery and 1511 post a new message. 1513 4. The SMTP Specifications 1515 4.1. SMTP Commands 1517 4.1.1. Command Semantics and Syntax 1519 The SMTP commands define the mail transfer or the mail system 1520 function requested by the user. SMTP commands are character strings 1521 terminated by . The commands themselves are alphabetic 1522 characters terminated by if parameters follow and 1523 otherwise. (In the interest of improved interoperability, SMTP 1524 receivers SHOULD tolerate trailing white space before the terminating 1525 .) The syntax of the local part of a mailbox MUST conform to 1526 receiver site conventions and the syntax specified in Section 4.1.2. 1527 The SMTP commands are discussed below. The SMTP replies are 1528 discussed in Section 4.2. 1530 A mail transaction involves several data objects that are 1531 communicated as arguments to different commands. The reverse-path is 1532 the argument of the MAIL command, the forward-path is the argument of 1533 the RCPT command, and the mail data is the argument of the DATA 1534 command. These arguments or data objects must be transmitted and 1535 held, pending the confirmation communicated by the end of mail data 1536 indication that finalizes the transaction. The model for this is 1537 that distinct buffers are provided to hold the types of data objects; 1538 that is, there is a reverse-path buffer, a forward-path buffer, and a 1539 mail data buffer. Specific commands cause information to be appended 1540 to a specific buffer, or cause one or more buffers to be cleared. 1542 Several commands (RSET, DATA, QUIT) are specified as not permitting 1543 parameters. In the absence of specific extensions offered by the 1544 server and accepted by the client, clients MUST NOT send such 1545 parameters and servers SHOULD reject commands containing them as 1546 having invalid syntax. 1548 4.1.1.1. Extended HELLO (EHLO) or HELLO (HELO) 1550 These commands are used to identify the SMTP client to the SMTP 1551 server. The argument clause contains the fully-qualified domain name 1552 of the SMTP client, if one is available. In situations in which the 1553 SMTP client system does not have a meaningful domain name (e.g., when 1554 its address is dynamically allocated and no reverse mapping record is 1555 available), the client SHOULD send an address literal (see 1556 Section 4.1.3). 1558 RFC 2821, and some earlier informal practices, encouraged following 1559 the literal by information that would help to identify the client 1560 system. That convention was not widely supported, and many SMTP 1561 servers considered it an error. In the interest of interoperability, 1562 it is probably wise for servers to be prepared for this string to 1563 occur, but SMTP clients SHOULD NOT send it. 1565 The SMTP server identifies itself to the SMTP client in the 1566 connection greeting reply and in the response to this command. 1568 A client SMTP SHOULD start an SMTP session by issuing the EHLO 1569 command. If the SMTP server supports the SMTP service extensions, it 1570 will give a successful response, a failure response, or an error 1571 response. If the SMTP server, in violation of this specification, 1572 does not support any SMTP service extensions, it will generate an 1573 error response. Older client SMTP systems MAY, as discussed above, 1574 use HELO (as specified in RFC 821) instead of EHLO, and servers MUST 1575 support the HELO command and reply properly to it. In any event, a 1576 client MUST issue HELO or EHLO before starting a mail transaction. 1578 These commands, and a "250 OK" reply to one of them, confirm that 1579 both the SMTP client and the SMTP server are in the initial state, 1580 that is, there is no transaction in progress and all state tables and 1581 buffers are cleared. 1583 Syntax: 1585 ehlo = "EHLO" SP ( Domain / address-literal ) CRLF 1587 helo = "HELO" SP Domain CRLF 1589 Normally, the response to EHLO will be a multiline reply. Each line 1590 of the response contains a keyword and, optionally, one or more 1591 parameters. Following the normal syntax for multiline replies, these 1592 keywords follow the code (250) and a hyphen for all but the last 1593 line, and the code and a space for the last line. The syntax for a 1594 positive response, using the ABNF notation and terminal symbols of 1595 RFC 5234 [5], is: 1597 ehlo-ok-rsp = ( "250" SP Domain [ SP ehlo-greet ] CRLF ) 1598 / ( "250-" Domain [ SP ehlo-greet ] CRLF 1599 *( "250-" ehlo-line CRLF ) 1600 "250" SP ehlo-line CRLF ) 1602 ehlo-greet = 1*(%d0-9 / %d11-12 / %d14-127) 1603 ; string of any characters other than CR or LF 1605 ehlo-line = ehlo-keyword *( SP ehlo-param ) 1607 ehlo-keyword = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-") 1608 ; additional syntax of ehlo-params depends on 1609 ; ehlo-keyword 1611 ehlo-param = 1*(%d33-126) 1612 ; any CHAR excluding and all 1613 ; control characters (US-ASCII 0-31 and 127 1614 ; inclusive) 1616 Although EHLO keywords may be specified in upper, lower, or mixed 1617 case, they MUST always be recognized and processed in a case- 1618 insensitive manner. This is simply an extension of practices 1619 specified in RFC 821 and Section 2.4. 1621 The EHLO response MUST contain keywords (and associated parameters if 1622 required) for all commands not listed as "required" in Section 4.5.1 1623 excepting only private-use commands as described in Section 4.1.5. 1624 Private-use commands MAY be listed. 1626 4.1.1.2. MAIL (MAIL) 1628 This command is used to initiate a mail transaction in which the mail 1629 data is delivered to an SMTP server that may, in turn, deliver it to 1630 one or more mailboxes or pass it on to another system (possibly using 1631 SMTP). The argument clause contains a reverse-path and may contain 1632 optional parameters. In general, the MAIL command may be sent only 1633 when no mail transaction is in progress, see Section 4.1.4. 1635 The reverse-path consists of the sender mailbox. Historically, that 1636 mailbox might optionally have been preceded by a list of hosts, but 1637 that behavior is now deprecated (see Appendix C). In some types of 1638 reporting messages for which a reply is likely to cause a mail loop 1639 (for example, mail delivery and non-delivery notifications), the 1640 reverse-path may be null (see Section 3.6). 1642 This command clears the reverse-path buffer, the forward-path buffer, 1643 and the mail data buffer, and it inserts the reverse-path information 1644 from its argument clause into the reverse-path buffer. 1646 If service extensions were negotiated, the MAIL command may also 1647 carry parameters associated with a particular service extension. 1649 Syntax: 1651 mail = "MAIL FROM:" Reverse-path 1652 [SP Mail-parameters] CRLF 1654 4.1.1.3. RECIPIENT (RCPT) 1656 This command is used to identify an individual recipient of the mail 1657 data; multiple recipients are specified by multiple uses of this 1658 command. The argument clause contains a forward-path and may contain 1659 optional parameters. 1661 The forward-path normally consists of the required destination 1662 mailbox. Sending systems SHOULD NOT generate the optional list of 1663 hosts known as a source route. Receiving systems MUST recognize 1664 source route syntax but SHOULD strip off the source route 1665 specification and utilize the domain name associated with the mailbox 1666 as if the source route had not been provided. 1668 Similarly, relay hosts SHOULD strip or ignore source routes, and 1669 names MUST NOT be copied into the reverse-path. When mail reaches 1670 its ultimate destination (the forward-path contains only a 1671 destination mailbox), the SMTP server inserts it into the destination 1672 mailbox in accordance with its host mail conventions. 1674 This command appends its forward-path argument to the forward-path 1675 buffer; it does not change the reverse-path buffer nor the mail data 1676 buffer. 1678 For example, mail received at relay host xyz.com with envelope 1679 commands 1681 MAIL FROM: 1682 RCPT TO:<@hosta.int,@jkl.org:userc@d.bar.org> 1684 will normally be sent directly on to host d.bar.org with envelope 1685 commands 1687 MAIL FROM: 1688 RCPT TO: 1690 As provided in Appendix C, xyz.com MAY also choose to relay the 1691 message to hosta.int, using the envelope commands 1693 MAIL FROM: 1694 RCPT TO:<@hosta.int,@jkl.org:userc@d.bar.org> 1696 or to jkl.org, using the envelope commands 1698 MAIL FROM: 1699 RCPT TO:<@jkl.org:userc@d.bar.org> 1701 Attempting to use relaying this way is now strongly discouraged. 1702 Since hosts are not required to relay mail at all, xyz.com MAY also 1703 reject the message entirely when the RCPT command is received, using 1704 a 550 code (since this is a "policy reason"). 1706 If service extensions were negotiated, the RCPT command may also 1707 carry parameters associated with a particular service extension 1708 offered by the server. The client MUST NOT transmit parameters other 1709 than those associated with a service extension offered by the server 1710 in its EHLO response. 1712 Syntax: 1714 rcpt = "RCPT TO:" ( "" / "" / 1715 Forward-path ) [SP Rcpt-parameters] CRLF 1717 Note that, in a departure from the usual rules for 1718 local-parts, the "Postmaster" string shown above is 1719 treated as case-insensitive. 1721 4.1.1.4. DATA (DATA) 1723 The receiver normally sends a 354 response to DATA, and then treats 1724 the lines (strings ending in sequences, as described in 1725 Section 2.3.7) following the command as mail data from the sender. 1726 This command causes the mail data to be appended to the mail data 1727 buffer. The mail data may contain any of the 128 ASCII character 1728 codes, although experience has indicated that use of control 1729 characters other than SP, HT, CR, and LF may cause problems and 1730 SHOULD be avoided when possible. 1732 The mail data are terminated by a line containing only a period, that 1733 is, the character sequence ".", where the first is 1734 actually the terminator of the previous line (see Section 4.5.2). 1735 This is the end of mail data indication. The first of this 1736 terminating sequence is also the that ends the final line of 1737 the data (message text) or, if there was no mail data, ends the DATA 1738 command itself (the "no mail data" case does not conform to this 1739 specification since it would require that neither the trace header 1740 fields required by this specification nor the message header section 1741 required by RFC 5322 [11] be transmitted). An extra MUST NOT 1742 be added, as that would cause an empty line to be added to the 1743 message. The only exception to this rule would arise if the message 1744 body were passed to the originating SMTP-sender with a final "line" 1745 that did not end in ; in that case, the originating SMTP system 1746 MUST either reject the message as invalid or add in order to 1747 have the receiving SMTP server recognize the "end of data" condition. 1749 The custom of accepting lines ending only in , as a concession to 1750 non-conforming behavior on the part of some UNIX systems, has proven 1751 to cause more interoperability problems than it solves, and SMTP 1752 server systems MUST NOT do this, even in the name of improved 1753 robustness. In particular, the sequence "." (bare line 1754 feeds, without carriage returns) MUST NOT be treated as equivalent to 1755 . as the end of mail data indication. 1757 Receipt of the end of mail data indication requires the server to 1758 process the stored mail transaction information. This processing 1759 consumes the information in the reverse-path buffer, the forward-path 1760 buffer, and the mail data buffer, and on the completion of this 1761 command these buffers are cleared. If the processing is successful, 1762 the receiver MUST send an OK reply. If the processing fails, the 1763 receiver MUST send a failure reply. The SMTP model does not allow 1764 for partial failures at this point: either the message is accepted by 1765 the server for delivery and a positive response is returned or it is 1766 not accepted and a failure reply is returned. In sending a positive 1767 "250 OK" completion reply to the end of data indication, the receiver 1768 takes full responsibility for the message (see Section 6.1). Errors 1769 that are diagnosed subsequently MUST be reported in a mail message, 1770 as discussed in Section 4.4. 1772 When the SMTP server accepts a message either for relaying or for 1773 final delivery, it inserts a trace record (also referred to 1774 interchangeably as a "time stamp line" or "Received" line) at the top 1775 of the mail data. This trace record indicates the identity of the 1776 host that sent the message, the identity of the host that received 1777 the message (and is inserting this time stamp), and the date and time 1778 the message was received. Relayed messages will have multiple time 1779 stamp lines. Details for formation of these lines, including their 1780 syntax, is specified in Section 4.4. 1782 Additional discussion about the operation of the DATA command appears 1783 in Section 3.3. 1785 Syntax: 1787 data = "DATA" CRLF 1789 4.1.1.5. RESET (RSET) 1791 This command specifies that the current mail transaction will be 1792 aborted. Any stored sender, recipients, and mail data MUST be 1793 discarded, and all buffers and state tables cleared. The receiver 1794 MUST send a "250 OK" reply to a RSET command with no arguments. A 1795 reset command may be issued by the client at any time. It is 1796 effectively equivalent to a NOOP (i.e., it has no effect) if issued 1797 immediately after EHLO, before EHLO is issued in the session, after 1798 an end of data indicator has been sent and acknowledged, or 1799 immediately before a QUIT. An SMTP server MUST NOT close the 1800 connection as the result of receiving a RSET; that action is reserved 1801 for QUIT (see Section 4.1.1.10). 1803 Since EHLO implies some additional processing and response by the 1804 server, RSET will normally be more efficient than reissuing that 1805 command, even though the formal semantics are the same. 1807 There are circumstances, contrary to the intent of this 1808 specification, in which an SMTP server may receive an indication that 1809 the underlying TCP connection has been closed or reset. To preserve 1810 the robustness of the mail system, SMTP servers SHOULD be prepared 1811 for this condition and SHOULD treat it as if a QUIT had been received 1812 before the connection disappeared. 1814 Syntax: 1816 rset = "RSET" CRLF 1818 4.1.1.6. VERIFY (VRFY) 1820 This command asks the receiver to confirm that the argument 1821 identifies a user or mailbox. If it is a user name, information is 1822 returned as specified in Section 3.5. 1824 This command has no effect on the reverse-path buffer, the forward- 1825 path buffer, or the mail data buffer. 1827 Syntax: 1829 vrfy = "VRFY" SP String CRLF 1831 4.1.1.7. EXPAND (EXPN) 1833 This command asks the receiver to confirm that the argument 1834 identifies a mailing list, and if so, to return the membership of 1835 that list. If the command is successful, a reply is returned 1836 containing information as described in Section 3.5. This reply will 1837 have multiple lines except in the trivial case of a one-member list. 1839 This command has no effect on the reverse-path buffer, the forward- 1840 path buffer, or the mail data buffer, and it may be issued at any 1841 time. 1843 Syntax: 1845 expn = "EXPN" SP String CRLF 1847 4.1.1.8. HELP (HELP) 1849 This command causes the server to send helpful information to the 1850 client. The command MAY take an argument (e.g., any command name) 1851 and return more specific information as a response. 1853 This command has no effect on the reverse-path buffer, the forward- 1854 path buffer, or the mail data buffer, and it may be issued at any 1855 time. 1857 SMTP servers SHOULD support HELP without arguments and MAY support it 1858 with arguments. 1860 Syntax: 1862 help = "HELP" [ SP String ] CRLF 1864 4.1.1.9. NOOP (NOOP) 1866 This command does not affect any parameters or previously entered 1867 commands. It specifies no action other than that the receiver send a 1868 "250 OK" reply. 1870 This command has no effect on the reverse-path buffer, the forward- 1871 path buffer, or the mail data buffer, and it may be issued at any 1872 time. If a parameter string is specified, servers SHOULD ignore it. 1874 Syntax: 1876 noop = "NOOP" [ SP String ] CRLF 1878 4.1.1.10. QUIT (QUIT) 1880 This command specifies that the receiver MUST send a "221 OK" reply, 1881 and then close the transmission channel. 1883 The receiver MUST NOT intentionally close the transmission channel 1884 until it receives and replies to a QUIT command (even if there was an 1885 error). The sender MUST NOT intentionally close the transmission 1886 channel until it sends a QUIT command, and it SHOULD wait until it 1887 receives the reply (even if there was an error response to a previous 1888 command). If the connection is closed prematurely due to violations 1889 of the above or system or network failure, the server MUST cancel any 1890 pending transaction, but not undo any previously completed 1891 transaction, and generally MUST act as if the command or transaction 1892 in progress had received a temporary error (i.e., a 4yz response). 1894 The QUIT command may be issued at any time. Any current uncompleted 1895 mail transaction will be aborted. 1897 Syntax: 1899 quit = "QUIT" CRLF 1901 4.1.1.11. Mail-Parameter and Rcpt-Parameter Error Responses 1903 If the server SMTP does not recognize or cannot implement one or more 1904 of the parameters associated with a particular MAIL FROM or RCPT TO 1905 command, it will return code 555. 1907 If, for some reason, the server is temporarily unable to accommodate 1908 one or more of the parameters associated with a MAIL FROM or RCPT TO 1909 command, and if the definition of the specific parameter does not 1910 mandate the use of another code, it should return code 455. 1912 Errors specific to particular parameters and their values will be 1913 specified in the parameter's defining RFC. 1915 4.1.2. Command Argument Syntax 1917 The syntax of the argument clauses of the above commands (using the 1918 syntax specified in RFC 5234 [5] where applicable) is given below. 1919 Some of the productions given below are used only in conjunction with 1920 source routes as described in Appendix C. Terminals not defined in 1921 this document, such as ALPHA, DIGIT, SP, CR, LF, CRLF, are as defined 1922 in the "core" syntax in Appendix B of RFC 5234 [5] or in the message 1923 format syntax in RFC 5322 [11]. 1925 Reverse-path = Path / "<>" 1927 Forward-path = Path 1929 Path = "<" [ A-d-l ":" ] Mailbox ">" 1931 A-d-l = At-domain *( "," At-domain ) 1932 ; Note that this form, the so-called "source 1933 ; route", MUST BE accepted, SHOULD NOT be 1934 ; generated, and SHOULD be ignored. 1936 At-domain = "@" Domain 1938 Mail-parameters = esmtp-param *(SP esmtp-param) 1940 Rcpt-parameters = esmtp-param *(SP esmtp-param) 1942 esmtp-param = esmtp-keyword ["=" esmtp-value] 1944 esmtp-keyword = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-") 1946 esmtp-value = 1*(%d33-60 / %d62-126) 1947 ; any CHAR excluding "=", SP, and control 1948 ; characters. If this string is an email address, 1949 ; i.e., a Mailbox, then the "xtext" syntax [12] 1950 ; SHOULD be used. 1952 Keyword = Ldh-str 1954 Argument = Atom 1956 Domain = sub-domain *("." sub-domain) 1958 sub-domain = Let-dig [Ldh-str] 1960 Let-dig = ALPHA / DIGIT 1962 Ldh-str = *( ALPHA / DIGIT / "-" ) Let-dig 1964 address-literal = "[" ( IPv4-address-literal / 1965 IPv6-address-literal / 1966 General-address-literal ) "]" 1967 ; See Section 4.1.3 1969 Mailbox = Local-part "@" ( Domain / address-literal ) 1971 Local-part = Dot-string / Quoted-string 1972 ; MAY be case-sensitive 1974 Dot-string = Atom *("." Atom) 1976 Atom = 1*atext 1978 Quoted-string = DQUOTE 1*QcontentSMTP DQUOTE 1980 QcontentSMTP = qtextSMTP / quoted-pairSMTP 1982 quoted-pairSMTP = %d92 %d32-126 1983 ; i.e., backslash followed by any ASCII 1984 ; graphic (including itself) or SPace 1986 qtextSMTP = %d32-33 / %d35-91 / %d93-126 1987 ; i.e., within a quoted string, any 1988 ; ASCII graphic or space is permitted 1989 ; without blackslash-quoting except 1990 ; double-quote and the backslash itself. 1992 String = Atom / Quoted-string 1994 While the above definition for Local-part is relatively permissive, 1995 for maximum interoperability, a host that expects to receive mail 1996 SHOULD avoid defining mailboxes where the Local-part requires (or 1997 uses) the Quoted-string form or where the Local-part is case- 1998 sensitive. For any purposes that require generating or comparing 1999 Local-parts (e.g., to specific mailbox names), all quoted forms MUST 2000 be treated as equivalent, and the sending system SHOULD transmit the 2001 form that uses the minimum quoting possible. 2003 Systems MUST NOT define mailboxes in such a way as to require the use 2004 in SMTP of non-ASCII characters (octets with the high order bit set 2005 to one) or ASCII "control characters" (decimal value 0-31 and 127). 2006 These characters MUST NOT be used in MAIL or RCPT commands or other 2007 commands that require mailbox names. 2009 Note that the backslash, "\", is a quote character, which is used to 2010 indicate that the next character is to be used literally (instead of 2011 its normal interpretation). For example, "Joe\,Smith" indicates a 2012 single nine-character user name string with the comma being the 2013 fourth character of that string. 2015 To promote interoperability and consistent with long-standing 2016 guidance about conservative use of the DNS in naming and applications 2017 (e.g., see Section 2.3.1 of the base DNS document, RFC 1035 [7]), 2018 characters outside the set of alphabetic characters, digits, and 2019 hyphen MUST NOT appear in domain name labels for SMTP clients or 2020 servers. In particular, the underscore character is not permitted. 2021 SMTP servers that receive a command in which invalid character codes 2022 have been employed, and for which there are no other reasons for 2023 rejection, MUST reject that command with a 501 response (this rule, 2024 like others, could be overridden by appropriate SMTP extensions). 2026 4.1.3. Address Literals 2028 Sometimes a host is not known to the domain name system and 2029 communication (and, in particular, communication to report and repair 2030 the error) is blocked. To bypass this barrier, a special literal 2031 form of the address is allowed as an alternative to a domain name. 2032 For IPv4 addresses, this form uses four small decimal integers 2033 separated by dots and enclosed by brackets such as [123.255.37.2], 2034 which indicates an (IPv4) Internet Address in sequence-of-octets 2035 form. For IPv6 and other forms of addressing that might eventually 2036 be standardized, the form consists of a standardized "tag" that 2037 identifies the address syntax, a colon, and the address itself, in a 2038 format specified as part of the relevant standards (i.e., RFC 4291 2039 [6] for IPv6). 2040 [[CREF10: [5321bis] Proposed erratum 4315 (2015-03-27) suggests yet 2041 another modification to the IPv6 address literal syntax, based on 2042 part on RFC 5952. We should consider whether those, or other, 2043 modifications are appropriate and/or whether, given both the issues 2044 of spam/malware and servers supporting multiple domains, it it time 2045 to deprecate mailboxes containing address literals entirely (EHLO 2046 fields may be a different issue). If we are going to allow IPv6 2047 address literals, it may be time to incorporate something by 2048 reference rather than including specific syntax here (RFC 5952 is 14 2049 pages long and does not contain any ABNF).]] 2051 Specifically: 2053 IPv4-address-literal = Snum 3("." Snum) 2055 IPv6-address-literal = "IPv6:" IPv6-addr 2057 General-address-literal = Standardized-tag ":" 1*dcontent 2059 Standardized-tag = Ldh-str 2060 ; Standardized-tag MUST be specified in a 2061 ; Standards-Track RFC and registered with IANA 2063 dcontent = %d33-90 / ; Printable US-ASCII 2064 %d94-126 ; excl. "[", "\", "]" 2066 Snum = 1*3DIGIT 2067 ; representing a decimal integer 2068 ; value in the range 0 through 255 2070 IPv6-addr = 6( h16 ":" ) ls32 2071 / "::" 5( h16 ":" ) ls32 2072 / [ h16 ] "::" 4( h16 ":" ) ls32 2073 / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32 2074 / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32 2075 / [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32 2076 / [ *4( h16 ":" ) h16 ] "::" ls32 2077 / [ *5( h16 ":" ) h16 ] "::" h16 2078 / [ *6( h16 ":" ) h16 ] "::" 2079 ; This definition is consistent with the one for 2080 ; URIs [47]. 2082 ls32 = ( h16 ":" h16 ) / IPv4address 2083 ; least-significant 32 bits of address 2085 h16 = 1*4HEXDIG 2086 ; 16 bits of address represented in hexadecimal 2087 [[CREF11: [5321bis](2821ter) 2821bis Last Call 2088 comment]] 2090 4.1.4. Order of Commands 2092 There are restrictions on the order in which these commands may be 2093 used. 2095 A session that will contain mail transactions MUST first be 2096 initialized by the use of the EHLO command. An SMTP server SHOULD 2097 accept commands for non-mail transactions (e.g., VRFY or EXPN) 2098 without this initialization. 2100 An EHLO command MAY be issued by a client later in the session. If 2101 it is issued after the session begins and the EHLO command is 2102 acceptable to the SMTP server, the SMTP server MUST clear all buffers 2103 and reset the state exactly as if a RSET command had been issued. In 2104 other words, the sequence of RSET followed immediately by EHLO is 2105 redundant, but not harmful other than in the performance cost of 2106 executing unnecessary commands. 2108 If the EHLO command is not acceptable to the SMTP server, 501, 500, 2109 502, or 550 failure replies MUST be returned as appropriate. The 2110 SMTP server MUST stay in the same state after transmitting these 2111 replies that it was in before the EHLO was received. 2113 The SMTP client MUST, if possible, ensure that the domain parameter 2114 to the EHLO command is a primary host name as specified for this 2115 command in Section 2.3.5. If this is not possible (e.g., when the 2116 client's address is dynamically assigned and the client does not have 2117 an obvious name), an address literal SHOULD be substituted for the 2118 domain name. 2120 An SMTP server MAY verify that the domain name argument in the EHLO 2121 command actually corresponds to the IP address of the client. 2122 [[CREF12: [5321bis] [[Note in draft -- proposed change to "An SMTP 2123 server MAY verify that the domain name argument in the EHLO command 2124 has an address record matching the IP address of the client." --David 2125 MacQuigg, david_macquigg@yahoo.com, Friday, 20090130 0637 -0700]]]] 2126 However, if the verification fails, the server MUST NOT refuse to 2127 accept a message on that basis. Information captured in the 2128 verification attempt is for logging and tracing purposes. Note that 2129 this prohibition applies to the matching of the parameter to its IP 2130 address only; see Section 7.9 for a more extensive discussion of 2131 rejecting incoming connections or mail messages. 2133 The NOOP, HELP, EXPN, VRFY, and RSET commands can be used at any time 2134 during a session, or without previously initializing a session. SMTP 2135 servers SHOULD process these normally (that is, not return a 503 2136 code) even if no EHLO command has yet been received; clients SHOULD 2137 open a session with EHLO before sending these commands. 2139 If these rules are followed, the example in RFC 821 that shows "550 2140 access denied to you" in response to an EXPN command is incorrect 2141 unless an EHLO command precedes the EXPN or the denial of access is 2142 based on the client's IP address or other authentication or 2143 authorization-determining mechanisms. 2145 The MAIL command (or the obsolete SEND, SOML, or SAML commands) 2146 [[5321bis Editor's Note: is there any reason to not clean those 2147 commands out of this entirely, replacing them with, e.g., a strong 2148 reference to Appendix F.6]] 2149 begins a mail transaction. Once started, a mail transaction consists 2150 of a transaction beginning command, one or more RCPT commands, and a 2151 DATA command, in that order. A mail transaction may be aborted by 2152 the RSET, a new EHLO, or the QUIT command. There may be zero or more 2153 transactions in a session. MAIL (or SEND, SOML, or SAML) MUST NOT be 2154 sent if a mail transaction is already open, i.e., it should be sent 2155 only if no mail transaction had been started in the session, or if 2156 the previous one successfully concluded with a successful DATA 2157 command, or if the previous one was aborted, e.g., with a RSET or new 2158 EHLO. [[CREF13: [5321bis] 2821ter note: see comment about changing 2159 this convoluted discussion to talk about 'mail transaction' above. 2160 --Jck]] 2162 If the transaction beginning command argument is not acceptable, a 2163 501 failure reply MUST be returned and the SMTP server MUST stay in 2164 the same state. If the commands in a transaction are out of order to 2165 the degree that they cannot be processed by the server, a 503 failure 2166 reply MUST be returned and the SMTP server MUST stay in the same 2167 state. 2169 The last command in a session MUST be the QUIT command. The QUIT 2170 command SHOULD be used by the client SMTP to request connection 2171 closure, even when no session opening command was sent and accepted. 2173 4.1.5. Private-Use Commands 2175 As specified in Section 2.2.2, commands starting in "X" may be used 2176 by bilateral agreement between the client (sending) and server 2177 (receiving) SMTP agents. An SMTP server that does not recognize such 2178 a command is expected to reply with "500 Command not recognized". An 2179 extended SMTP server MAY list the feature names associated with these 2180 private commands in the response to the EHLO command. 2182 Commands sent or accepted by SMTP systems that do not start with "X" 2183 MUST conform to the requirements of Section 2.2.2. 2185 4.2. SMTP Replies 2187 Replies to SMTP commands serve to ensure the synchronization of 2188 requests and actions in the process of mail transfer and to guarantee 2189 that the SMTP client always knows the state of the SMTP server. 2190 Every command MUST generate exactly one reply. 2192 The details of the command-reply sequence are described in 2193 Section 4.3. 2195 An SMTP reply consists of a three digit number (transmitted as three 2196 numeric characters) followed by some text unless specified otherwise 2197 in this document. The number is for use by automata to determine 2198 what state to enter next; the text is for the human user. The three 2199 digits contain enough encoded information that the SMTP client need 2200 not examine the text and may either discard it or pass it on to the 2201 user, as appropriate. Exceptions are as noted elsewhere in this 2202 document. In particular, the 220, 221, 251, 421, and 551 reply codes 2203 are associated with message text that must be parsed and interpreted 2204 by machines. In the general case, the text may be receiver dependent 2205 and context dependent, so there are likely to be varying texts for 2206 each reply code. A discussion of the theory of reply codes is given 2207 in Section 4.2.1. Formally, a reply is defined to be the sequence: a 2208 three-digit code, , one line of text, and , or a multiline 2209 reply (as defined in the same section). Since, in violation of this 2210 specification, the text is sometimes not sent, clients that do not 2211 receive it SHOULD be prepared to process the code alone (with or 2212 without a trailing space character). Only the EHLO, EXPN, and HELP 2213 commands are expected to result in multiline replies in normal 2214 circumstances; however, multiline replies are allowed for any 2215 command. 2217 In ABNF, server responses are: 2219 Greeting = ( "220 " (Domain / address-literal) 2220 [ SP textstring ] CRLF ) / 2221 ( "220-" (Domain / address-literal) 2222 [ SP textstring ] CRLF 2223 *( "220-" [ textstring ] CRLF ) 2224 "220" [ SP textstring ] CRLF ) 2226 textstring = 1*(%d09 / %d32-126) ; HT, SP, Printable US-ASCII 2228 Reply-line = *( Reply-code "-" [ textstring ] CRLF ) 2229 Reply-code [ SP textstring ] CRLF 2231 Reply-code = %x32-35 %x30-35 %x30-39 2232 where "Greeting" appears only in the 220 response that announces that 2233 the server is opening its part of the connection. (Other possible 2234 server responses upon connection follow the syntax of Reply-line.) 2236 An SMTP server SHOULD send only the reply codes listed in this 2237 document or additions to the list as discussed below. 2238 [[CREF14: [5321bis] 20140804: New text to clear up ambiguity.]] 2239 An SMTP server SHOULD use the text shown in the examples whenever 2240 appropriate. 2242 An SMTP client MUST determine its actions only by the reply code, not 2243 by the text (except for the "change of address" 251 and 551 and, if 2244 necessary, 220, 221, and 421 replies); in the general case, any text, 2245 including no text at all (although senders SHOULD NOT send bare 2246 codes), MUST be acceptable. The space (blank) following the reply 2247 code is considered part of the text. Whenever possible, a sender- 2248 SMTP SHOULD test the first digit (severity indication) of a reply 2249 code it receives. 2250 [[CREF15: [5321bis] 20141209 [[Note in Draft: What is that sentence 2251 supposed to be tell us? Test the first digit and examine the others 2252 only if necessary? Note the interaction between this and various 2253 flaps about adding new codes.]]]] 2255 The list of codes that appears below MUST NOT be construed as 2256 permanent. While the addition of new codes should be a rare and 2257 significant activity, with supplemental information in the textual 2258 part of the response (including enhanced status codes [38] and the 2259 successors to that specification) 2260 [[CREF16: [5321bis] 20140802: New text for clarity]] 2261 being preferred, new codes may be added as the result of new 2262 Standards or Standards-Track specifications. Consequently, a sender- 2263 SMTP MUST be prepared to handle codes not specified in this document 2264 and MUST do so by interpreting the first digit only. 2266 In the absence of extensions negotiated with the client, SMTP servers 2267 MUST NOT send reply codes whose first digits are other than 2, 3, 4, 2268 or 5. Clients that receive such out-of-range codes SHOULD normally 2269 treat them as fatal errors and terminate the mail transaction. 2271 4.2.1. Reply Code Severities and Theory 2273 The three digits of the reply each have a special significance. The 2274 first digit denotes whether the response is good, bad, or incomplete. 2275 An unsophisticated SMTP client, or one that receives an unexpected 2276 code, will be able to determine its next action (proceed as planned, 2277 redo, retrench, etc.) by examining this first digit. An SMTP client 2278 that wants to know approximately what kind of error occurred (e.g., 2279 mail system error, command syntax error) may examine the second 2280 digit. The third digit and any supplemental information that may be 2281 present is reserved for the finest gradation of information. 2283 There are four values for the first digit of the reply code: 2285 2yz Positive Completion reply 2286 The requested action has been successfully completed. A new 2287 request may be initiated. 2289 3yz Positive Intermediate reply 2290 The command has been accepted, but the requested action is being 2291 held in abeyance, pending receipt of further information. The 2292 SMTP client should send another command specifying this 2293 information. This reply is used in command sequence groups (i.e., 2294 in DATA). 2296 4yz Transient Negative Completion reply 2297 The command was not accepted, and the requested action did not 2298 occur. However, the error condition is temporary, and the action 2299 may be requested again. The sender should return to the beginning 2300 of the command sequence (if any). It is difficult to assign a 2301 meaning to "transient" when two different sites (receiver- and 2302 sender-SMTP agents) must agree on the interpretation. Each reply 2303 in this category might have a different time value, but the SMTP 2304 client SHOULD try again. A rule of thumb to determine whether a 2305 reply fits into the 4yz or the 5yz category (see below) is that 2306 replies are 4yz if they can be successful if repeated without any 2307 change in command form or in properties of the sender or receiver 2308 (that is, the command is repeated identically and the receiver 2309 does not put up a new implementation). 2311 5yz Permanent Negative Completion reply 2312 The command was not accepted and the requested action did not 2313 occur. The SMTP client SHOULD NOT repeat the exact request (in 2314 the same sequence). Even some "permanent" error conditions can be 2315 corrected, so the human user may want to direct the SMTP client to 2316 reinitiate the command sequence by direct action at some point in 2317 the future (e.g., after the spelling has been changed, or the user 2318 has altered the account status). 2320 It is worth noting that the file transfer protocol (FTP) [18] uses a 2321 very similar code architecture and that the SMTP codes are based on 2322 the FTP model. However, SMTP uses a one-command, one-response model 2323 (while FTP is asynchronous) and FTP's 1yz codes are not part of the 2324 SMTP model. 2326 The second digit encodes responses in specific categories: 2328 x0z Syntax: These replies refer to syntax errors, syntactically 2329 correct commands that do not fit any functional category, and 2330 unimplemented or superfluous commands. 2332 x1z Information: These are replies to requests for information, such 2333 as status or help. 2335 x2z Connections: These are replies referring to the transmission 2336 channel. 2338 x3z Unspecified. 2340 x4z Unspecified. 2342 x5z Mail system: These replies indicate the status of the receiver 2343 mail system vis-a-vis the requested transfer or other mail system 2344 action. 2346 The third digit gives a finer gradation of meaning in each category 2347 specified by the second digit. The list of replies illustrates this. 2348 Each reply text is recommended rather than mandatory, and may even 2349 change according to the command with which it is associated. On the 2350 other hand, the reply codes must strictly follow the specifications 2351 in this section. Receiver implementations should not invent new 2352 codes for slightly different situations from the ones described here, 2353 but rather adapt codes already defined. 2355 For example, a command such as NOOP, whose successful execution does 2356 not offer the SMTP client any new information, will return a 250 2357 reply. The reply is 502 when the command requests an unimplemented 2358 non-site-specific action. A refinement of that is the 504 reply for 2359 a command that is implemented, but that requests an unimplemented 2360 parameter. 2362 The reply text may be longer than a single line; in these cases the 2363 complete text must be marked so the SMTP client knows when it can 2364 stop reading the reply. This requires a special format to indicate a 2365 multiple line reply. 2367 The format for multiline replies requires that every line, except the 2368 last, begin with the reply code, followed immediately by a hyphen, 2369 "-" (also known as minus), followed by text. The last line will 2370 begin with the reply code, followed immediately by , optionally 2371 some text, and . As noted above, servers SHOULD send the 2372 if subsequent text is not sent, but clients MUST be prepared for it 2373 to be omitted. 2375 For example: 2377 250-First line 2378 250-Second line 2379 250-234 Text beginning with numbers 2380 250 The last line 2382 In a multiline reply, the reply code on each of the lines MUST be the 2383 same. It is reasonable for the client to rely on this, so it can 2384 make processing decisions based on the code in any line, assuming 2385 that all others will be the same. In a few cases, there is important 2386 data for the client in the reply "text". The client will be able to 2387 identify these cases from the current context. 2389 4.2.2. Reply Codes by Function Groups 2391 500 Syntax error, command unrecognized (This may include errors such 2392 as command line too long) 2394 501 Syntax error in parameters or arguments 2396 502 Command not implemented (see Section 4.2.4.1) 2398 503 Bad sequence of commands 2400 504 Command parameter not implemented 2402 211 System status, or system help reply 2404 214 Help message (Information on how to use the receiver or the 2405 meaning of a particular non-standard command; this reply is useful 2406 only to the human user) 2408 220 Service ready 2410 221 Service closing transmission channel 2412 421 Service not available, closing transmission channel 2413 (This may be a reply to any command if the service knows it must 2414 shut down) 2416 hangText="521"> No mail service here. [[CREF17: 2417 [5321bis]20140804: Specific code introduced with RFC 1846, updated 2418 and specified in draft-klensin-smtp-521code.]] 2420 556 No mail service at this domain. [[CREF18: [5321bis] 20140912: 2421 Specific code introduced in draft-klensin-smtp-521code-02, largely 2422 for nullMX]] 2424 250 Requested mail action okay, completed 2426 251 User not local; will forward to (See Section 3.4) 2428 252 Cannot VRFY user, but will accept message and attempt delivery 2429 (See Section 3.5.3) 2431 455 Server unable to accommodate parameters 2433 555 MAIL FROM/RCPT TO parameters not recognized or not implemented 2435 450 Requested mail action not taken: mailbox unavailable (e.g., 2436 mailbox busy or temporarily blocked for policy reasons) 2438 550 Requested action not taken: mailbox unavailable (e.g., mailbox 2439 not found, no access, or command rejected for policy reasons) 2441 451 Requested action aborted: error in processing 2443 551 User not local; please try (See Section 3.4) 2445 452 Requested action not taken: insufficient system storage 2447 552 Requested mail action aborted: exceeded storage allocation 2449 553 Requested action not taken: mailbox name not allowed (e.g., 2450 mailbox syntax incorrect) 2452 354 Start mail input; end with . 2454 554 Transaction failed (Or, in the case of a connection-opening 2455 response, "No SMTP service here") 2456 [[CREF19: [5321bis] [[Note in Draft: Revise above statement in the 2457 light of new 521 code??]] ]] 2459 4.2.3. Reply Codes in Numeric Order 2461 211 System status, or system help reply 2463 214 Help message (Information on how to use the receiver or the 2464 meaning of a particular non-standard command; this reply is useful 2465 only to the human user) 2467 220 Service ready 2469 221 Service closing transmission channel 2471 250 Requested mail action okay, completed 2472 251 User not local; will forward to (See Section 3.4) 2474 252 Cannot VRFY user, but will accept message and attempt delivery 2475 (See Section 3.5.3) 2477 354 Start mail input; end with . 2479 421 Service not available, closing transmission channel 2480 (This may be a reply to any command if the service knows it must 2481 shut down) 2483 450 Requested mail action not taken: mailbox unavailable (e.g., 2484 mailbox busy or temporarily blocked for policy reasons) 2486 451 Requested action aborted: local error in processing 2488 452 Requested action not taken: insufficient system storage 2490 455 Server unable to accommodate parameters 2492 500 Syntax error, command unrecognized (This may include errors such 2493 as command line too long) 2495 501 Syntax error in parameters or arguments 2497 502 Command not implemented (see Section 4.2.4.1) 2499 503 Bad sequence of commands 2501 504 Command parameter not implemented 2503 521 No mail service 2505 550 Requested action not taken: mailbox unavailable (e.g., mailbox 2506 not found, no access, or command rejected for policy reasons) 2508 551 User not local; please try (See Section 3.4) 2510 552 Requested mail action aborted: exceeded storage allocation 2512 553 Requested action not taken: mailbox name not allowed (e.g., 2513 mailbox syntax incorrect) 2515 554 Transaction failed (Or, in the case of a connection-opening 2516 response, "No SMTP service here") 2518 555 MAIL FROM/RCPT TO parameters not recognized or not implemented 2520 556 No mail service at this domain. 2522 4.2.4. Some specific code situations and relationships 2524 4.2.4.1. Reply Code 502 2526 Questions have been raised as to when reply code 502 (Command not 2527 implemented) SHOULD be returned in preference to other codes. 502 2528 SHOULD be used when the command is actually recognized by the SMTP 2529 server, but not implemented. If the command is not recognized, code 2530 500 SHOULD be returned. Extended SMTP systems MUST NOT list 2531 capabilities in response to EHLO for which they will return 502 (or 2532 500) replies. 2534 4.2.4.2. "No mail accepted" situations and the 521, 554, and 556 codes 2536 [[CREF20: [5321bis] This section is new with 5321bis. ]] 2538 Codes 521, 554, and 556 are all used to report different types of "no 2539 mail accepted" situations. They differ as follows. 521 is an 2540 indication from a system answering on the SMTP port that it does not 2541 support SMTP service (a so-called "dummy server" as discussed in RFC 2542 1846 [24] and elsewhere). Obviously, it requires that system exist 2543 and that a connection can be made successfully to it. Because a 2544 system that does not accept any mail cannot meaningfully accept a 2545 RCPT command, any commands (other than QUIT) issued after an SMTP 2546 server has issued a 521 reply are client (sender) errors. 556 is 2547 used by a message submission or intermediate SMTP system (see 2548 Section 1.1) to report that it cannot forward the message further 2549 because it knows (e.g., from a DNS entry [51]) that the recipient 2550 domain does not accept mail. It would normally be used in response 2551 to a RCPT or similar (extension) command when the SMTP system 2552 identifies a domain that it can (or has) determined never accepts 2553 mail. Other codes, including 554 and the temporary 450, are used for 2554 more transient situations and situations in which an SMTP server 2555 cannot or will not deliver to (or accept mail for) a particular 2556 system or mailbox for policy reasons rather than ones directly 2557 related to SMTP processing. 2559 4.2.4.3. Reply Codes after DATA and the Subsequent . 2561 When an SMTP server returns a positive completion status (2yz code) 2562 after the DATA command is completed with ., it accepts 2563 responsibility for: 2565 o delivering the message (if the recipient mailbox exists), or 2567 o if attempts to deliver the message fail due to transient 2568 conditions, retrying delivery some reasonable number of times at 2569 intervals as specified in Section 4.5.4. 2571 o if attempts to deliver the message fail due to permanent 2572 conditions, or if repeated attempts to deliver the message fail 2573 due to transient conditions, returning appropriate notification to 2574 the sender of the original message (using the address in the SMTP 2575 MAIL command). 2577 When an SMTP server returns a temporary error status (4yz) code after 2578 the DATA command is completed with ., it MUST NOT make a 2579 subsequent attempt to deliver that message. The SMTP client retains 2580 responsibility for the delivery of that message and may either return 2581 it to the user or requeue it for a subsequent attempt (see 2582 Section 4.5.4.1). 2584 The user who originated the message SHOULD be able to interpret the 2585 return of a transient failure status (by mail message or otherwise) 2586 as a non-delivery indication, just as a permanent failure would be 2587 interpreted. If the client SMTP successfully handles these 2588 conditions, the user will not receive such a reply. 2590 When an SMTP server returns a permanent error status (5yz) code after 2591 the DATA command is completed with ., it MUST NOT make 2592 any subsequent attempt to deliver the message. As with temporary 2593 error status codes, the SMTP client retains responsibility for the 2594 message, but SHOULD not again attempt delivery to the same server 2595 without user review of the message and response and appropriate 2596 intervention. 2598 4.3. Sequencing of Commands and Replies 2600 4.3.1. Sequencing Overview 2602 The communication between the sender and receiver is an alternating 2603 dialogue, controlled by the sender. As such, the sender issues a 2604 command and the receiver responds with a reply. Unless other 2605 arrangements are negotiated through service extensions, the sender 2606 MUST wait for this response before sending further commands. One 2607 important reply is the connection greeting. Normally, a receiver 2608 will send a 220 "Service ready" reply when the connection is 2609 completed. The sender SHOULD wait for this greeting message before 2610 sending any commands. 2612 Note: all the greeting-type replies have the official name (the 2613 fully-qualified primary domain name) of the server host as the first 2614 word following the reply code. Sometimes the host will have no 2615 meaningful name. See Section 4.1.3 for a discussion of alternatives 2616 in these situations. 2618 For example, 2620 220 ISIF.USC.EDU Service ready 2622 or 2624 220 mail.example.com SuperSMTP v 6.1.2 Service ready 2626 or 2628 220 [10.0.0.1] Clueless host service ready 2630 The table below lists alternative success and failure replies for 2631 each command. These SHOULD be strictly adhered to. A receiver MAY 2632 substitute text in the replies, but the meanings and actions implied 2633 by the code numbers and by the specific command reply sequence MUST 2634 be preserved. However, in order to provide robustness as SMTP is 2635 extended and evolves, the discussion in Section 4.2.1 still applies: 2636 all SMTP clients MUST be prepared to accept any code that conforms to 2637 the discussion in that section and MUST be prepared to interpret it 2638 on the basis of its first digit only. [[CREF21: [5321bis] 20140914: 2639 Above sentence is new text based on yet another round of discussions 2640 about "invalid codes".]] 2642 4.3.2. Command-Reply Sequences 2644 Each command is listed with its usual possible replies. The prefixes 2645 used before the possible replies are "I" for intermediate, "S" for 2646 success, and "E" for error. Since some servers may generate other 2647 replies under special circumstances, and to allow for future 2648 extension, SMTP clients SHOULD, when possible, interpret only the 2649 first digit of the reply and MUST be prepared to deal with 2650 unrecognized reply codes by interpreting the first digit only. 2651 Unless extended using the mechanisms described in Section 2.2, SMTP 2652 servers MUST NOT transmit reply codes to an SMTP client that are 2653 other than three digits or that do not start in a digit between 2 and 2654 5 inclusive. 2656 These sequencing rules and, in principle, the codes themselves, can 2657 be extended or modified by SMTP extensions offered by the server and 2658 accepted (requested) by the client. However, if the target is more 2659 precise granularity in the codes, rather than codes for completely 2660 new purposes, the system described in RFC 3463 [38] SHOULD be used in 2661 preference to the invention of new codes. 2663 In addition to the codes listed below, any SMTP command can return 2664 any of the following codes if the corresponding unusual circumstances 2665 are encountered: 2667 500 For the "command line too long" case or if the command name was 2668 not recognized. Note that producing a "command not recognized" 2669 error in response to the required subset of these commands is a 2670 violation of this specification. Similarly, producing a "command 2671 too long" message for a command line shorter than 512 characters 2672 would violate the provisions of Section 4.5.3.1.4. 2674 501 Syntax error in command or arguments. In order to provide for 2675 future extensions, commands that are specified in this document as 2676 not accepting arguments (DATA, RSET, QUIT) SHOULD return a 501 2677 message if arguments are supplied in the absence of EHLO- 2678 advertised extensions. 2680 421 Service shutting down and closing transmission channel 2682 Specific sequences are: 2684 CONNECTION ESTABLISHMENT 2686 S: 220 2687 E: 521, 554 2689 EHLO or HELO 2691 S: 250 2692 E: 504 (a conforming implementation could return this code only 2693 in fairly obscure cases), 550, 502 (permitted only with an old- 2694 style server that does not support EHLO) 2696 MAIL 2698 S: 250 2699 E: 552, 451, 452, 550, 553, 503, 455, 555 2701 RCPT 2703 S: 250, 251 (but see Section 3.4 for discussion of 251 and 551) 2704 E: 550, 551, 552, 553, 450, 451, 452, 503, 455, 555 2706 DATA 2708 I: 354 -> data -> S: 250 2710 E: 552, 554, 451, 452 2712 E: 450, 550 (rejections for policy reasons) 2714 E: 503, 554 2716 RSET 2718 S: 250 2720 VRFY 2722 S: 250, 251, 252 2723 E: 550, 551, 553, 502, 504 2725 EXPN 2727 S: 250, 252 2728 E: 550, 500, 502, 504 2730 HELP 2732 S: 211, 214 2733 E: 502, 504 2735 NOOP 2737 S: 250 2739 QUIT 2741 S: 221 2743 4.4. Trace Information 2745 When an SMTP server receives a message for delivery or further 2746 processing, it MUST insert trace (often referred to as "time stamp" 2747 or "Received" information) [[CREF22: [5321bis] See note on 2748 rfc5321bis-00c above]] information at the beginning of the message 2749 content, as discussed in Section 4.1.1.4. 2751 This line MUST be structured as follows: 2753 o The FROM clause, which MUST be supplied in an SMTP environment, 2754 SHOULD contain both (1) the name of the source host as presented 2755 in the EHLO command and (2) an address literal containing the IP 2756 address of the source, determined from the TCP connection. 2758 o The ID clause MAY contain an "@" as suggested in RFC 822, but this 2759 is not required. 2761 o If the FOR clause appears, it MUST contain exactly one 2762 entry, even when multiple RCPT commands have been given. Multiple 2763 s raise some security issues and have been deprecated, see 2764 Section 7.2. 2766 An Internet mail program MUST NOT change or delete a Received: line 2767 that was previously added to the message header section. SMTP 2768 servers MUST prepend Received lines to messages; they MUST NOT change 2769 the order of existing lines or insert Received lines in any other 2770 location. 2772 As the Internet grows, comparability of Received header fields is 2773 important for detecting problems, especially slow relays. SMTP 2774 servers that create Received header fields SHOULD use explicit 2775 offsets in the dates (e.g., -0800), rather than time zone names of 2776 any type. Local time (with an offset) SHOULD be used rather than UT 2777 when feasible. This formulation allows slightly more information 2778 about local circumstances to be specified. If UT is needed, the 2779 receiver need merely do some simple arithmetic to convert the values. 2780 Use of UT loses information about the time zone-location of the 2781 server. If it is desired to supply a time zone name, it SHOULD be 2782 included in a comment. 2784 When the delivery SMTP server makes the "final delivery" of a 2785 message, it inserts a return-path line at the beginning of the mail 2786 data. This use of return-path is required; mail systems MUST support 2787 it. The return-path line preserves the information in the from the MAIL command. Here, final delivery means the message 2789 has left the SMTP environment. Normally, this would mean it had been 2790 delivered to the destination user or an associated mail drop, but in 2791 some cases it may be further processed and transmitted by another 2792 mail system. 2794 It is possible for the mailbox in the return path to be different 2795 from the actual sender's mailbox, for example, if error responses are 2796 to be delivered to a special error handling mailbox rather than to 2797 the message sender. When mailing lists are involved, this 2798 arrangement is common and useful as a means of directing errors to 2799 the list maintainer rather than the message originator. 2801 The text above implies that the final mail data will begin with a 2802 return path line, followed by one or more time stamp lines. These 2803 lines will be followed by the rest of the mail data: first the 2804 balance of the mail header section and then the body (RFC 5322 [11]). 2806 It is sometimes difficult for an SMTP server to determine whether or 2807 not it is making final delivery since forwarding or other operations 2808 may occur after the message is accepted for delivery. Consequently, 2809 any further (forwarding, gateway, or relay) systems MAY remove the 2810 return path and rebuild the MAIL command as needed to ensure that 2811 exactly one such line appears in a delivered message. 2813 A message-originating SMTP system SHOULD NOT send a message that 2814 already contains a Return-path header field. SMTP servers performing 2815 a relay function MUST NOT inspect the message data, and especially 2816 not to the extent needed to determine if Return-path header fields 2817 are present. SMTP servers making final delivery MAY remove Return- 2818 path header fields before adding their own. 2820 The primary purpose of the Return-path is to designate the address to 2821 which messages indicating non-delivery or other mail system failures 2822 are to be sent. For this to be unambiguous, exactly one return path 2823 SHOULD be present when the message is delivered. Systems using RFC 2824 822 syntax with non-SMTP transports SHOULD designate an unambiguous 2825 address, associated with the transport envelope, to which error 2826 reports (e.g., non-delivery messages) should be sent. 2828 Historical note: Text in RFC 822 that appears to contradict the use 2829 of the Return-path header field (or the envelope reverse-path address 2830 from the MAIL command) as the destination for error messages is not 2831 applicable on the Internet. The reverse-path address (as copied into 2832 the Return-path) MUST be used as the target of any mail containing 2833 delivery error messages. 2835 In particular: 2837 o a gateway from SMTP -> elsewhere SHOULD insert a return-path 2838 header field, unless it is known that the "elsewhere" transport 2839 also uses Internet domain addresses and maintains the envelope 2840 sender address separately. 2842 o a gateway from elsewhere -> SMTP SHOULD delete any return-path 2843 header field present in the message, and either copy that 2844 information to the SMTP envelope or combine it with information 2845 present in the envelope of the other transport system to construct 2846 the reverse-path argument to the MAIL command in the SMTP 2847 envelope. 2849 The server must give special treatment to cases in which the 2850 processing following the end of mail data indication is only 2851 partially successful. This could happen if, after accepting several 2852 recipients and the mail data, the SMTP server finds that the mail 2853 data could be successfully delivered to some, but not all, of the 2854 recipients. In such cases, the response to the DATA command MUST be 2855 an OK reply. However, the SMTP server MUST compose and send an 2856 "undeliverable mail" notification message to the originator of the 2857 message. 2859 A single notification listing all of the failed recipients or 2860 separate notification messages MUST be sent for each failed 2861 recipient. For economy of processing by the sender, the former 2862 SHOULD be used when possible. Note that the key difference between 2863 handling aliases (Section 3.9.1) and forwarding (this subsection) is 2864 the change to the backward-pointing address in this case. All 2865 notification messages about undeliverable mail MUST be sent using the 2866 MAIL command (even if they result from processing the obsolete SEND, 2867 SOML, or SAML commands) and MUST use a null return path as discussed 2868 in Section 3.6. 2870 The time stamp line and the return path line are formally defined as 2871 follows (the definitions for "FWS" and "CFWS" appear in RFC 5322 2872 [11]): 2874 Return-path-line = "Return-Path:" FWS Reverse-path 2876 Time-stamp-line = "Received:" FWS Stamp 2878 Stamp = From-domain By-domain Opt-info [CFWS] ";" 2879 FWS date-time 2880 ; where "date-time" is as defined in RFC 5322 [11] 2881 ; but the "obs-" forms, especially two-digit 2882 ; years, are prohibited in SMTP and MUST NOT be used. 2884 From-domain = "FROM" FWS Extended-Domain 2886 By-domain = CFWS "BY" FWS Extended-Domain 2888 Extended-Domain = Domain / 2889 ( Domain FWS "(" TCP-info ")" ) / 2890 ( address-literal FWS "(" TCP-info ")" ) 2892 TCP-info = address-literal / ( Domain FWS address-literal ) 2893 ; Information derived by server from TCP connection 2894 ; not client EHLO. 2896 Opt-info = [Via] [With] [ID] [For] 2897 [Additional-Registered-Clauses] 2899 Via = CFWS "VIA" FWS Link 2901 With = CFWS "WITH" FWS Protocol 2903 ID = CFWS "ID" FWS ( Atom / msg-id ) 2904 ; msg-id is defined in RFC 5322 [11] 2906 For = CFWS "FOR" FWS ( Path / Mailbox ) 2908 Additional-Registered-Clauses = 1* (CFWS Atom FWS String) 2909 [[CREF23: [5321bis] 5321 errata #1683, 20090215, 2910 Roberto Javier Godoy, rjgodoy@fich.unl.edu.ar]] 2911 ; Additional standard clauses may be added in this 2912 ; location by future standards and registration with 2913 ; IANA. SMTP servers SHOULD NOT use unregistered 2914 ; names. See Section 8. 2916 Link = "TCP" / Addtl-Link 2918 Addtl-Link = Atom 2919 ; Additional standard names for links are 2920 ; registered with the Internet Assigned Numbers 2921 ; Authority (IANA). "Via" is primarily of value 2922 ; with non-Internet transports. SMTP servers 2923 ; SHOULD NOT use unregistered names. 2925 Protocol = "ESMTP" / "SMTP" / Attdl-Protocol 2927 Addtl-Protocol = Atom 2928 ; Additional standard names for protocols are 2929 ; registered with the Internet Assigned Numbers 2930 ; Authority (IANA) in the "mail parameters" 2931 ; registry [9]. SMTP servers SHOULD NOT 2932 ; use unregistered names. 2934 4.5. Additional Implementation Issues 2936 4.5.1. Minimum Implementation 2938 In order to make SMTP workable, the following minimum implementation 2939 MUST be provided by all receivers. The following commands MUST be 2940 supported to conform to this specification: 2942 EHLO 2943 HELO 2944 MAIL 2945 RCPT 2946 DATA 2947 RSET 2948 NOOP 2949 QUIT 2950 VRFY 2952 Any system that includes an SMTP server supporting mail relaying or 2953 delivery MUST support the reserved mailbox "postmaster" as a case- 2954 insensitive local name. This postmaster address is not strictly 2955 necessary if the server always returns 554 on connection opening (as 2956 described in Section 3.1). The requirement to accept mail for 2957 postmaster implies that RCPT commands that specify a mailbox for 2958 postmaster at any of the domains for which the SMTP server provides 2959 mail service, as well as the special case of "RCPT TO:" 2960 (with no domain specification), MUST be supported. 2962 SMTP systems are expected to make every reasonable effort to accept 2963 mail directed to Postmaster from any other system on the Internet. 2964 In extreme cases -- such as to contain a denial of service attack or 2965 other breach of security -- an SMTP server may block mail directed to 2966 Postmaster. However, such arrangements SHOULD be narrowly tailored 2967 so as to avoid blocking messages that are not part of such attacks. 2969 4.5.2. Transparency 2971 Without some provision for data transparency, the character sequence 2972 "." ends the mail text and cannot be sent by the user. 2973 In general, users are not aware of such "forbidden" sequences. To 2974 allow all user composed text to be transmitted transparently, the 2975 following procedures are used: 2977 o Before sending a line of mail text, the SMTP client checks the 2978 first character of the line. If it is a period, one additional 2979 period is inserted at the beginning of the line. 2981 o When a line of mail text is received by the SMTP server, it checks 2982 the line. If the line is composed of a single period, it is 2983 treated as the end of mail indicator. If the first character is a 2984 period and there are other characters on the line, the first 2985 character is deleted. 2987 The mail data may contain any of the 128 ASCII characters. All 2988 characters are to be delivered to the recipient's mailbox, including 2989 spaces, vertical and horizontal tabs, and other control characters. 2991 If the transmission channel provides an 8-bit byte (octet) data 2992 stream, the 7-bit ASCII codes are transmitted, right justified, in 2993 the octets, with the high-order bits cleared to zero. See 2994 Section 3.6 for special treatment of these conditions in SMTP systems 2995 serving a relay function. 2997 In some systems, it may be necessary to transform the data as it is 2998 received and stored. This may be necessary for hosts that use a 2999 different character set than ASCII as their local character set, that 3000 store data in records rather than strings, or which use special 3001 character sequences as delimiters inside mailboxes. If such 3002 transformations are necessary, they MUST be reversible, especially if 3003 they are applied to mail being relayed. 3005 4.5.3. Sizes and Timeouts 3007 4.5.3.1. Size Limits and Minimums 3009 There are several objects that have required minimum/maximum sizes. 3010 Every implementation MUST be able to receive objects of at least 3011 these sizes. Objects larger than these sizes SHOULD be avoided when 3012 possible. However, some Internet mail constructs such as encoded 3013 X.400 addresses (RFC 2156 [31]) will often require larger objects. 3014 Clients MAY attempt to transmit these, but MUST be prepared for a 3015 server to reject them if they cannot be handled by it. To the 3016 maximum extent possible, implementation techniques that impose no 3017 limits on the length of these objects should be used. 3019 Extensions to SMTP may involve the use of characters that occupy more 3020 than a single octet each. This section therefore specifies lengths 3021 in octets where absolute lengths, rather than character counts, are 3022 intended. 3024 [[CREF24: [5321bis] [[Note in Draft: Klensin 20191126: Given the 3025 controversy on the SMTP mailing list between 20191123 and now about 3026 maximum lengths, is the above adequate or is further tuning of the 3027 limit text below needed? ]]]] 3029 4.5.3.1.1. Local-part 3031 The maximum total length of a user name or other local-part is 64 3032 octets. 3034 4.5.3.1.2. Domain 3036 The maximum total length of a domain name or number is 255 octets. 3038 4.5.3.1.3. Path 3040 The maximum total length of a reverse-path or forward-path is 256 3041 octets (including the punctuation and element separators). 3043 4.5.3.1.4. Command Line 3045 The maximum total length of a command line including the command word 3046 and the is 512 octets. SMTP extensions may be used to 3047 increase this limit. 3049 4.5.3.1.5. Reply Line 3051 The maximum total length of a reply line including the reply code and 3052 the is 512 octets. More information may be conveyed through 3053 multiple-line replies. 3055 4.5.3.1.6. Text Line 3057 The maximum total length of a text line including the is 1000 3058 octets (not counting the leading dot duplicated for transparency). 3059 This number may be increased by the use of SMTP Service Extensions. 3061 4.5.3.1.7. Message Content 3063 The maximum total length of a message content (including any message 3064 header section as well as the message body) MUST BE at least 64K 3065 octets. Since the introduction of Internet Standards for multimedia 3066 mail (RFC 2045 [29]), message lengths on the Internet have grown 3067 dramatically, and message size restrictions should be avoided if at 3068 all possible. SMTP server systems that must impose restrictions 3069 SHOULD implement the "SIZE" service extension of RFC 1870 [4], and 3070 SMTP client systems that will send large messages SHOULD utilize it 3071 when possible. 3073 4.5.3.1.8. Recipient Buffer 3075 The minimum total number of recipients that MUST be buffered is 100 3076 recipients. Rejection of messages (for excessive recipients) with 3077 fewer than 100 RCPT commands is a violation of this specification. 3078 The general principle that relaying SMTP server MUST NOT, and 3079 delivery SMTP servers SHOULD NOT, perform validation tests on message 3080 header fields suggests that messages SHOULD NOT be rejected based on 3081 the total number of recipients shown in header fields. A server that 3082 imposes a limit on the number of recipients MUST behave in an orderly 3083 fashion, such as rejecting additional addresses over its limit rather 3084 than silently discarding addresses previously accepted. A client 3085 that needs to deliver a message containing over 100 RCPT commands 3086 SHOULD be prepared to transmit in 100-recipient "chunks" if the 3087 server declines to accept more than 100 recipients in a single 3088 message. 3090 4.5.3.1.9. Treatment When Limits Exceeded 3092 Errors due to exceeding these limits may be reported by using the 3093 reply codes. Some examples of reply codes are: 3095 500 Line too long. 3097 or 3099 501 Path too long 3101 or 3103 452 Too many recipients (see below) 3105 or 3107 552 Too much mail data. 3109 4.5.3.1.10. Too Many Recipients Code 3111 RFC 821 [8] incorrectly listed the error where an SMTP server 3112 exhausts its implementation limit on the number of RCPT commands 3113 ("too many recipients") as having reply code 552. The correct reply 3114 code for this condition is 452. Clients SHOULD treat a 552 code in 3115 this case as a temporary, rather than permanent, failure so the logic 3116 below works. 3118 When a conforming SMTP server encounters this condition, it has at 3119 least 100 successful RCPT commands in its recipient buffer. If the 3120 server is able to accept the message, then at least these 100 3121 addresses will be removed from the SMTP client's queue. When the 3122 client attempts retransmission of those addresses that received 452 3123 responses, at least 100 of these will be able to fit in the SMTP 3124 server's recipient buffer. Each retransmission attempt that is able 3125 to deliver anything will be able to dispose of at least 100 of these 3126 recipients. 3128 If an SMTP server has an implementation limit on the number of RCPT 3129 commands and this limit is exhausted, it MUST use a response code of 3130 452 (but the client SHOULD also be prepared for a 552, as noted 3131 above). If the server has a configured site-policy limitation on the 3132 number of RCPT commands, it MAY instead use a 5yz response code. In 3133 particular, if the intent is to prohibit messages with more than a 3134 site-specified number of recipients, rather than merely limit the 3135 number of recipients in a given mail transaction, it would be 3136 reasonable to return a 503 response to any DATA command received 3137 subsequent to the 452 (or 552) code or to simply return the 503 after 3138 DATA without returning any previous negative response. 3140 4.5.3.2. Timeouts 3142 An SMTP client MUST provide a timeout mechanism. It MUST use per- 3143 command timeouts rather than somehow trying to time the entire mail 3144 transaction. Timeouts SHOULD be easily reconfigurable, preferably 3145 without recompiling the SMTP code. To implement this, a timer is set 3146 for each SMTP command and for each buffer of the data transfer. The 3147 latter means that the overall timeout is inherently proportional to 3148 the size of the message. 3150 Based on extensive experience with busy mail-relay hosts, the minimum 3151 per-command timeout values SHOULD be as follows: 3153 4.5.3.2.1. Initial 220 Message: 5 Minutes 3155 An SMTP client process needs to distinguish between a failed TCP 3156 connection and a delay in receiving the initial 220 greeting message. 3157 Many SMTP servers accept a TCP connection but delay delivery of the 3158 220 message until their system load permits more mail to be 3159 processed. 3161 4.5.3.2.2. MAIL Command: 5 Minutes 3163 4.5.3.2.3. RCPT Command: 5 Minutes 3165 A longer timeout is required if processing of mailing lists and 3166 aliases is not deferred until after the message was accepted. 3168 4.5.3.2.4. DATA Initiation: 2 Minutes 3170 This is while awaiting the "354 Start Input" reply to a DATA command. 3172 4.5.3.2.5. Data Block: 3 Minutes 3174 This is while awaiting the completion of each TCP SEND call 3175 transmitting a chunk of data. 3177 4.5.3.2.6. DATA Termination: 10 Minutes. 3179 This is while awaiting the "250 OK" reply. When the receiver gets 3180 the final period terminating the message data, it typically performs 3181 processing to deliver the message to a user mailbox. A spurious 3182 timeout at this point would be very wasteful and would typically 3183 result in delivery of multiple copies of the message, since it has 3184 been successfully sent and the server has accepted responsibility for 3185 delivery. See Section 6.1 for additional discussion. 3187 4.5.3.2.7. Server Timeout: 5 Minutes. 3189 An SMTP server SHOULD have a timeout of at least 5 minutes while it 3190 is awaiting the next command from the sender. 3192 4.5.4. Retry Strategies 3194 The common structure of a host SMTP implementation includes user 3195 mailboxes, one or more areas for queuing messages in transit, and one 3196 or more daemon processes for sending and receiving mail. The exact 3197 structure will vary depending on the needs of the users on the host 3198 and the number and size of mailing lists supported by the host. We 3199 describe several optimizations that have proved helpful, particularly 3200 for mailers supporting high traffic levels. 3202 Any queuing strategy MUST include timeouts on all activities on a 3203 per-command basis. A queuing strategy MUST NOT send error messages 3204 in response to error messages under any circumstances. 3206 4.5.4.1. Sending Strategy 3208 The general model for an SMTP client is one or more processes that 3209 periodically attempt to transmit outgoing mail. In a typical system, 3210 the program that composes a message has some method for requesting 3211 immediate attention for a new piece of outgoing mail, while mail that 3212 cannot be transmitted immediately MUST be queued and periodically 3213 retried by the sender. A mail queue entry will include not only the 3214 message itself but also the envelope information. 3216 The sender MUST delay retrying a particular destination after one 3217 attempt has failed. In general, the retry interval SHOULD be at 3218 least 30 minutes; however, more sophisticated and variable strategies 3219 will be beneficial when the SMTP client can determine the reason for 3220 non-delivery. 3222 Retries continue until the message is transmitted or the sender gives 3223 up; the give-up time generally needs to be at least 4-5 days. It MAY 3224 be appropriate to set a shorter maximum number of retries for non- 3225 delivery notifications and equivalent error messages than for 3226 standard messages. The parameters to the retry algorithm MUST be 3227 configurable. 3229 A client SHOULD keep a list of hosts it cannot reach and 3230 corresponding connection timeouts, rather than just retrying queued 3231 mail items. 3233 Experience suggests that failures are typically transient (the target 3234 system or its connection has crashed), favoring a policy of two 3235 connection attempts in the first hour the message is in the queue, 3236 and then backing off to one every two or three hours. 3238 The SMTP client can shorten the queuing delay in cooperation with the 3239 SMTP server. For example, if mail is received from a particular 3240 address, it is likely that mail queued for that host can now be sent. 3241 Application of this principle may, in many cases, eliminate the 3242 requirement for an explicit "send queues now" function such as ETRN, 3243 RFC 1985 [28]. 3245 The strategy may be further modified as a result of multiple 3246 addresses per host (see below) to optimize delivery time versus 3247 resource usage. 3249 An SMTP client may have a large queue of messages for each 3250 unavailable destination host. If all of these messages were retried 3251 in every retry cycle, there would be excessive Internet overhead and 3252 the sending system would be blocked for a long period. Note that an 3253 SMTP client can generally determine that a delivery attempt has 3254 failed only after a timeout of several minutes, and even a one-minute 3255 timeout per connection will result in a very large delay if retries 3256 are repeated for dozens, or even hundreds, of queued messages to the 3257 same host. 3259 At the same time, SMTP clients SHOULD use great care in caching 3260 negative responses from servers. In an extreme case, if EHLO is 3261 issued multiple times during the same SMTP connection, different 3262 answers may be returned by the server. More significantly, 5yz 3263 responses to the MAIL command MUST NOT be cached. 3265 When a mail message is to be delivered to multiple recipients, and 3266 the SMTP server to which a copy of the message is to be sent is the 3267 same for multiple recipients, then only one copy of the message 3268 SHOULD be transmitted. That is, the SMTP client SHOULD use the 3269 command sequence: MAIL, RCPT, RCPT, ..., RCPT, DATA instead of the 3270 sequence: MAIL, RCPT, DATA, ..., MAIL, RCPT, DATA. However, if there 3271 are very many addresses, a limit on the number of RCPT commands per 3272 MAIL command MAY be imposed. This efficiency feature SHOULD be 3273 implemented. 3275 Similarly, to achieve timely delivery, the SMTP client MAY support 3276 multiple concurrent outgoing mail transactions. However, some limit 3277 may be appropriate to protect the host from devoting all its 3278 resources to mail. 3280 4.5.4.2. Receiving Strategy 3282 The SMTP server SHOULD attempt to keep a pending listen on the SMTP 3283 port (specified by IANA as port 25) at all times. This requires the 3284 support of multiple incoming TCP connections for SMTP. Some limit 3285 MAY be imposed, but servers that cannot handle more than one SMTP 3286 transaction at a time are not in conformance with the intent of this 3287 specification. 3289 As discussed above, when the SMTP server receives mail from a 3290 particular host address, it could activate its own SMTP queuing 3291 mechanisms to retry any mail pending for that host address. 3293 4.5.5. Messages with a Null Reverse-Path 3295 There are several types of notification messages that are required by 3296 existing and proposed Standards to be sent with a null reverse-path, 3297 namely non-delivery notifications as discussed in Section 3.7, other 3298 kinds of Delivery Status Notifications (DSNs, RFC 3461 [12]), and 3299 Message Disposition Notifications (MDNs, RFC 3798 [40]). All of 3300 these kinds of messages are notifications about a previous message, 3301 and they are sent to the reverse-path of the previous mail message. 3302 (If the delivery of such a notification message fails, that usually 3303 indicates a problem with the mail system of the host to which the 3304 notification message is addressed. For this reason, at some hosts 3305 the MTA is set up to forward such failed notification messages to 3306 someone who is able to fix problems with the mail system, e.g., via 3307 the postmaster alias.) 3309 All other types of messages (i.e., any message which is not required 3310 by a Standards-Track RFC to have a null reverse-path) SHOULD be sent 3311 with a valid, non-null reverse-path. 3313 Implementers of automated email processors should be careful to make 3314 sure that the various kinds of messages with a null reverse-path are 3315 handled correctly. In particular, such systems SHOULD NOT reply to 3316 messages with a null reverse-path, and they SHOULD NOT add a non-null 3317 reverse-path, or change a null reverse-path to a non-null one, to 3318 such messages when forwarding. 3320 5. Address Resolution and Mail Handling 3321 5.1. Locating the Target Host 3323 Once an SMTP client lexically identifies a domain to which mail will 3324 be delivered for processing (as described in Sections 2.3.5 and 3.6), 3325 a DNS lookup MUST be performed to resolve the domain name (RFC 1035 3326 [7]). The names are expected to be fully-qualified domain names 3327 (FQDNs): mechanisms for inferring FQDNs from partial names or local 3328 aliases are outside of this specification. Due to a history of 3329 problems, SMTP servers used for initial submission of messages SHOULD 3330 NOT make such inferences (Message Submission Servers [43] have 3331 somewhat more flexibility) and intermediate (relay) SMTP servers MUST 3332 NOT make them. 3334 The lookup first attempts to locate an MX record associated with the 3335 name. If a CNAME record is found, the resulting name is processed as 3336 if it were the initial name. If a non-existent domain error is 3337 returned, this situation MUST be reported as an error. If a 3338 temporary error is returned, the message MUST be queued and retried 3339 later (see Section 4.5.4.1). If an empty list of MXs is returned, 3340 the address is treated as if it was associated with an implicit MX 3341 RR, with a preference of 0, pointing to that host. If MX records are 3342 present, but none of them are usable, or the implicit MX is unusable, 3343 this situation MUST be reported as an error. 3345 If one or more MX RRs are found for a given name, SMTP systems MUST 3346 NOT utilize any address RRs associated with that name unless they are 3347 located using the MX RRs; the "implicit MX" rule above applies only 3348 if there are no MX records present. If MX records are present, but 3349 none of them are usable, this situation MUST be reported as an error. 3351 When a domain name associated with an MX RR is looked up and the 3352 associated data field obtained, the data field of that response MUST 3353 contain a domain name that conforms to the specifications of 3354 Section 2.3.5. 3355 [[5321bis Editor's Note: Depending on how the "null MX" discussion 3356 unfolds, some additional text may be in order here (20140718)]] 3357 That domain name, when queried, MUST return at least one address 3358 record (e.g., A or AAAA RR) that gives the IP address of the SMTP 3359 server to which the message should be directed. Any other response, 3360 specifically including a value that will return a CNAME record when 3361 queried, lies outside the scope of this Standard. The prohibition on 3362 labels in the data that resolve to CNAMEs is discussed in more detail 3363 in RFC 2181, Section 10.3 [32]. 3365 When the lookup succeeds, the mapping can result in a list of 3366 alternative delivery addresses rather than a single address, because 3367 of multiple MX records, multihoming, or both. To provide reliable 3368 mail transmission, the SMTP client MUST be able to try (and retry) 3369 each of the relevant addresses in this list in order, until a 3370 delivery attempt succeeds. However, there MAY also be a configurable 3371 limit on the number of alternate addresses that can be tried. In any 3372 case, the SMTP client SHOULD try at least two addresses. 3374 Two types of information are used to rank the host addresses: 3375 multiple MX records, and multihomed hosts. 3377 MX records contain a preference indication that MUST be used in 3378 sorting if more than one such record appears (see below). Lower 3379 numbers are more preferred than higher ones. If there are multiple 3380 destinations with the same preference and there is no clear reason to 3381 favor one (e.g., by recognition of an easily reached address), then 3382 the sender-SMTP MUST randomize them to spread the load across 3383 multiple mail exchangers for a specific organization. 3385 The destination host (perhaps taken from the preferred MX record) may 3386 be multihomed, in which case the domain name resolver will return a 3387 list of alternative IP addresses. It is the responsibility of the 3388 domain name resolver interface to have ordered this list by 3389 decreasing preference if necessary, and the SMTP sender MUST try them 3390 in the order presented. 3392 Although the capability to try multiple alternative addresses is 3393 required, specific installations may want to limit or disable the use 3394 of alternative addresses. The question of whether a sender should 3395 attempt retries using the different addresses of a multihomed host 3396 has been controversial. The main argument for using the multiple 3397 addresses is that it maximizes the probability of timely delivery, 3398 and indeed sometimes the probability of any delivery; the counter- 3399 argument is that it may result in unnecessary resource use. Note 3400 that resource use is also strongly determined by the sending strategy 3401 discussed in Section 4.5.4.1. 3403 If an SMTP server receives a message with a destination for which it 3404 is a designated Mail eXchanger, it MAY relay the message (potentially 3405 after having rewritten the MAIL FROM and/or RCPT TO addresses), make 3406 final delivery of the message, or hand it off using some mechanism 3407 outside the SMTP-provided transport environment. Of course, neither 3408 of the latter require that the list of MX records be examined 3409 further. 3411 If it determines that it should relay the message without rewriting 3412 the address, it MUST sort the MX records to determine candidates for 3413 delivery. The records are first ordered by preference, with the 3414 lowest-numbered records being most preferred. The relay host MUST 3415 then inspect the list for any of the names or addresses by which it 3416 might be known in mail transactions. If a matching record is found, 3417 all records at that preference level and higher-numbered ones MUST be 3418 discarded from consideration. If there are no records left at that 3419 point, it is an error condition, and the message MUST be returned as 3420 undeliverable. If records do remain, they SHOULD be tried, best 3421 preference first, as described above. 3423 5.2. IPv6 and MX Records 3425 In the contemporary Internet, SMTP clients and servers may be hosted 3426 on IPv4 systems, IPv6 systems, or dual-stack systems that are 3427 compatible with either version of the Internet Protocol. The host 3428 domains to which MX records point may, consequently, contain "A RR"s 3429 (IPv4), "AAAA RR"s (IPv6), or any combination of them. While RFC 3430 3974 [14] discusses some operational experience in mixed 3431 environments, it was not comprehensive enough to justify 3432 standardization, and some of its recommendations appear to be 3433 inconsistent with this specification. The appropriate actions to be 3434 taken either will depend on local circumstances, such as performance 3435 of the relevant networks and any conversions that might be necessary, 3436 or will be obvious (e.g., an IPv6-only client need not attempt to 3437 look up A RRs or attempt to reach IPv4-only servers). Designers of 3438 SMTP implementations that might run in IPv6 or dual-stack 3439 environments should study the procedures above, especially the 3440 comments about multihomed hosts, and, preferably, provide mechanisms 3441 to facilitate operational tuning and mail interoperability between 3442 IPv4 and IPv6 systems while considering local circumstances. 3444 6. Problem Detection and Handling 3446 6.1. Reliable Delivery and Replies by Email 3448 When the receiver-SMTP accepts a piece of mail (by sending a "250 OK" 3449 message in response to DATA), it is accepting responsibility for 3450 delivering or relaying the message. It must take this responsibility 3451 seriously. It MUST NOT lose the message for frivolous reasons, such 3452 as because the host later crashes or because of a predictable 3453 resource shortage. Some reasons that are not considered frivolous 3454 are discussed in the next subsection and in Section 7.8. 3456 If there is a delivery failure after acceptance of a message, the 3457 receiver-SMTP MUST formulate and mail a notification message. This 3458 notification MUST be sent using a null ("<>") reverse-path in the 3459 envelope. The recipient of this notification MUST be the address 3460 from the envelope return path (or the Return-Path: line). However, 3461 if this address is null ("<>"), the receiver-SMTP MUST NOT send a 3462 notification. Obviously, nothing in this section can or should 3463 prohibit local decisions (i.e., as part of the same system 3464 environment as the receiver-SMTP) to log or otherwise transmit 3465 information about null address events locally if that is desired. If 3466 the address is an explicit source route, it MUST be stripped down to 3467 its final hop. 3469 For example, suppose that an error notification must be sent for a 3470 message that arrived with: 3472 MAIL FROM:<@a,@b:user@d> 3474 The notification message MUST be sent using: 3476 RCPT TO: 3478 Some delivery failures after the message is accepted by SMTP will be 3479 unavoidable. For example, it may be impossible for the receiving 3480 SMTP server to validate all the delivery addresses in RCPT command(s) 3481 due to a "soft" domain system error, because the target is a mailing 3482 list (see earlier discussion of RCPT), or because the server is 3483 acting as a relay and has no immediate access to the delivering 3484 system. 3486 To avoid receiving duplicate messages as the result of timeouts, a 3487 receiver-SMTP MUST seek to minimize the time required to respond to 3488 the final . end of data indicator. See RFC 1047 [20] for 3489 a discussion of this problem. 3491 6.2. Unwanted, Unsolicited, and "Attack" Messages 3493 Utility and predictability of the Internet mail system requires that 3494 messages that can be delivered should be delivered, regardless of any 3495 syntax or other faults associated with those messages and regardless 3496 of their content. If they cannot be delivered, and cannot be 3497 rejected by the SMTP server during the SMTP transaction, they should 3498 be "bounced" (returned with non-delivery notification messages) as 3499 described above. In today's world, in which many SMTP server 3500 operators have discovered that the quantity of undesirable bulk email 3501 vastly exceeds the quantity of desired mail and in which accepting a 3502 message may trigger additional undesirable traffic by providing 3503 verification of the address, those principles may not be practical. 3505 As discussed in Section 7.8 and Section 7.9 below, dropping mail 3506 without notification of the sender is permitted in practice. 3507 However, it is extremely dangerous and violates a long tradition and 3508 community expectations that mail is either delivered or returned. If 3509 silent message-dropping is misused, it could easily undermine 3510 confidence in the reliability of the Internet's mail systems. So 3511 silent dropping of messages should be considered only in those cases 3512 where there is very high confidence that the messages are seriously 3513 fraudulent or otherwise inappropriate. 3515 To stretch the principle of delivery if possible even further, it may 3516 be a rational policy to not deliver mail that has an invalid return 3517 address, although the history of the network is that users are 3518 typically better served by delivering any message that can be 3519 delivered. Reliably determining that a return address is invalid can 3520 be a difficult and time-consuming process, especially if the putative 3521 sending system is not directly accessible or does not fully and 3522 accurately support VRFY and, even if a "drop messages with invalid 3523 return addresses" policy is adopted, it SHOULD be applied only when 3524 there is near-certainty that the return addresses are, in fact, 3525 invalid. 3527 Conversely, if a message is rejected because it is found to contain 3528 hostile content (a decision that is outside the scope of an SMTP 3529 server as defined in this document), rejection ("bounce") messages 3530 SHOULD NOT be sent unless the receiving site is confident that those 3531 messages will be usefully delivered. The preference and default in 3532 these cases is to avoid sending non-delivery messages when the 3533 incoming message is determined to contain hostile content. 3535 6.3. Loop Detection 3537 Simple counting of the number of "Received:" header fields in a 3538 message has proven to be an effective, although rarely optimal, 3539 method of detecting loops in mail systems. SMTP servers using this 3540 technique SHOULD use a large rejection threshold, normally at least 3541 100 Received entries. Whatever mechanisms are used, servers MUST 3542 contain provisions for detecting and stopping trivial loops. 3544 6.4. Compensating for Irregularities 3546 Unfortunately, variations, creative interpretations, and outright 3547 violations of Internet mail protocols do occur; some would suggest 3548 that they occur quite frequently. The debate as to whether a well- 3549 behaved SMTP receiver or relay should reject a malformed message, 3550 attempt to pass it on unchanged, or attempt to repair it to increase 3551 the odds of successful delivery (or subsequent reply) began almost 3552 with the dawn of structured network mail and shows no signs of 3553 abating. Advocates of rejection claim that attempted repairs are 3554 rarely completely adequate and that rejection of bad messages is the 3555 only way to get the offending software repaired. Advocates of 3556 "repair" or "deliver no matter what" argue that users prefer that 3557 mail go through it if at all possible and that there are significant 3558 market pressures in that direction. In practice, these market 3559 pressures may be more important to particular vendors than strict 3560 conformance to the standards, regardless of the preference of the 3561 actual developers. 3563 The problems associated with ill-formed messages were exacerbated by 3564 the introduction of the split-UA mail reading protocols (Post Office 3565 Protocol (POP) version 2 [17], Post Office Protocol (POP) version 3 3566 [27], IMAP version 2 [22], and PCMAIL [21]). These protocols 3567 encouraged the use of SMTP as a posting (message submission) 3568 protocol, and SMTP servers as relay systems for these client hosts 3569 (which are often only intermittently connected to the Internet). 3570 Historically, many of those client machines lacked some of the 3571 mechanisms and information assumed by SMTP (and indeed, by the mail 3572 format protocol, RFC 822 [16]). Some could not keep adequate track 3573 of time; others had no concept of time zones; still others could not 3574 identify their own names or addresses; and, of course, none could 3575 satisfy the assumptions that underlay RFC 822's conception of 3576 authenticated addresses. 3578 In response to these weak SMTP clients, many SMTP systems now 3579 complete messages that are delivered to them in incomplete or 3580 incorrect form. This strategy is generally considered appropriate 3581 when the server can identify or authenticate the client, and there 3582 are prior agreements between them. By contrast, there is at best 3583 great concern about fixes applied by a relay or delivery SMTP server 3584 that has little or no knowledge of the user or client machine. Many 3585 of these issues are addressed by using a separate protocol, such as 3586 that defined in RFC 4409 [43], for message submission, rather than 3587 using originating SMTP servers for that purpose. 3589 The following changes to a message being processed MAY be applied 3590 when necessary by an originating SMTP server, or one used as the 3591 target of SMTP as an initial posting (message submission) protocol: 3593 o Addition of a message-id field when none appears 3595 o Addition of a date, time, or time zone when none appears 3597 o Correction of addresses to proper FQDN format 3599 The less information the server has about the client, the less likely 3600 these changes are to be correct and the more caution and conservatism 3601 should be applied when considering whether or not to perform fixes 3602 and how. These changes MUST NOT be applied by an SMTP server that 3603 provides an intermediate relay function. 3605 In all cases, properly operating clients supplying correct 3606 information are preferred to corrections by the SMTP server. In all 3607 cases, documentation SHOULD be provided in trace header fields and/or 3608 header field comments for actions performed by the servers. 3610 7. Security Considerations 3612 7.1. Mail Security and Spoofing 3614 SMTP mail is inherently insecure in that it is feasible for even 3615 fairly casual users to negotiate directly with receiving and relaying 3616 SMTP servers and create messages that will trick a naive recipient 3617 into believing that they came from somewhere else. Constructing such 3618 a message so that the "spoofed" behavior cannot be detected by an 3619 expert is somewhat more difficult, but not sufficiently so as to be a 3620 deterrent to someone who is determined and knowledgeable. 3621 Consequently, as knowledge of Internet mail increases, so does the 3622 knowledge that SMTP mail inherently cannot be authenticated, or 3623 integrity checks provided, at the transport level. Real mail 3624 security lies only in end-to-end methods involving the message 3625 bodies, such as those that use digital signatures (see RFC 1847 [25] 3626 and, e.g., Pretty Good Privacy (PGP) in RFC 4880 [15] or Secure/ 3627 Multipurpose Internet Mail Extensions (S/MIME) in RFC 3851 [41]). 3629 Various protocol extensions and configuration options that provide 3630 authentication at the transport level (e.g., from an SMTP client to 3631 an SMTP server) improve somewhat on the traditional situation 3632 described above. However, in general, they only authenticate one 3633 server to another rather than a chain of relays and servers, much 3634 less authenticating users or user machines. Consequently, unless 3635 they are accompanied by careful handoffs of responsibility in a 3636 carefully designed trust environment, they remain inherently weaker 3637 than end-to-end mechanisms that use digitally signed messages rather 3638 than depending on the integrity of the transport system. 3640 Efforts to make it more difficult for users to set envelope return 3641 path and header "From" fields to point to valid addresses other than 3642 their own are largely misguided: they frustrate legitimate 3643 applications in which mail is sent by one user on behalf of another, 3644 in which error (or normal) replies should be directed to a special 3645 address, or in which a single message is sent to multiple recipients 3646 on different hosts. (Systems that provide convenient ways for users 3647 to alter these header fields on a per-message basis should attempt to 3648 establish a primary and permanent mailbox address for the user so 3649 that Sender header fields within the message data can be generated 3650 sensibly.) 3652 This specification does not further address the authentication issues 3653 associated with SMTP other than to advocate that useful functionality 3654 not be disabled in the hope of providing some small margin of 3655 protection against a user who is trying to fake mail. 3657 7.2. "Blind" Copies 3659 Addresses that do not appear in the message header section may appear 3660 in the RCPT commands to an SMTP server for a number of reasons. The 3661 two most common involve the use of a mailing address as a "list 3662 exploder" (a single address that resolves into multiple addresses) 3663 and the appearance of "blind copies". Especially when more than one 3664 RCPT command is present, and in order to avoid defeating some of the 3665 purpose of these mechanisms, SMTP clients and servers SHOULD NOT copy 3666 the full set of RCPT command arguments into the header section, 3667 either as part of trace header fields or as informational or private- 3668 extension header fields. [[CREF25: [rfc5321bis] [[Note in draft - 3669 Suggestion from 20070124 that got lost: delete "especially" and "the 3670 full set of" -- copying the first one can be as harmful as copying 3671 all of them, at least without verifying that the addresses do appear 3672 in the headers.]] Arnt Gulbrandsen, arnt@oryx.com, 2007.01.24 3673 1121+0100]] Since this rule is often violated in practice, and cannot 3674 be enforced, sending SMTP systems that are aware of "bcc" use MAY 3675 find it helpful to send each blind copy as a separate message 3676 transaction containing only a single RCPT command. 3678 There is no inherent relationship between either "reverse" (from 3679 MAIL, SAML, etc., commands) or "forward" (RCPT) addresses in the SMTP 3680 transaction ("envelope") and the addresses in the header section. 3681 Receiving systems SHOULD NOT attempt to deduce such relationships and 3682 use them to alter the header section of the message for delivery. 3683 The popular "Apparently-to" header field is a violation of this 3684 principle as well as a common source of unintended information 3685 disclosure and SHOULD NOT be used. 3687 7.3. VRFY, EXPN, and Security 3689 As discussed in Section 3.5, individual sites may want to disable 3690 either or both of VRFY or EXPN for security reasons (see below). As 3691 a corollary to the above, implementations that permit this MUST NOT 3692 appear to have verified addresses that are not, in fact, verified. 3693 If a site disables these commands for security reasons, the SMTP 3694 server MUST return a 252 response, rather than a code that could be 3695 confused with successful or unsuccessful verification. 3697 Returning a 250 reply code with the address listed in the VRFY 3698 command after having checked it only for syntax violates this rule. 3699 Of course, an implementation that "supports" VRFY by always returning 3700 550 whether or not the address is valid is equally not in 3701 conformance. 3703 On the public Internet, the contents of mailing lists have become 3704 popular as an address information source for so-called "spammers." 3705 The use of EXPN to "harvest" addresses has increased as list 3706 administrators have installed protections against inappropriate uses 3707 of the lists themselves. However, VRFY and EXPN are still useful for 3708 authenticated users and within an administrative domain. For 3709 example, VRFY and EXPN are useful for performing internal audits of 3710 how email gets routed to check and to make sure no one is 3711 automatically forwarding sensitive mail outside the organization. 3712 Sites implementing SMTP authentication may choose to make VRFY and 3713 EXPN available only to authenticated requestors. Implementations 3714 SHOULD still provide support for EXPN, but sites SHOULD carefully 3715 evaluate the tradeoffs. 3717 Whether disabling VRFY provides any real marginal security depends on 3718 a series of other conditions. In many cases, RCPT commands can be 3719 used to obtain the same information about address validity. On the 3720 other hand, especially in situations where determination of address 3721 validity for RCPT commands is deferred until after the DATA command 3722 is received, RCPT may return no information at all, while VRFY is 3723 expected to make a serious attempt to determine validity before 3724 generating a response code (see discussion above). 3726 7.4. Mail Rerouting Based on the 251 and 551 Response Codes 3728 Before a client uses the 251 or 551 reply codes from a RCPT command 3729 to automatically update its future behavior (e.g., updating the 3730 user's address book), it should be certain of the server's 3731 authenticity. If it does not, it may be subject to a man in the 3732 middle attack. 3734 7.5. Information Disclosure in Announcements 3736 There has been an ongoing debate about the tradeoffs between the 3737 debugging advantages of announcing server type and version (and, 3738 sometimes, even server domain name) in the greeting response or in 3739 response to the HELP command and the disadvantages of exposing 3740 information that might be useful in a potential hostile attack. The 3741 utility of the debugging information is beyond doubt. Those who 3742 argue for making it available point out that it is far better to 3743 actually secure an SMTP server rather than hope that trying to 3744 conceal known vulnerabilities by hiding the server's precise identity 3745 will provide more protection. Sites are encouraged to evaluate the 3746 tradeoff with that issue in mind; implementations SHOULD minimally 3747 provide for making type and version information available in some way 3748 to other network hosts. 3750 7.6. Information Disclosure in Trace Fields 3752 In some circumstances, such as when mail originates from within a LAN 3753 whose hosts are not directly on the public Internet, trace 3754 ("Received") header fields produced in conformance with this 3755 specification may disclose host names and similar information that 3756 would not normally be available. This ordinarily does not pose a 3757 problem, but sites with special concerns about name disclosure should 3758 be aware of it. Also, the optional FOR clause should be supplied 3759 with caution or not at all when multiple recipients are involved lest 3760 it inadvertently disclose the identities of "blind copy" recipients 3761 to others. 3763 7.7. Information Disclosure in Message Forwarding 3765 As discussed in Section 3.4, use of the 251 or 551 reply codes to 3766 identify the replacement address associated with a mailbox may 3767 inadvertently disclose sensitive information. Sites that are 3768 concerned about those issues should ensure that they select and 3769 configure servers appropriately. 3771 7.8. Resistance to Attacks 3773 In recent years, there has been an increase of attacks on SMTP 3774 servers, either in conjunction with attempts to discover addresses 3775 for sending unsolicited messages or simply to make the servers 3776 inaccessible to others (i.e., as an application-level denial of 3777 service attack). While the means of doing so are beyond the scope of 3778 this Standard, rational operational behavior requires that servers be 3779 permitted to detect such attacks and take action to defend 3780 themselves. For example, if a server determines that a large number 3781 of RCPT TO commands are being sent, most or all with invalid 3782 addresses, as part of such an attack, it would be reasonable for the 3783 server to close the connection after generating an appropriate number 3784 of 5yz (normally 550) replies. 3786 7.9. Scope of Operation of SMTP Servers 3788 It is a well-established principle that an SMTP server may refuse to 3789 accept mail for any operational or technical reason that makes sense 3790 to the site providing the server. However, cooperation among sites 3791 and installations makes the Internet possible. If sites take 3792 excessive advantage of the right to reject traffic, the ubiquity of 3793 email availability (one of the strengths of the Internet) will be 3794 threatened; considerable care should be taken and balance maintained 3795 if a site decides to be selective about the traffic it will accept 3796 and process. 3798 In recent years, use of the relay function through arbitrary sites 3799 has been used as part of hostile efforts to hide the actual origins 3800 of mail. Some sites have decided to limit the use of the relay 3801 function to known or identifiable sources, and implementations SHOULD 3802 provide the capability to perform this type of filtering. When mail 3803 is rejected for these or other policy reasons, a 550 code SHOULD be 3804 used in response to EHLO (or HELO), MAIL, or RCPT as appropriate. 3806 8. IANA Considerations 3808 IANA maintains three registries in support of this specification, all 3809 of which were created for RFC 2821 or earlier. This document expands 3810 the third one as specified below. The registry references listed are 3811 as of the time of publication; IANA does not guarantee the locations 3812 associated with the URLs. The registries are as follows: 3814 o The first, "Simple Mail Transfer Protocol (SMTP) Service 3815 Extensions" [49], consists of SMTP service extensions with the 3816 associated keywords, and, as needed, parameters and verbs. As 3817 specified in Section 2.2.2, no entry may be made in this registry 3818 that starts in an "X". Entries may be made only for service 3819 extensions (and associated keywords, parameters, or verbs) that 3820 are defined in Standards-Track or Experimental RFCs specifically 3821 approved by the IESG for this purpose. 3823 o The second registry, "Address Literal Tags" [50], consists of 3824 "tags" that identify forms of domain literals other than those for 3825 IPv4 addresses (specified in RFC 821 and in this document). The 3826 initial entry in that registry is for IPv6 addresses (specified in 3827 this document). Additional literal types require standardization 3828 before being used; none are anticipated at this time. 3830 o The third, "Mail Transmission Types" [49], established by RFC 821 3831 and renewed by this specification, is a registry of link and 3832 protocol identifiers to be used with the "via" and "with" 3833 subclauses of the time stamp ("Received:" header field) described 3834 in Section 4.4. Link and protocol identifiers in addition to 3835 those specified in this document may be registered only by 3836 standardization or by way of an RFC-documented, IESG-approved, 3837 Experimental protocol extension. This name space is for 3838 identification and not limited in size: the IESG is encouraged to 3839 approve on the basis of clear documentation and a distinct method 3840 rather than preferences about the properties of the method itself. 3842 An additional subsection has been added to the "VIA link types" 3843 and "WITH protocol types" subsections of this registry to contain 3844 registrations of "Additional-registered-clauses" as described 3845 above. The registry will contain clause names, a description, a 3846 summary of the syntax of the associated String, and a reference. 3847 As new clauses are defined, they may, in principle, specify 3848 creation of their own registries if the Strings consist of 3849 reserved terms or keywords rather than less restricted strings. 3850 As with link and protocol identifiers, additional clauses may be 3851 registered only by standardization or by way of an RFC-documented, 3852 IESG-approved, Experimental protocol extension. The additional 3853 clause name space is for identification and is not limited in 3854 size: the IESG is encouraged to approve on the basis of clear 3855 documentation, actual use or strong signs that the clause will be 3856 used, and a distinct requirement rather than preferences about the 3857 properties of the clause itself. 3859 In addition, if additional trace header fields (i.e., in addition to 3860 Return-path and Received) are ever created, those trace fields MUST 3861 be added to the IANA registry established by BCP 90 (RFC 3864) [10] 3862 for use with RFC 5322 [11]. 3864 9. Acknowledgments 3866 Many people contributed to the development of RFCs 2821 and 5321. 3867 Those documents should be consulted for those acknowledgments. 3869 Neither this document nor RFCs 2821 or 5321 would have been possible 3870 without the many contribution and insights of the late Jon Postel. 3871 Those contributions of course include the original specification of 3872 SMTP in RFC 821. A considerable quantity of text from RFC 821 still 3873 appears in this document as do several of Jon's original examples 3874 that have been updated only as needed to reflect other changes in the 3875 specification. 3877 The following filed errata against RFC 5321 that were not rejected at 3878 the time of submission: Jasen Betts, Adrien de Croy Guillaume Fortin- 3879 Debigare Roberto Javier Godoy, David Romerstein, Dominic Sayers, 3880 Rodrigo Speller, Alessandro Vesely, and Brett Watson. In addition, 3881 specific suggestions that led to corrections and improvements in this 3882 version were received from Ned Freed, Barry Leiba, Ivar Lumi, Pete 3883 Resnick, and others. 3885 10. References 3887 10.1. Normative References 3889 [1] Bradner, S., "Key words for use in RFCs to Indicate 3890 Requirement Levels", BCP 14, RFC 2119, 3891 DOI 10.17487/RFC2119, March 1997, 3892 . 3894 [2] American National Standards Institute (formerly United 3895 States of America Standards Institute), "USA Code for 3896 Information Interchange", ANSI X3.4-1968, 1968. 3898 ANSI X3.4-1968 has been replaced by newer versions with 3899 slight modifications, but the 1968 version remains 3900 definitive for the Internet. 3902 [3] Braden, R., Ed., "Requirements for Internet Hosts - 3903 Application and Support", STD 3, RFC 1123, 3904 DOI 10.17487/RFC1123, October 1989, 3905 . 3907 [4] Klensin, J., Freed, N., and K. Moore, "SMTP Service 3908 Extension for Message Size Declaration", STD 10, RFC 1870, 3909 DOI 10.17487/RFC1870, November 1995, 3910 . 3912 [5] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 3913 Specifications: ABNF", STD 68, RFC 5234, 3914 DOI 10.17487/RFC5234, January 2008, 3915 . 3917 [6] Hinden, R. and S. Deering, "IP Version 6 Addressing 3918 Architecture", RFC 4291, DOI 10.17487/RFC4291, February 3919 2006, . 3921 [7] Mockapetris, P., "Domain names - implementation and 3922 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 3923 November 1987, . 3925 [8] Postel, J., "Simple Mail Transfer Protocol", STD 10, 3926 RFC 821, DOI 10.17487/RFC0821, August 1982, 3927 . 3929 [9] Newman, C., "ESMTP and LMTP Transmission Types 3930 Registration", RFC 3848, DOI 10.17487/RFC3848, July 2004, 3931 . 3933 [10] Klyne, G., Nottingham, M., and J. Mogul, "Registration 3934 Procedures for Message Header Fields", BCP 90, RFC 3864, 3935 DOI 10.17487/RFC3864, September 2004, 3936 . 3938 [11] Resnick, P., "Internet Message Format", RFC 5322, 3939 September 2008. 3941 10.2. Informative References 3943 [12] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service 3944 Extension for Delivery Status Notifications (DSNs)", 3945 RFC 3461, DOI 10.17487/RFC3461, January 2003, 3946 . 3948 [13] Moore, K. and G. Vaudreuil, "An Extensible Message Format 3949 for Delivery Status Notifications", RFC 3464, 3950 DOI 10.17487/RFC3464, January 2003, 3951 . 3953 [14] Nakamura, M. and J. Hagino, "SMTP Operational Experience 3954 in Mixed IPv4/v6 Environments", RFC 3974, 3955 DOI 10.17487/RFC3974, January 2005, 3956 . 3958 [15] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. 3959 Thayer, "OpenPGP Message Format", RFC 4880, 3960 DOI 10.17487/RFC4880, November 2007, 3961 . 3963 [16] Crocker, D., "STANDARD FOR THE FORMAT OF ARPA INTERNET 3964 TEXT MESSAGES", STD 11, RFC 822, DOI 10.17487/RFC0822, 3965 August 1982, . 3967 [17] Butler, M., Postel, J., Chase, D., Goldberger, J., and J. 3968 Reynolds, "Post Office Protocol: Version 2", RFC 937, 3969 DOI 10.17487/RFC0937, February 1985, 3970 . 3972 [18] Postel, J. and J. Reynolds, "File Transfer Protocol", 3973 STD 9, RFC 959, DOI 10.17487/RFC0959, October 1985, 3974 . 3976 [19] Partridge, C., "Mail routing and the domain system", 3977 STD 10, RFC 974, DOI 10.17487/RFC0974, January 1986, 3978 . 3980 [20] Partridge, C., "Duplicate messages and SMTP", RFC 1047, 3981 DOI 10.17487/RFC1047, February 1988, 3982 . 3984 [21] Lambert, M., "PCMAIL: A distributed mail system for 3985 personal computers", RFC 1056, DOI 10.17487/RFC1056, June 3986 1988, . 3988 [22] Crispin, M., "Interactive Mail Access Protocol: Version 3989 2", RFC 1176, DOI 10.17487/RFC1176, August 1990, 3990 . 3992 [23] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D. 3993 Crocker, "SMTP Service Extension for 8bit-MIMEtransport", 3994 RFC 1652, DOI 10.17487/RFC1652, July 1994, 3995 . 3997 [24] Durand, A. and F. Dupont, "SMTP 521 Reply Code", RFC 1846, 3998 DOI 10.17487/RFC1846, September 1995, 3999 . 4001 [25] Galvin, J., Murphy, S., Crocker, S., and N. Freed, 4002 "Security Multiparts for MIME: Multipart/Signed and 4003 Multipart/Encrypted", RFC 1847, DOI 10.17487/RFC1847, 4004 October 1995, . 4006 [26] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D. 4007 Crocker, "SMTP Service Extensions", STD 10, RFC 1869, 4008 DOI 10.17487/RFC1869, November 1995, 4009 . 4011 [27] Myers, J. and M. Rose, "Post Office Protocol - Version 3", 4012 STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996, 4013 . 4015 [28] De Winter, J., "SMTP Service Extension for Remote Message 4016 Queue Starting", RFC 1985, DOI 10.17487/RFC1985, August 4017 1996, . 4019 [29] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 4020 Extensions (MIME) Part One: Format of Internet Message 4021 Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996, 4022 . 4024 [30] Moore, K., "MIME (Multipurpose Internet Mail Extensions) 4025 Part Three: Message Header Extensions for Non-ASCII Text", 4026 RFC 2047, DOI 10.17487/RFC2047, November 1996, 4027 . 4029 [31] Kille, S., "MIXER (Mime Internet X.400 Enhanced Relay): 4030 Mapping between X.400 and RFC 822/MIME", RFC 2156, 4031 DOI 10.17487/RFC2156, January 1998, 4032 . 4034 [32] Elz, R. and R. Bush, "Clarifications to the DNS 4035 Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997, 4036 . 4038 [33] Freed, N. and K. Moore, "MIME Parameter Value and Encoded 4039 Word Extensions: Character Sets, Languages, and 4040 Continuations", RFC 2231, DOI 10.17487/RFC2231, November 4041 1997, . 4043 [34] Klensin, J., Ed., "Simple Mail Transfer Protocol", 4044 RFC 2821, DOI 10.17487/RFC2821, April 2001, 4045 . 4047 [35] Freed, N., "SMTP Service Extension for Command 4048 Pipelining", STD 60, RFC 2920, DOI 10.17487/RFC2920, 4049 September 2000, . 4051 [36] Freed, N., "Behavior of and Requirements for Internet 4052 Firewalls", RFC 2979, DOI 10.17487/RFC2979, October 2000, 4053 . 4055 [37] Vaudreuil, G., "SMTP Service Extensions for Transmission 4056 of Large and Binary MIME Messages", RFC 3030, 4057 DOI 10.17487/RFC3030, December 2000, 4058 . 4060 [38] Vaudreuil, G., "Enhanced Mail System Status Codes", 4061 RFC 3463, DOI 10.17487/RFC3463, January 2003, 4062 . 4064 [39] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4065 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, 4066 . 4068 [40] Hansen, T., Ed. and G. Vaudreuil, Ed., "Message 4069 Disposition Notification", RFC 3798, DOI 10.17487/RFC3798, 4070 May 2004, . 4072 [41] Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail 4073 Extensions (S/MIME) Version 3.1 Message Specification", 4074 RFC 3851, DOI 10.17487/RFC3851, July 2004, 4075 . 4077 [42] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF) 4078 for Authorizing Use of Domains in E-Mail, Version 1", 4079 RFC 4408, DOI 10.17487/RFC4408, April 2006, 4080 . 4082 [43] Gellens, R. and J. Klensin, "Message Submission for Mail", 4083 RFC 4409, DOI 10.17487/RFC4409, April 2006, 4084 . 4086 [44] Fenton, J., "Analysis of Threats Motivating DomainKeys 4087 Identified Mail (DKIM)", RFC 4686, DOI 10.17487/RFC4686, 4088 September 2006, . 4090 [45] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, 4091 J., and M. Thomas, "DomainKeys Identified Mail (DKIM) 4092 Signatures", RFC 4871, DOI 10.17487/RFC4871, May 2007, 4093 . 4095 [46] Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced 4096 Mail System Status Codes", BCP 138, RFC 5248, 4097 DOI 10.17487/RFC5248, June 2008, 4098 . 4100 [47] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 4101 Resource Identifier (URI): Generic Syntax", STD 66, 4102 RFC 3986, DOI 10.17487/RFC3986, January 2005, 4103 . 4105 [48] Klensin, J., "SMTP 521 and 556 Reply Codes", RFC 7504, 4106 DOI 10.17487/RFC7504, June 2015, 4107 . 4109 [49] Internet Assigned Number Authority (IANA), "IANA Mail 4110 Parameters", 2007, 4111 . 4113 [50] Internet Assigned Number Authority (IANA), "Address 4114 Literal Tags", 2007, 4115 . 4117 [51] Levine, J. and M. Delany, "A "Null MX" No Service Resource 4118 Record for Domains that Accept No Mail", September 2014, 4119 . 4122 [52] RFC Editor, "RFC Errata - RFC 5321", 2019, 4123 . 4125 Captured 2019-11-19 4127 Appendix A. TCP Transport Service 4129 The TCP connection supports the transmission of 8-bit bytes. The 4130 SMTP data is 7-bit ASCII characters. Each character is transmitted 4131 as an 8-bit byte with the high-order bit cleared to zero. Service 4132 extensions may modify this rule to permit transmission of full 8-bit 4133 data bytes as part of the message body, or, if specifically designed 4134 to do so, in SMTP commands or responses. 4136 Appendix B. Generating SMTP Commands from RFC 822 Header Fields 4138 Some systems use an RFC 822 header section (only) in a mail 4139 submission protocol, or otherwise generate SMTP commands from RFC 822 4140 header fields when such a message is handed to an MTA from a UA. 4141 While the MTA-UA protocol is a private matter, not covered by any 4142 Internet Standard, there are problems with this approach. For 4143 example, there have been repeated problems with proper handling of 4144 "bcc" copies and redistribution lists when information that 4145 conceptually belongs to the mail envelope is not separated early in 4146 processing from header field information (and kept separate). 4148 It is recommended that the UA provide its initial ("submission 4149 client") MTA with an envelope separate from the message itself. 4150 However, if the envelope is not supplied, SMTP commands SHOULD be 4151 generated as follows: 4153 1. Each recipient address from a TO, CC, or BCC header field SHOULD 4154 be copied to a RCPT command (generating multiple message copies 4155 if that is required for queuing or delivery). This includes any 4156 addresses listed in a RFC 822 "group". Any BCC header fields 4157 SHOULD then be removed from the header section. Once this 4158 process is completed, the remaining header fields SHOULD be 4159 checked to verify that at least one TO, CC, or BCC header field 4160 remains. If none do, then a BCC header field with no additional 4161 information SHOULD be inserted as specified in [11]. 4163 2. The return address in the MAIL command SHOULD, if possible, be 4164 derived from the system's identity for the submitting (local) 4165 user, and the "From:" header field otherwise. If there is a 4166 system identity available, it SHOULD also be copied to the Sender 4167 header field if it is different from the address in the From 4168 header field. (Any Sender header field that was already there 4169 SHOULD be removed.) Systems may provide a way for submitters to 4170 override the envelope return address, but may want to restrict 4171 its use to privileged users. This will not prevent mail forgery, 4172 but may lessen its incidence; see Section 7.1. 4174 When an MTA is being used in this way, it bears responsibility for 4175 ensuring that the message being transmitted is valid. The mechanisms 4176 for checking that validity, and for handling (or returning) messages 4177 that are not valid at the time of arrival, are part of the MUA-MTA 4178 interface and not covered by this specification. 4180 A submission protocol based on Standard RFC 822 information alone 4181 MUST NOT be used to gateway a message from a foreign (non-SMTP) mail 4182 system into an SMTP environment. Additional information to construct 4183 an envelope must come from some source in the other environment, 4184 whether supplemental header fields or the foreign system's envelope. 4186 Attempts to gateway messages using only their header "To" and "Cc" 4187 fields have repeatedly caused mail loops and other behavior adverse 4188 to the proper functioning of the Internet mail environment. These 4189 problems have been especially common when the message originates from 4190 an Internet mailing list and is distributed into the foreign 4191 environment using envelope information. When these messages are then 4192 processed by a header-section-only remailer, loops back to the 4193 Internet environment (and the mailing list) are almost inevitable. 4195 Appendix C. Source Routes 4197 Historically, the was a reverse source routing list of 4198 hosts and a source mailbox. The first host in the was 4199 historically the host sending the MAIL command; today, source routes 4200 SHOULD NOT appear in the reverse-path. Similarly, the 4201 may be a source routing lists of hosts and a destination mailbox. 4202 However, in general, the SHOULD contain only a mailbox 4203 and domain name, relying on the domain name system to supply routing 4204 information if required. The use of source routes is deprecated (see 4205 Appendix F.2); while servers MUST be prepared to receive and handle 4206 them as discussed in Section 3.3 and Appendix F.2, clients SHOULD NOT 4207 transmit them and this section is included in the current 4208 specification only to provide context. It has been modified somewhat 4209 from the material in RFC 821 to prevent server actions that might 4210 confuse clients or subsequent servers that do not expect a full 4211 source route implementation. 4213 Historically, for relay purposes, the forward-path may have been a 4214 source route of the form "@ONE,@TWO:JOE@THREE", where ONE, TWO, and 4215 THREE MUST be fully-qualified domain names. This form was used to 4216 emphasize the distinction between an address and a route. The 4217 mailbox (here, JOE@THREE) is an absolute address, and the route is 4218 information about how to get there. The two concepts should not be 4219 confused.[[CREF26: [5321bis]JcK 20090123: Tightened this and the next 4220 paragraph to be clear that this doesn't authorize source route use.]] 4221 If source routes are used contrary to requirements and 4222 recommendations elsewhere in this specfiication, RFC 821 and the text 4223 below should be consulted for the mechanisms for constructing and 4224 updating the forward-path. A server that is reached by means of a 4225 source route (e.g., its domain name appears first in the list in the 4226 forward-path) MUST remove its domain name from any forward-paths in 4227 which that domain name appears before forwarding the message and MAY 4228 remove all other source routing information. The reverse-path SHOULD 4229 NOT be updated by servers conforming to this specification. 4231 Notice that the forward-path and reverse-path appear in the SMTP 4232 commands and replies, but not necessarily in the message. That is, 4233 there is no need for these paths and especially this syntax to appear 4234 in the "To:" , "From:", "CC:", etc. fields of the message header 4235 section. Conversely, SMTP servers MUST NOT derive final message 4236 routing information from message header fields. 4238 When the list of hosts is present despite the recommendations and 4239 requirements [[CREF27: [5321bis]JcK 20090123 "and requrements" 4240 added]] above, it is a "reverse" source route and indicates that the 4241 mail was relayed through each host on the list (the first host in the 4242 list was the most recent relay). This list is used as a source route 4243 to return non-delivery notices to the sender. If, contrary to the 4244 recommendations here, a relay host adds itself to the beginning of 4245 the list, it MUST use its name as known in the transport environment 4246 to which it is relaying the mail rather than that of the transport 4247 environment from which the mail came (if they are different). Note 4248 that a situation could easily arise in which some relay hosts add 4249 their names to the reverse source route and others do not, generating 4250 discontinuities in the routing list. This is another reason why 4251 servers needing to return a message SHOULD ignore the source route 4252 entirely and simply use the domain as specified in the Mailbox. 4254 Appendix D. Scenarios 4256 This section presents complete scenarios of several types of SMTP 4257 sessions. In the examples, "C:" indicates what is said by the SMTP 4258 client, and "S:" indicates what is said by the SMTP server. 4260 D.1. A Typical SMTP Transaction Scenario 4262 This SMTP example shows mail sent by Smith at host bar.com, and to 4263 Jones, Green, and Brown at host foo.com. Here we assume that host 4264 bar.com contacts host foo.com directly. The mail is accepted for 4265 Jones and Brown. Green does not have a mailbox at host foo.com. 4267 S: 220 foo.com Simple Mail Transfer Service Ready 4268 C: EHLO bar.com 4269 S: 250-foo.com greets bar.com 4270 S: 250-8BITMIME 4271 S: 250-SIZE 4272 S: 250-DSN 4273 S: 250 HELP 4274 C: MAIL FROM: 4275 S: 250 OK 4276 C: RCPT TO: 4277 S: 250 OK 4278 C: RCPT TO: 4279 S: 550 No such user here 4280 C: RCPT TO: 4281 S: 250 OK 4282 C: DATA 4283 S: 354 Start mail input; end with . 4284 C: Blah blah blah... 4285 C: ...etc. etc. etc. 4286 C: . 4287 S: 250 OK 4288 C: QUIT 4289 S: 221 foo.com Service closing transmission channel 4291 D.2. Aborted SMTP Transaction Scenario 4293 S: 220 foo.com Simple Mail Transfer Service Ready 4294 C: EHLO bar.com 4295 S: 250-foo.com greets bar.com 4296 S: 250-8BITMIME 4297 S: 250-SIZE 4298 S: 250-DSN 4299 S: 250 HELP 4300 C: MAIL FROM: 4301 S: 250 OK 4302 C: RCPT TO: 4303 S: 250 OK 4304 C: RCPT TO: 4305 S: 550 No such user here 4306 C: RSET 4307 S: 250 OK 4308 C: QUIT 4309 S: 221 foo.com Service closing transmission channel 4311 D.3. Relayed Mail Scenario 4313 Step 1 -- Source Host to Relay Host 4315 The source host performs a DNS lookup on XYZ.COM (the destination 4316 address) and finds DNS MX records specifying xyz.com as the best 4317 preference and foo.com as a lower preference. It attempts to open a 4318 connection to xyz.com and fails. It then opens a connection to 4319 foo.com, with the following dialogue: 4321 S: 220 foo.com Simple Mail Transfer Service Ready 4322 C: EHLO bar.com 4323 S: 250-foo.com greets bar.com 4324 S: 250-8BITMIME 4325 S: 250-SIZE 4326 S: 250-DSN 4327 S: 250 HELP 4328 C: MAIL FROM: 4329 S: 250 OK 4330 C: RCPT TO: 4331 S: 250 OK 4332 C: DATA 4333 S: 354 Start mail input; end with . 4334 C: Date: Thu, 21 May 1998 05:33:29 -0700 4335 C: From: John Q. Public 4336 C: Subject: The Next Meeting of the Board 4337 C: To: Jones@xyz.com 4338 C: 4339 C: Bill: 4340 C: The next meeting of the board of directors will be 4341 C: on Tuesday. 4342 C: John. 4343 C: . 4344 S: 250 OK 4345 C: QUIT 4346 S: 221 foo.com Service closing transmission channel 4348 Step 2 -- Relay Host to Destination Host 4350 foo.com, having received the message, now does a DNS lookup on 4351 xyz.com. It finds the same set of MX records, but cannot use the one 4352 that points to itself (or to any other host as a worse preference). 4353 It tries to open a connection to xyz.com itself and succeeds. Then 4354 we have: 4356 S: 220 xyz.com Simple Mail Transfer Service Ready 4357 C: EHLO foo.com 4358 S: 250 xyz.com is on the air 4359 C: MAIL FROM: 4360 S: 250 OK 4361 C: RCPT TO: 4362 S: 250 OK 4363 C: DATA 4364 S: 354 Start mail input; end with . 4365 C: Received: from bar.com by foo.com ; Thu, 21 May 1998 4366 C: 05:33:29 -0700 4367 C: Date: Thu, 21 May 1998 05:33:29 -0700 4368 C: From: John Q. Public 4369 C: Subject: The Next Meeting of the Board 4370 C: To: Jones@xyz.com 4371 C: 4372 C: Bill: 4373 C: The next meeting of the board of directors will be 4374 C: on Tuesday. 4375 C: John. 4376 C: . 4377 S: 250 OK 4378 C: QUIT 4379 S: 221 xyz.com Service closing transmission channel 4381 D.4. Verifying and Sending Scenario 4383 S: 220 foo.com Simple Mail Transfer Service Ready 4384 C: EHLO bar.com 4385 S: 250-foo.com greets bar.com 4386 S: 250-8BITMIME 4387 S: 250-SIZE 4388 S: 250-DSN 4389 S: 250-VRFY 4390 S: 250 HELP 4391 C: VRFY Crispin 4392 S: 250 Mark Crispin 4393 C: MAIL FROM: 4394 S: 250 OK 4395 C: RCPT TO: 4396 S: 250 OK 4397 C: DATA 4398 S: 354 Start mail input; end with . 4399 C: Blah blah blah... 4400 C: ...etc. etc. etc. 4401 C: . 4402 S: 250 OK 4403 C: QUIT 4404 S: 221 foo.com Service closing transmission channel 4406 Appendix E. Other Gateway Issues 4408 In general, gateways between the Internet and other mail systems 4409 SHOULD attempt to preserve any layering semantics across the 4410 boundaries between the two mail systems involved. Gateway- 4411 translation approaches that attempt to take shortcuts by mapping 4412 (such as mapping envelope information from one system to the message 4413 header section or body of another) have generally proven to be 4414 inadequate in important ways. Systems translating between 4415 environments that do not support both envelopes and a header section 4416 and Internet mail must be written with the understanding that some 4417 information loss is almost inevitable. 4419 Appendix F. Deprecated Features of RFC 821 4421 A few features of RFC 821 have proven to be problematic and SHOULD 4422 NOT be used in Internet mail. Some of these features were deprecated 4423 in RFC 2821 in 2001; source routing and two-digit years in dates were 4424 deprecated by RFC 1123 in 1989. Of the domain literal forms, RFC 4425 1123 required support only for the dotted decimal form. With the 4426 possible exception of old, hardware-embedded, applications, there is 4427 no longer any excuse for these features to appear on the contemporary 4428 Internet. [[CREF28: [5321bis] (2821ter) 2821bis Last Call Comment]] 4430 F.1. TURN 4432 This command, described in RFC 821, raises important security issues 4433 since, in the absence of strong authentication of the host requesting 4434 that the client and server switch roles, it can easily be used to 4435 divert mail from its correct destination. Its use is deprecated; 4436 SMTP systems SHOULD NOT use it unless the server can authenticate the 4437 client. 4439 F.2. Source Routing 4441 RFC 821 utilized the concept of explicit source routing to get mail 4442 from one host to another via a series of relays. The requirement to 4443 utilize source routes in regular mail traffic was eliminated by the 4444 introduction of the domain name system "MX" record and the last 4445 significant justification for them was eliminated by the 4446 introduction, in RFC 1123, of a clear requirement that addresses 4447 following an "@" must all be fully-qualified domain names. 4448 Consequently, the only remaining justifications for the use of source 4449 routes are support for very old SMTP clients or MUAs and in mail 4450 system debugging. They can, however, still be useful in the latter 4451 circumstance and for routing mail around serious, but temporary, 4452 problems such as problems with the relevant DNS records. 4454 SMTP servers MUST continue to accept source route syntax as specified 4455 in the main body of this document and in RFC 1123. They MAY, if 4456 necessary, ignore the routes and utilize only the target domain in 4457 the address. If they do utilize the source route, the message MUST 4458 be sent to the first domain shown in the address. In particular, a 4459 server MUST NOT guess at shortcuts within the source route. 4461 Clients SHOULD NOT utilize explicit source routing except under 4462 unusual circumstances, such as debugging or potentially relaying 4463 around firewall or mail system configuration errors. 4465 F.3. HELO 4467 As discussed in Sections 3.1 and 4.1.1, EHLO SHOULD be used rather 4468 than HELO when the server will accept the former. Servers MUST 4469 continue to accept and process HELO in order to support older 4470 clients. 4472 F.4. #-literals 4474 RFC 821 provided for specifying an Internet address as a decimal 4475 integer host number prefixed by a pound sign, "#". In practice, that 4476 form has been obsolete since the introduction of TCP/IP. It is 4477 deprecated and MUST NOT be used. 4479 F.5. Dates and Years 4481 When dates are inserted into messages by SMTP clients or servers 4482 (e.g., in trace header fields), four-digit years MUST BE used. Two- 4483 digit years are deprecated; three-digit years were never permitted in 4484 the Internet mail system. 4486 F.6. Sending versus Mailing 4488 In addition to specifying a mechanism for delivering messages to 4489 user's mailboxes, RFC 821 provided additional, optional, commands to 4490 deliver messages directly to the user's terminal screen. These 4491 commands (SEND, SAML, SOML) were rarely implemented, and changes in 4492 workstation technology and the introduction of other protocols may 4493 have rendered them obsolete even where they are implemented. 4494 [[5321bis Editor's Note: does this need a stronger reference to 821, 4495 2821, and/or 5321?]] 4497 Clients SHOULD NOT provide SEND, SAML, or SOML as services. Servers 4498 MAY implement them. If they are implemented by servers, the 4499 implementation model specified in RFC 821 MUST be used and the 4500 command names MUST be published in the response to the EHLO command. 4502 Appendix G. Change log for RFC 5321bis 4504 [[RFC Editor: Please remove this section before publication.]] 4506 G.1. RFC 5321 Errata Summary 4508 This document addresses the following errata filed against RFC 5321 4509 since its publication in October 2008 [52]. [[CREF29: [[Note in 4510 Draft: Items with comments below have not yet been resolved.]]]] 4512 1683 ABNF error. Section 4.4 4514 4198 Description error. Section 4.2 4516 2578 Syntax description error. Section 4.1.2 4518 1543 Wrong code in description Section 3.8 4520 4315 ABNF - IPv6 Section 4.1.3. [[CREF30: [5321bis]The IPv6 syntax 4521 has been adjusted since 5321 was published. See the rewritten 4522 form and the comment in the section cited in the previous 4523 sentence. The editor awaits instructions. See https://www.rfc- 4524 editor.org/errata/eid4315]] 4526 5414 ABNF for Quoted-string Section 4.1.2 4528 1851 Location of text on unexpected close Section 4.1.1.5. 4529 [[CREF31: [5321bis]Matter of taste, editor seeks advice.]] 4531 3447 Use of normative language (e.g., more "MUST"s), possible 4532 confusion in some sections Section 4.4. [[CREF32: [5321bis]As 4533 Barry notes in his verifier comments on the erratum (see 4534 https://www.rfc-editor.org/errata/eid3447), the comments and 4535 suggestions here raise a number of interesting (and difficult) 4536 issues. One of the issues is that the core of RFCs 5321 (and 4537 2821) is text carried over from Jon Postel's RFC 821, a document 4538 that was not only written in a different style than the IETF uses 4539 today but that was written at a time when no one had dreamt of RFC 4540 2119 or even the IETF itself. It appears to me that trying to 4541 patch that style might easily result in a document that is harder 4542 to read as well as being error prone. If we want to get the 4543 document entirely into contemporary style, we really should bite 4544 the bullet and do a complete rewrite. To respond to a different 4545 point in Barry's discussion, I think an explicit statement that 4546 5321/5322 and their predecessors differ in places and why would be 4547 helpful. Text, and suggestions about where to put it, are 4548 solicited. A list of differences might be a good idea too, but 4549 getting it right might be more work than there is available energy 4550 to do correctly. ]] 4552 5711 Missing leading spaces in example Appendix D.3. [[CREF33: 4553 [5321bis]Well, this is interesting because the XML is correct and 4554 the spaces are there, embedded in artwork. So either the XML2RFC 4555 processor at the time took those leading spaces out or the RFC 4556 Editor improved on the document and the change was not caught in 4557 AUTH48, perhaps because rfcdiff ignores white space. We just need 4558 to watch for future iterations. ]] 4560 [[CREF34: [5321bis]Note that rejected errata have _not_ been reviewed 4561 to see if they contain anything useful that should be discussed again 4562 with the possibility of rethinking and changing text. Volunteers 4563 sought.]] 4565 G.2. Changes from RFC 5321 (published October 2008) to the initial 4566 (-00) version of this draft 4568 o Acknowledgments section (Section 9) trimmed back for new document. 4570 o Introductory paragraph to Appendix F extended to make it clear 4571 that these features were deprecated a long time ago and really 4572 should not be in use any more. 4574 o Adjusted some language to clarify that source routes really, 4575 really, should not be used or depended upon. 4577 o IPv6 address syntax replaced by a copy of the IPv6 URI syntax and 4578 a note added. 4580 o Production index added as a first step in tying all productions to 4581 their sources. As part of the effort to make the document more 4582 easily navigable, table of contents entries have been created for 4583 the individual command descriptions. 4585 o Clarified the relationship between the SMTP "letters, digits, and 4586 hyphens" and DNS "preferred name syntax" (Section 2.3.5). 4588 o Revised the reply code sections to add new 521 and 556 codes, 4589 clarify relationships, and be explicit about the requirement for 4590 clients to rely on first digits rather than the sequences in 4591 Section 4.3.2. 4593 o In conjunction with the above, explicitly obsolete RFCs 1846 and 4594 7504. 4596 o Incorporated a correction reflecting Errata ID 2578. 4598 o Some small editorial changes made to eliminate redundant 4599 statements that were very close together. Other, equally small, 4600 editorial changes have been made to improve grammar or clarity. 4602 o A few questions, marked "[[5321bis Editor's Note:", or "[[Note in 4603 Draft" have been added for the group to resolve. Other questions, 4604 especially those in the errata summary, are simply included in 4605 narrative comments in CREFs. 4607 o Checked and rationalized "response" (to a command) and "reply 4608 code" terminology. One can talk about a "999 response" but only a 4609 "999 reply code". There is no such thing as a "response code". 4611 o Added note about length limit on mailbox names ("email 4612 addresses"). 4614 o Added an "errata summary" subsection to this change log/ 4615 comparison to 5321 in this Appendix. The entire Appendix will, of 4616 course, disappear at the time of RFC publication unless someone 4617 wants to make a strong case for retaining it. 4619 o Rationalized CREFs to 2821, 5321, 5321bis etc.; added note to 4620 readers below the Abstract. 4622 o Temporarily added a "Note on Reading This Working Draft" after the 4623 Abstract. 4625 G.3. Changes Among Versions of Rfc5321Bis 4627 G.3.1. Changes from draft-klensin-rfc5321bis-00 (posted 2012-12-02) to 4628 -01 4630 Substantively, these two versions differ only by suppression of the 4631 CREF and other discussion associated with the evolution from RFC 2821 4632 to RFC 5321. That change includes an update to the document's Note 4633 to Readers, the date, the file name, and the addition of this change 4634 log subsection. 4636 Index 4638 A 4639 Argument Syntax 4640 A-d-l 41 4641 Additional-Registered-Clauses 62 4642 address-literal 42 4643 Addtl-Link 62 4644 Addtl-Protocol 62 4645 Argument 42 4646 At-domain 41 4647 Atom 42 4648 By-domain 61 4649 dcontent 44 4650 Domain 42 4651 Dot-string 42 4652 esmtp-keyword 41 4653 esmtp-param 41 4654 esmtp-value 42 4655 Extended-Domain 61 4656 For 62 4657 Forward-Path 41 4658 From-domain 61 4659 General-address-literal 44 4660 Greeting 47 4661 h16 44 4662 ID 62 4663 IPv4-address-literal 44 4664 IPv6-addr 44 4665 IPv6-address-literal 44 4666 Keyword 42 4667 Ldh-str 42 4668 Let-dig 42 4669 Link 62 4670 Local-part 42 4671 ls32 44 4672 Mail-parameters 41 4673 Mailbox 42 4674 Opt-info 62 4675 Path 41 4676 Protocol 62 4677 QcontentSMTP 42 4678 qtextSMTP 42 4679 quoted-pairSMTP 42 4680 Quoted-string 42 4681 Rcpt-parameters 41 4682 Reply-code 47 4683 Reply-line 47 4684 Return-path-line 61 4685 Reverse-Path 41 4686 Snum 44 4687 Stamp 61 4688 Standardized-tag 44 4689 String 42 4690 sub-domain 42 4691 TCP-info 62 4692 textstring 47 4693 Time-stamp-line 61 4694 Via 62 4695 With 62 4697 C 4698 Command Syntax 4699 data 38 4700 expn 39 4701 help 40 4702 mail 35 4703 noop 40 4704 quit 40 4705 rcpt 37 4706 rset 39 4707 vrfy 39 4709 Author's Address 4711 John C. Klensin 4712 1770 Massachusetts Ave, Suite 322 4713 Cambridge, MA 02140 4714 USA 4716 EMail: john-ietf@jck.com