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If you are able to get all authors (current and original) to grant those rights, you can and should remove the disclaimer; otherwise, the disclaimer is needed and you can ignore this comment. (See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (6 October 2020) is 1291 days in the past. Is this intentional? 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Resnick, Ed. 3 Internet-Draft Episteme 4 Obsoletes: 5322 (if approved) 6 October 2020 5 Updates: 4021 (if approved) 6 Intended status: Standards Track 7 Expires: 9 April 2021 9 Internet Message Format 10 draft-ietf-emailcore-rfc5322bis-00 12 Abstract 14 This document specifies the Internet Message Format (IMF), a syntax 15 for text messages that are sent between computer users, within the 16 framework of "electronic mail" messages. This specification is a 17 revision of Request For Comments (RFC) 5322, itself a revision of 18 Request For Comments (RFC) 2822, all of which supersede Request For 19 Comments (RFC) 822, "Standard for the Format of ARPA Internet Text 20 Messages", updating it to reflect current practice and incorporating 21 incremental changes that were specified in other RFCs. 23 Status of This Memo 25 This Internet-Draft is submitted in full conformance with the 26 provisions of BCP 78 and BCP 79. 28 Internet-Drafts are working documents of the Internet Engineering 29 Task Force (IETF). Note that other groups may also distribute 30 working documents as Internet-Drafts. The list of current Internet- 31 Drafts is at https://datatracker.ietf.org/drafts/current/. 33 Internet-Drafts are draft documents valid for a maximum of six months 34 and may be updated, replaced, or obsoleted by other documents at any 35 time. It is inappropriate to use Internet-Drafts as reference 36 material or to cite them other than as "work in progress." 38 This Internet-Draft will expire on 9 April 2021. 40 Copyright Notice 42 Copyright (c) 2020 IETF Trust and the persons identified as the 43 document authors. All rights reserved. 45 This document is subject to BCP 78 and the IETF Trust's Legal 46 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 47 license-info) in effect on the date of publication of this document. 48 Please review these documents carefully, as they describe your rights 49 and restrictions with respect to this document. Code Components 50 extracted from this document must include Simplified BSD License text 51 as described in Section 4.e of the Trust Legal Provisions and are 52 provided without warranty as described in the Simplified BSD License. 54 This document may contain material from IETF Documents or IETF 55 Contributions published or made publicly available before November 56 10, 2008. The person(s) controlling the copyright in some of this 57 material may not have granted the IETF Trust the right to allow 58 modifications of such material outside the IETF Standards Process. 59 Without obtaining an adequate license from the person(s) controlling 60 the copyright in such materials, this document may not be modified 61 outside the IETF Standards Process, and derivative works of it may 62 not be created outside the IETF Standards Process, except to format 63 it for publication as an RFC or to translate it into languages other 64 than English. 66 Table of Contents 68 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 69 1.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 4 70 1.2. Notational Conventions . . . . . . . . . . . . . . . . . 5 71 1.2.1. Requirements Notation . . . . . . . . . . . . . . . . 5 72 1.2.2. Syntactic Notation . . . . . . . . . . . . . . . . . 5 73 1.2.3. Structure of This Document . . . . . . . . . . . . . 5 74 2. Lexical Analysis of Messages . . . . . . . . . . . . . . . . 6 75 2.1. General Description . . . . . . . . . . . . . . . . . . . 6 76 2.1.1. Line Length Limits . . . . . . . . . . . . . . . . . 7 77 2.2. Header Fields . . . . . . . . . . . . . . . . . . . . . . 8 78 2.2.1. Unstructured Header Field Bodies . . . . . . . . . . 8 79 2.2.2. Structured Header Field Bodies . . . . . . . . . . . 8 80 2.2.3. Long Header Fields . . . . . . . . . . . . . . . . . 9 81 2.3. Body . . . . . . . . . . . . . . . . . . . . . . . . . . 9 82 3. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 83 3.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 10 84 3.2. Lexical Tokens . . . . . . . . . . . . . . . . . . . . . 10 85 3.2.1. Quoted characters . . . . . . . . . . . . . . . . . . 11 86 3.2.2. Folding White Space and Comments . . . . . . . . . . 11 87 3.2.3. Atom . . . . . . . . . . . . . . . . . . . . . . . . 12 88 3.2.4. Quoted Strings . . . . . . . . . . . . . . . . . . . 13 89 3.2.5. Miscellaneous Tokens . . . . . . . . . . . . . . . . 14 90 3.3. Date and Time Specification . . . . . . . . . . . . . . . 15 91 3.4. Address Specification . . . . . . . . . . . . . . . . . . 16 92 3.4.1. Addr-Spec Specification . . . . . . . . . . . . . . . 17 93 3.5. Overall Message Syntax . . . . . . . . . . . . . . . . . 18 94 3.6. Field Definitions . . . . . . . . . . . . . . . . . . . . 19 95 3.6.1. The Origination Date Field . . . . . . . . . . . . . 22 96 3.6.2. Originator Fields . . . . . . . . . . . . . . . . . . 22 97 3.6.3. Destination Address Fields . . . . . . . . . . . . . 23 98 3.6.4. Identification Fields . . . . . . . . . . . . . . . . 25 99 3.6.5. Informational Fields . . . . . . . . . . . . . . . . 28 100 3.6.6. Resent Fields . . . . . . . . . . . . . . . . . . . . 29 101 3.6.7. Trace Fields . . . . . . . . . . . . . . . . . . . . 31 102 3.6.8. Optional Fields . . . . . . . . . . . . . . . . . . . 32 103 4. Obsolete Syntax . . . . . . . . . . . . . . . . . . . . . . . 32 104 4.1. Miscellaneous Obsolete Tokens . . . . . . . . . . . . . . 33 105 4.2. Obsolete Folding White Space . . . . . . . . . . . . . . 34 106 4.3. Obsolete Date and Time . . . . . . . . . . . . . . . . . 34 107 4.4. Obsolete Addressing . . . . . . . . . . . . . . . . . . . 36 108 4.5. Obsolete Header Fields . . . . . . . . . . . . . . . . . 37 109 4.5.1. Obsolete Origination Date Field . . . . . . . . . . . 38 110 4.5.2. Obsolete Originator Fields . . . . . . . . . . . . . 38 111 4.5.3. Obsolete Destination Address Fields . . . . . . . . . 38 112 4.5.4. Obsolete Identification Fields . . . . . . . . . . . 39 113 4.5.5. Obsolete Informational Fields . . . . . . . . . . . . 39 114 4.5.6. Obsolete Resent Fields . . . . . . . . . . . . . . . 40 115 4.5.7. Obsolete Trace Fields . . . . . . . . . . . . . . . . 40 116 4.5.8. Obsolete optional fields . . . . . . . . . . . . . . 40 117 5. Security Considerations . . . . . . . . . . . . . . . . . . . 41 118 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 42 119 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 45 120 7.1. Normative References . . . . . . . . . . . . . . . . . . 45 121 7.2. Informative References . . . . . . . . . . . . . . . . . 45 122 Appendix A. Example Messages . . . . . . . . . . . . . . . . . . 47 123 A.1. Addressing Examples . . . . . . . . . . . . . . . . . . . 47 124 A.1.1. A Message from One Person to Another with Simple 125 Addressing . . . . . . . . . . . . . . . . . . . . . 47 126 A.1.2. Different Types of Mailboxes . . . . . . . . . . . . 48 127 A.1.3. Group Addresses . . . . . . . . . . . . . . . . . . . 48 128 A.2. Reply Messages . . . . . . . . . . . . . . . . . . . . . 49 129 A.3. Resent Messages . . . . . . . . . . . . . . . . . . . . . 50 130 A.4. Messages with Trace Fields . . . . . . . . . . . . . . . 51 131 A.5. White Space, Comments, and Other Oddities . . . . . . . . 51 132 A.6. Obsoleted Forms . . . . . . . . . . . . . . . . . . . . . 52 133 A.6.1. Obsolete Addressing . . . . . . . . . . . . . . . . . 52 134 A.6.2. Obsolete Dates . . . . . . . . . . . . . . . . . . . 52 135 A.6.3. Obsolete White Space and Comments . . . . . . . . . . 53 136 Appendix B. Differences from Earlier Specifications . . . . . . 53 137 Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 56 138 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 56 140 1. Introduction 141 1.1. Scope 143 This document specifies the Internet Message Format (IMF), a syntax 144 for text messages that are sent between computer users, within the 145 framework of "electronic mail" messages. This specification is an 146 update to [RFC5322], itself a revision of [RFC2822], all of which 147 supersede [RFC0822], updating it to reflect current practice and 148 incorporating incremental changes that were specified in other RFCs 149 such as [RFC1123]. 151 This document specifies a syntax only for text messages. In 152 particular, it makes no provision for the transmission of images, 153 audio, or other sorts of structured data in electronic mail messages. 154 There are several extensions published, such as the MIME document 155 series ([RFC2045], [RFC2046], [RFC2049]), which describe mechanisms 156 for the transmission of such data through electronic mail, either by 157 extending the syntax provided here or by structuring such messages to 158 conform to this syntax. Those mechanisms are outside of the scope of 159 this specification. 161 In the context of electronic mail, messages are viewed as having an 162 envelope and contents. The envelope contains whatever information is 163 needed to accomplish transmission and delivery. (See 164 [I-D.klensin-rfc5321bis] for a discussion of the envelope.) The 165 contents comprise the object to be delivered to the recipient. This 166 specification applies only to the format and some of the semantics of 167 message contents. It contains no specification of the information in 168 the envelope. 170 However, some message systems may use information from the contents 171 to create the envelope. It is intended that this specification 172 facilitate the acquisition of such information by programs. 174 This specification is intended as a definition of what message 175 content format is to be passed between systems. Though some message 176 systems locally store messages in this format (which eliminates the 177 need for translation between formats) and others use formats that 178 differ from the one specified in this specification, local storage is 179 outside of the scope of this specification. 181 | Note: This specification is not intended to dictate the 182 | internal formats used by sites, the specific message system 183 | features that they are expected to support, or any of the 184 | characteristics of user interface programs that create or read 185 | messages. In addition, this document does not specify an 186 | encoding of the characters for either transport or storage; 187 | that is, it does not specify the number of bits used or how 188 | those bits are specifically transferred over the wire or stored 189 | on disk. 191 1.2. Notational Conventions 193 1.2.1. Requirements Notation 195 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 196 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 197 "OPTIONAL" in this document are to be interpreted as described in 198 [BCP14] RFC2119 RFC8174 when, and only when, they appear in all 199 capitals, as shown here. 201 1.2.2. Syntactic Notation 203 This specification uses the Augmented Backus-Naur Form (ABNF) [STD68] 204 notation for the formal definitions of the syntax of messages. 205 Characters will be specified either by a decimal value (e.g., the 206 value %d65 for uppercase A and %d97 for lowercase A) or by a case- 207 insensitive literal value enclosed in quotation marks (e.g., "A" for 208 either uppercase or lowercase A). 210 1.2.3. Structure of This Document 212 This document is divided into several sections. 214 This section, section 1, is a short introduction to the document. 216 Section 2 lays out the general description of a message and its 217 constituent parts. This is an overview to help the reader understand 218 some of the general principles used in the later portions of this 219 document. Any examples in this section MUST NOT be taken as 220 specification of the formal syntax of any part of a message. 222 Section 3 specifies formal ABNF rules for the structure of each part 223 of a message (the syntax) and describes the relationship between 224 those parts and their meaning in the context of a message (the 225 semantics). That is, it lays out the actual rules for the structure 226 of each part of a message (the syntax) as well as a description of 227 the parts and instructions for their interpretation (the semantics). 228 This includes analysis of the syntax and semantics of subparts of 229 messages that have specific structure. The syntax included in 230 section 3 represents messages as they MUST be created. There are 231 also notes in section 3 to indicate if any of the options specified 232 in the syntax SHOULD be used over any of the others. 234 Both sections 2 and 3 describe messages that are legal to generate 235 for purposes of this specification. 237 Section 4 of this document specifies an "obsolete" syntax. There are 238 references in section 3 to these obsolete syntactic elements. The 239 rules of the obsolete syntax are elements that have appeared in 240 earlier versions of this specification or have previously been widely 241 used in Internet messages. As such, these elements MUST be 242 interpreted by parsers of messages in order to be conformant to this 243 specification. However, since items in this syntax have been 244 determined to be non-interoperable or to cause significant problems 245 for recipients of messages, they MUST NOT be generated by creators of 246 conformant messages. 248 Section 5 details security considerations to take into account when 249 implementing this specification. 251 Appendix A lists examples of different sorts of messages. These 252 examples are not exhaustive of the types of messages that appear on 253 the Internet, but give a broad overview of certain syntactic forms. 255 Appendix B lists the differences between this specification and 256 earlier specifications for Internet messages. 258 Appendix C contains acknowledgements. 260 2. Lexical Analysis of Messages 262 2.1. General Description 264 At the most basic level, a message is a series of characters. A 265 message that is conformant with this specification is composed of 266 characters with values in the range of 1 through 127 and interpreted 267 as US-ASCII [ANSI.X3-4.1986] characters. For brevity, this document 268 sometimes refers to this range of characters as simply "US-ASCII 269 characters". 271 | Note: This document specifies that messages are made up of 272 | characters in the US-ASCII range of 1 through 127. There are 273 | other documents, specifically the MIME document series 274 | ([RFC2045], [RFC2046], [RFC2047], [RFC2049], [BCP13]) and the 275 | Internationalized Email Headers specification ([RFC6532]), that 276 | extend this specification to allow for values outside of that 277 | range. Discussion of those mechanisms is not within the scope 278 | of this specification. 280 Messages are divided into lines of characters. A line is a series of 281 characters that is delimited with the two characters carriage-return 282 and line-feed; that is, the carriage return (CR) character (ASCII 283 value 13) followed immediately by the line feed (LF) character (ASCII 284 value 10). (The carriage return/line feed pair is usually written in 285 this document as "CRLF".) 287 A message consists of header fields (collectively called "the header 288 section of the message") followed, optionally, by a body. The header 289 section is a sequence of lines of characters with special syntax as 290 defined in this specification. The body is simply a sequence of 291 characters that follows the header section and is separated from the 292 header section by an empty line (i.e., a line with nothing preceding 293 the CRLF). 295 | Note: Common parlance and earlier versions of this 296 | specification use the term "header" to either refer to the 297 | entire header section or to refer to an individual header 298 | field. To avoid ambiguity, this document does not use the 299 | terms "header" or "headers" in isolation, but instead always 300 | uses "header field" to refer to the individual field and 301 | "header section" to refer to the entire collection. 303 2.1.1. Line Length Limits 305 There are two limits that this specification places on the number of 306 characters in a line. Each line of characters MUST be no more than 307 998 characters, and SHOULD be no more than 78 characters, excluding 308 the CRLF. 310 The 998 character limit is due to limitations in many implementations 311 that send, receive, or store IMF messages which simply cannot handle 312 more than 998 characters on a line. Receiving implementations would 313 do well to handle an arbitrarily large number of characters in a line 314 for robustness sake. However, there are so many implementations that 315 (in compliance with the transport requirements of 316 [I-D.klensin-rfc5321bis]) do not accept messages containing more than 317 1000 characters including the CR and LF per line, it is important for 318 implementations not to create such messages. 320 The more conservative 78 character recommendation is to accommodate 321 the many implementations of user interfaces that display these 322 messages which may truncate, or disastrously wrap, the display of 323 more than 78 characters per line, in spite of the fact that such 324 implementations are non-conformant to the intent of this 325 specification (and that of [I-D.klensin-rfc5321bis] if they actually 326 cause information to be lost). Again, even though this limitation is 327 put on messages, it is incumbent upon implementations that display 328 messages to handle an arbitrarily large number of characters in a 329 line (certainly at least up to the 998 character limit) for the sake 330 of robustness. 332 2.2. Header Fields 334 Header fields are lines beginning with a field name, followed by a 335 colon (":"), followed by a field body, and terminated by CRLF. A 336 field name MUST be composed of visible US-ASCII characters (i.e., 337 characters that have values between 33 and 126, inclusive), except 338 colon. A field body may be composed of visible US-ASCII characters 339 as well as the space (SP, ASCII value 32) and horizontal tab (HTAB, 340 ASCII value 9) characters (together known as the white space 341 characters, WSP). A field body MUST NOT include CR and LF except 342 when used in "folding" and "unfolding", as described in section 343 2.2.3. All field bodies MUST conform to the syntax described in 344 sections 3 and 4 of this specification. 346 2.2.1. Unstructured Header Field Bodies 348 Some field bodies in this specification are defined simply as 349 "unstructured" (which is specified in section 3.2.5 as any visible 350 US-ASCII characters plus white space characters) with no further 351 restrictions. These are referred to as unstructured field bodies. 352 Semantically, unstructured field bodies are simply to be treated as a 353 single line of characters with no further processing (except for 354 "folding" and "unfolding" as described in section 2.2.3). 356 2.2.2. Structured Header Field Bodies 358 Some field bodies in this specification have a syntax that is more 359 restrictive than the unstructured field bodies described above. 360 These are referred to as "structured" field bodies. Structured field 361 bodies are sequences of specific lexical tokens as described in 362 sections 3 and 4 of this specification. Many of these tokens are 363 allowed (according to their syntax) to be introduced or end with 364 comments (as described in section 3.2.2) as well as the white space 365 characters, and those white space characters are subject to "folding" 366 and "unfolding" as described in section 2.2.3. Semantic analysis of 367 structured field bodies is given along with their syntax. 369 2.2.3. Long Header Fields 371 Each header field is logically a single line of characters comprising 372 the field name, the colon, and the field body. For convenience 373 however, and to deal with the 998/78 character limitations per line, 374 the field body portion of a header field can be split into a 375 multiple-line representation; this is called "folding". The general 376 rule is that wherever this specification allows for folding white 377 space (not simply WSP characters), a CRLF may be inserted before any 378 WSP. 380 For example, the header field: 382 Subject: This is a test 384 can be represented as: 386 Subject: This 387 is a test 389 | Note: Though structured field bodies are defined in such a way 390 | that folding can take place between many of the lexical tokens 391 | (and even within some of the lexical tokens), folding SHOULD be 392 | limited to placing the CRLF at higher-level syntactic breaks. 393 | For instance, if a field body is defined as comma-separated 394 | values, it is recommended that folding occur after the comma 395 | separating the structured items in preference to other places 396 | where the field could be folded, even if it is allowed 397 | elsewhere. 399 The process of moving from this folded multiple-line representation 400 of a header field to its single line representation is called 401 "unfolding". Unfolding is accomplished by simply removing any CRLF 402 that is immediately followed by WSP. Each header field should be 403 treated in its unfolded form for further syntactic and semantic 404 evaluation. An unfolded header field has no length restriction and 405 therefore may be indeterminately long. 407 2.3. Body 409 The body of a message is simply lines of US-ASCII characters. The 410 only two limitations on the body are as follows: 412 * CR and LF MUST only occur together as CRLF; they MUST NOT appear 413 independently in the body. 415 * Lines of characters in the body MUST be limited to 998 characters, 416 and SHOULD be limited to 78 characters, excluding the CRLF. 418 | Note: As was stated earlier, there are other documents, 419 | specifically the MIME documents ([RFC2045], [RFC2046], 420 | [RFC2049], [BCP13]), that extend (and limit) this specification 421 | to allow for different sorts of message bodies. Again, these 422 | mechanisms are beyond the scope of this document. 424 3. Syntax 426 3.1. Introduction 428 The syntax as given in this section defines the legal syntax of 429 Internet messages. Messages that are conformant to this 430 specification MUST conform to the syntax in this section. If there 431 are options in this section where one option SHOULD be generated, 432 that is indicated either in the prose or in a comment next to the 433 syntax. 435 For the defined expressions, a short description of the syntax and 436 use is given, followed by the syntax in ABNF, followed by a semantic 437 analysis. The following primitive tokens that are used but otherwise 438 unspecified are taken from the "Core Rules" of [STD68], Appendix B.1: 439 CR, LF, CRLF, HTAB, SP, WSP, DQUOTE, DIGIT, ALPHA, and VCHAR. 441 In some of the definitions, there will be non-terminals whose names 442 start with "obs-". These "obs-" elements refer to tokens defined in 443 the obsolete syntax in section 4. In all cases, these productions 444 are to be ignored for the purposes of generating legal Internet 445 messages and MUST NOT be used as part of such a message. However, 446 when interpreting messages, these tokens MUST be honored as part of 447 the legal syntax. In this sense, section 3 defines a grammar for the 448 generation of messages, with "obs-" elements that are to be ignored, 449 while section 4 adds grammar for the interpretation of messages. 451 3.2. Lexical Tokens 453 The following rules are used to define an underlying lexical 454 analyzer, which feeds tokens to the higher-level parsers. This 455 section defines the tokens used in structured header field bodies. 457 | Note: Readers of this specification need to pay special 458 | attention to how these lexical tokens are used in both the 459 | lower-level and higher-level syntax later in the document. 460 | Particularly, the white space tokens and the comment tokens 461 | defined in section 3.2.2 get used in the lower-level tokens 462 | defined here, and those lower-level tokens are in turn used as 463 | parts of the higher-level tokens defined later. Therefore, 464 | white space and comments may be allowed in the higher-level 465 | tokens even though they may not explicitly appear in a 466 | particular definition. 468 3.2.1. Quoted characters 470 Some characters are reserved for special interpretation, such as 471 delimiting lexical tokens. To permit use of these characters as 472 uninterpreted data, a quoting mechanism is provided. 474 quoted-pair = ("\" (VCHAR / WSP)) / obs-qp 476 Where any quoted-pair appears, it is to be interpreted as the 477 character alone. That is to say, the "\" character that appears as 478 part of a quoted-pair is semantically "invisible". 480 | Note: The "\" character may appear in a message where it is not 481 | part of a quoted-pair. A "\" character that does not appear in 482 | a quoted-pair is not semantically invisible. The only places 483 | in this specification where quoted-pair currently appears are 484 | ccontent, qcontent, and in obs-dtext in section 4. 486 3.2.2. Folding White Space and Comments 488 White space characters, including white space used in folding 489 (described in section 2.2.3), may appear between many elements in 490 header field bodies. Also, strings of characters that are treated as 491 comments may be included in structured field bodies as characters 492 enclosed in parentheses. The following defines the folding white 493 space (FWS) and comment constructs. 495 Strings of characters enclosed in parentheses are considered comments 496 so long as they do not appear within a "quoted-string", as defined in 497 section 3.2.4. Comments may nest. 499 There are several places in this specification where comments and FWS 500 may be freely inserted. To accommodate that syntax, an additional 501 token for "CFWS" is defined for places where comments and/or FWS can 502 occur. However, where CFWS occurs in this specification, it MUST NOT 503 be inserted in such a way that any line of a folded header field is 504 made up entirely of WSP characters and nothing else. 506 FWS = ([*WSP CRLF] 1*WSP) / obs-FWS 507 ; Folding white space 509 ctext = %d33-39 / ; Visible US-ASCII 510 %d42-91 / ; characters not including 511 %d93-126 / ; "(", ")", or "\" 512 obs-ctext 514 ccontent = ctext / quoted-pair / comment 516 comment = "(" *([FWS] ccontent) [FWS] ")" 518 CFWS = (1*([FWS] comment) [FWS]) / FWS 520 Throughout this specification, where FWS (the folding white space 521 token) appears, it indicates a place where folding, as discussed in 522 section 2.2.3, may take place. Wherever folding appears in a message 523 (that is, a header field body containing a CRLF followed by any WSP), 524 unfolding (removal of the CRLF) is performed before any further 525 semantic analysis is performed on that header field according to this 526 specification. That is to say, any CRLF that appears in FWS is 527 semantically "invisible". 529 A comment is normally used in a structured field body to provide some 530 human-readable informational text. Since a comment is allowed to 531 contain FWS, folding is permitted within the comment. Also note that 532 since quoted-pair is allowed in a comment, the parentheses and 533 backslash characters may appear in a comment, so long as they appear 534 as a quoted-pair. Semantically, the enclosing parentheses are not 535 part of the comment; the comment is what is contained between the two 536 parentheses. As stated earlier, the "\" in any quoted-pair and the 537 CRLF in any FWS that appears within the comment are semantically 538 "invisible" and therefore not part of the comment either. 540 Runs of FWS, comment, or CFWS that occur between lexical tokens in a 541 structured header field are semantically interpreted as a single 542 space character. 544 3.2.3. Atom 546 Several productions in structured header field bodies are simply 547 strings of certain basic characters. Such productions are called 548 atoms. 550 Some of the structured header field bodies also allow the period 551 character (".", ASCII value 46) within runs of atext. An additional 552 "dot-atom" token is defined for those purposes. 554 | Note: The "specials" token does not appear anywhere else in 555 | this specification. It is simply the visible (i.e., non- 556 | control, non-white space) characters that do not appear in 557 | atext. It is provided only because it is useful for 558 | implementers who use tools that lexically analyze messages. 559 | Each of the characters in specials can be used to indicate a 560 | tokenization point in lexical analysis. 562 atext = ALPHA / DIGIT / ; Visible US-ASCII 563 "!" / "#" / ; characters not including 564 "$" / "%" / ; specials. Used for atoms. 565 "&" / "'" / 566 "*" / "+" / 567 "-" / "/" / 568 "=" / "?" / 569 "^" / "_" / 570 "`" / "{" / 571 "|" / "}" / 572 "~" 574 atom = [CFWS] 1*atext [CFWS] 576 dot-atom-text = 1*atext *("." 1*atext) 578 dot-atom = [CFWS] dot-atom-text [CFWS] 580 specials = "(" / ")" / ; Special characters that do 581 "<" / ">" / ; not appear in atext 582 "[" / "]" / 583 ":" / ";" / 584 "@" / "\" / 585 "," / "." / 586 DQUOTE 588 Both atom and dot-atom are interpreted as a single unit, comprising 589 the string of characters that make it up. Semantically, the optional 590 comments and FWS surrounding the rest of the characters are not part 591 of the atom; the atom is only the run of atext characters in an atom, 592 or the atext and "." characters in a dot-atom. 594 3.2.4. Quoted Strings 596 Strings of characters that include characters other than those 597 allowed in atoms can be represented in a quoted string format, where 598 the characters are surrounded by quote (DQUOTE, ASCII value 34) 599 characters. 601 qtext = %d33 / ; Visible US-ASCII 602 %d35-91 / ; characters not including 603 %d93-126 / ; "\" or the quote character 604 obs-qtext 606 qcontent = qtext / quoted-pair 608 quoted-string = [CFWS] 609 DQUOTE *([FWS] qcontent) [FWS] DQUOTE 610 [CFWS] 612 // Erratum 3135 (https://www.rfc-editor.org/errata/eid3135) wanted to 613 // disallow empty quoted strings. There doesn't appear to be 614 // consensus for that (e.g., see discussion starting at 615 // https://mailarchive.ietf.org/arch/msg/ietf- 616 // 822/9NByCGWq7_dOLRNhrPkZR24074g) and therefore this erratum 617 // probably should have been rejected. 619 A quoted-string is treated as a unit. That is, quoted-string is 620 identical to atom, semantically. Since a quoted-string is allowed to 621 contain FWS, folding is permitted. Also note that since quoted-pair 622 is allowed in a quoted-string, the quote and backslash characters may 623 appear in a quoted-string so long as they appear as a quoted-pair. 625 Semantically, neither the optional CFWS outside of the quote 626 characters nor the quote characters themselves are part of the 627 quoted-string; the quoted-string is what is contained between the two 628 quote characters. As stated earlier, the "\" in any quoted-pair and 629 the CRLF in any FWS/CFWS that appears within the quoted-string are 630 semantically "invisible" and therefore not part of the quoted-string 631 either. 633 3.2.5. Miscellaneous Tokens 635 Three additional tokens are defined: word and phrase for combinations 636 of atoms and/or quoted-strings, and unstructured for use in 637 unstructured header fields and in some places within structured 638 header fields. 640 word = atom / quoted-string 642 phrase = 1*word / obs-phrase 644 unstructured = (*([FWS] VCHAR) *WSP) / obs-unstruct 646 3.3. Date and Time Specification 648 Date and time values occur in several header fields. This section 649 specifies the syntax for a full date and time specification. Though 650 folding white space is permitted throughout the date-time 651 specification, it is RECOMMENDED that a single space be used in each 652 place that FWS appears (whether it is required or optional); some 653 older implementations will not interpret longer sequences of folding 654 white space correctly. 656 date-time = [ day-of-week "," ] date time [CFWS] 658 day-of-week = ([FWS] day-name) / obs-day-of-week 660 day-name = "Mon" / "Tue" / "Wed" / "Thu" / 661 "Fri" / "Sat" / "Sun" 663 date = day month year 665 day = ([FWS] 1*2DIGIT FWS) / obs-day 667 month = "Jan" / "Feb" / "Mar" / "Apr" / 668 "May" / "Jun" / "Jul" / "Aug" / 669 "Sep" / "Oct" / "Nov" / "Dec" 671 year = (FWS 4*DIGIT FWS) / obs-year 673 time = time-of-day zone 675 time-of-day = hour ":" minute [ ":" second ] 677 hour = 2DIGIT / obs-hour 679 minute = 2DIGIT / obs-minute 681 second = 2DIGIT / obs-second 683 zone = (FWS ( "+" / "-" ) 4DIGIT) / obs-zone 685 The day is the numeric day of the month. The year is any numeric 686 year 1900 or later. 688 The time-of-day specifies the number of hours, minutes, and 689 optionally seconds since midnight of the date indicated (at the 690 offset specified by the zone). 692 The date and time-of-day SHOULD express local time. 694 The zone specifies the offset from Coordinated Universal Time (UTC) 695 that the date and time-of-day represent. The "+" or "-" indicates 696 whether the time-of-day is ahead of (i.e., east of) or behind (i.e., 697 west of) Universal Time. The first two digits indicate the number of 698 hours difference from Universal Time, and the last two digits 699 indicate the number of additional minutes difference from Universal 700 Time. (Hence, +hhmm means +(hh * 60 + mm) minutes, and -hhmm means 701 -(hh * 60 + mm) minutes). The form "+0000" SHOULD be used to 702 indicate a time zone at Universal Time. Though "-0000" also 703 indicates Universal Time, it is used to indicate that the time was 704 generated on a system that may be in a local time zone other than 705 Universal Time and that the date-time contains no information about 706 the local time zone. 708 A date-time specification MUST be semantically valid. That is, the 709 day-of-week (if included) MUST be the day implied by the date, the 710 numeric day-of-month MUST be between 1 and the number of days allowed 711 for the specified month (in the specified year), the time-of-day MUST 712 be in the range 00:00:00 through 23:59:60 (the number of seconds 713 allowing for a leap second; see [RFC3339]), and the last two digits 714 of the zone MUST be within the range 00 through 59. 716 3.4. Address Specification 718 Addresses occur in several message header fields to indicate senders 719 and recipients of messages. An address may either be an individual 720 mailbox, or a group of mailboxes. 722 address = mailbox / group 724 mailbox = name-addr / addr-spec 726 name-addr = [display-name] angle-addr 728 angle-addr = [CFWS] "<" addr-spec ">" [CFWS] / 729 obs-angle-addr 731 group = display-name ":" [group-list] ";" [CFWS] 733 display-name = phrase 735 mailbox-list = (mailbox *("," mailbox)) / obs-mbox-list 737 address-list = (address *("," address)) / obs-addr-list 739 group-list = mailbox-list / CFWS / obs-group-list 740 A mailbox receives mail. It is a conceptual entity that does not 741 necessarily pertain to file storage. For example, some sites may 742 choose to print mail on a printer and deliver the output to the 743 addressee's desk. 745 Normally, a mailbox is composed of two parts: (1) an optional display 746 name that indicates the name of the recipient (which can be a person 747 or a system) that could be displayed to the user of a mail 748 application, and (2) an addr-spec address enclosed in angle brackets 749 ("<" and ">"). There is an alternate simple form of a mailbox where 750 the addr-spec address appears alone, without the recipient's name or 751 the angle brackets. The Internet addr-spec address is described in 752 section 3.4.1. 754 | Note: Some legacy implementations used the simple form where 755 | the addr-spec appears without the angle brackets, but included 756 | the name of the recipient in parentheses as a comment following 757 | the addr-spec. Since the meaning of the information in a 758 | comment is unspecified, implementations SHOULD use the full 759 | name-addr form of the mailbox, instead of the legacy form, to 760 | specify the display name associated with a mailbox. Also, 761 | because some legacy implementations interpret the comment, 762 | comments generally SHOULD NOT be used in address fields to 763 | avoid confusing such implementations. 765 When it is desirable to treat several mailboxes as a single unit 766 (i.e., in a distribution list), the group construct can be used. The 767 group construct allows the sender to indicate a named group of 768 recipients. This is done by giving a display name for the group, 769 followed by a colon, followed by a comma-separated list of any number 770 of mailboxes (including zero and one), and ending with a semicolon. 771 Because the list of mailboxes can be empty, using the group construct 772 is also a simple way to communicate to recipients that the message 773 was sent to one or more named sets of recipients, without actually 774 providing the individual mailbox address for any of those recipients. 776 3.4.1. Addr-Spec Specification 778 An addr-spec is a specific Internet identifier that contains a 779 locally interpreted string followed by the at-sign character ("@", 780 ASCII value 64) followed by an Internet domain. The locally 781 interpreted string is either a quoted-string or a dot-atom. If the 782 string can be represented as a dot-atom (that is, it contains no 783 characters other than atext characters or one or more of "." 784 surrounded by atext characters), then the dot-atom form SHOULD be 785 used and the quoted-string form SHOULD NOT be used. Comments and 786 folding white space SHOULD NOT be used around the "@" in the addr- 787 spec. 789 | Note: A liberal syntax for the domain portion of addr-spec is 790 | given here. However, the domain portion contains addressing 791 | information specified by and used in other protocols (e.g., 792 | [STD13], [RFC1123], [I-D.klensin-rfc5321bis]). It is therefore 793 | incumbent upon implementations to conform to the syntax of 794 | addresses for the context in which they are used. 796 addr-spec = local-part "@" domain 798 local-part = dot-atom / quoted-string / obs-local-part 800 domain = dot-atom / domain-literal / obs-domain 802 domain-literal = [CFWS] "[" *([FWS] dtext) [FWS] "]" [CFWS] 804 dtext = %d33-90 / ; Visible US-ASCII 805 %d94-126 / ; characters not including 806 obs-dtext ; "[", "]", or "\" 808 The domain portion identifies the point to which the mail is 809 delivered. In the dot-atom form, this is interpreted as an Internet 810 domain name (either a host name or a mail exchanger name) as 811 described in [STD13] and [RFC1123]. In the domain-literal form, the 812 domain is interpreted as the literal Internet address of the 813 particular host. In both cases, how addressing is used and how 814 messages are transported to a particular host is covered in separate 815 documents, such as [I-D.klensin-rfc5321bis]. These mechanisms are 816 outside of the scope of this document. 818 The local-part portion is a domain-dependent string. In addresses, 819 it is simply interpreted on the particular host as a name of a 820 particular mailbox. 822 3.5. Overall Message Syntax 824 A message consists of header fields, optionally followed by a message 825 body. Lines in a message MUST be a maximum of 998 characters 826 excluding the CRLF, but it is RECOMMENDED that lines be limited to 78 827 characters excluding the CRLF. (See section 2.1.1 for explanation.) 828 In a message body, though all of the characters listed in the text 829 rule MAY be used, the use of US-ASCII control characters (values 1 830 through 8, 11, 12, and 14 through 31) is discouraged since their 831 interpretation by receivers for display is not guaranteed. 833 message = (fields / obs-fields) 834 [CRLF body] 836 body = (*(*998text CRLF) *998text) / obs-body 838 text = %d1-9 / ; Characters excluding CR 839 %d11 / ; and LF 840 %d12 / 841 %d14-127 843 The header fields carry most of the semantic information and are 844 defined in section 3.6. The body is simply a series of lines of text 845 that are uninterpreted for the purposes of this specification. 847 3.6. Field Definitions 849 The header fields of a message are defined here. All header fields 850 have the same general syntactic structure: a field name, followed by 851 a colon, followed by the field body. The specific syntax for each 852 header field is defined in the subsequent sections. 854 | Note: In the ABNF syntax for each field in subsequent sections, 855 | each field name is followed by the required colon. However, 856 | for brevity, sometimes the colon is not referred to in the 857 | textual description of the syntax. It is, nonetheless, 858 | required. 860 It is important to note that the header fields are not guaranteed to 861 be in a particular order. They may appear in any order, and they 862 have been known to be reordered occasionally when transported over 863 the Internet. However, for the purposes of this specification, 864 header fields SHOULD NOT be reordered when a message is transported 865 or transformed. More importantly, the trace header fields and resent 866 header fields MUST NOT be reordered, and SHOULD be kept in blocks 867 prepended to the message. See sections 3.6.6 and 3.6.7 for more 868 information. 870 The only required header fields are the origination date field and 871 the originator address field(s). All other header fields are 872 syntactically optional. More information is contained in the table 873 following this definition. 875 fields = *(trace 876 *optional-field / 877 *(resent-date / 878 resent-from / 879 resent-sender / 880 resent-to / 881 resent-cc / 882 resent-bcc / 883 resent-msg-id)) 884 *(orig-date / 885 from / 886 sender / 887 reply-to / 888 to / 889 cc / 890 bcc / 891 message-id / 892 in-reply-to / 893 references / 894 subject / 895 comments / 896 keywords / 897 optional-field) 899 // Should there be a 1 in front of the resent fields as per erratum 900 // 2950 (https://www.rfc-editor.org/errata/eid2950)? 902 The following table indicates limits on the number of times each 903 field may occur in the header section of a message as well as any 904 special limitations on the use of those fields. An asterisk ("*") 905 next to a value in the minimum or maximum column indicates that a 906 special restriction appears in the Notes column. 908 +================+========+============+==========================+ 909 | Field | Min | Max number | Notes | 910 | | number | | | 911 +================+========+============+==========================+ 912 | trace | 0 | unlimited | Block prepended - see | 913 | | | | 3.6.7 | 914 +----------------+--------+------------+--------------------------+ 915 | resent-date | 0* | unlimited* | One per block, required | 916 | | | | if other resent fields | 917 | | | | are present - see 3.6.6 | 918 +----------------+--------+------------+--------------------------+ 919 | resent-from | 0 | unlimited* | One per block - see | 920 | | | | 3.6.6 | 921 +----------------+--------+------------+--------------------------+ 922 | resent-sender | 0* | unlimited* | One per block, MUST | 923 | | | | occur with multi-address | 924 | | | | resent-from - see 3.6.6 | 925 +----------------+--------+------------+--------------------------+ 926 | resent-to | 0 | unlimited* | One per block - see | 927 | | | | 3.6.6 | 928 +----------------+--------+------------+--------------------------+ 929 | resent-cc | 0 | unlimited* | One per block - see | 930 | | | | 3.6.6 | 931 +----------------+--------+------------+--------------------------+ 932 | resent-bcc | 0 | unlimited* | One per block - see | 933 | | | | 3.6.6 | 934 +----------------+--------+------------+--------------------------+ 935 | resent-msg-id | 0 | unlimited* | One per block - see | 936 | | | | 3.6.6 | 937 +----------------+--------+------------+--------------------------+ 938 | orig-date | 1 | 1 | | 939 +----------------+--------+------------+--------------------------+ 940 | from | 1 | 1 | See sender and 3.6.2 | 941 +----------------+--------+------------+--------------------------+ 942 | sender | 0* | 1 | MUST occur with multi- | 943 | | | | address from - see 3.6.2 | 944 +----------------+--------+------------+--------------------------+ 945 | reply-to | 0 | 1 | | 946 +----------------+--------+------------+--------------------------+ 947 | to | 0 | 1 | | 948 +----------------+--------+------------+--------------------------+ 949 | cc | 0 | 1 | | 950 +----------------+--------+------------+--------------------------+ 951 | bcc | 0 | 1 | | 952 +----------------+--------+------------+--------------------------+ 953 | message-id | 0* | 1 | SHOULD be present - see | 954 | | | | 3.6.4 | 955 +----------------+--------+------------+--------------------------+ 956 | in-reply-to | 0* | 1 | SHOULD occur in some | 957 | | | | replies - see 3.6.4 | 958 +----------------+--------+------------+--------------------------+ 959 | references | 0* | 1 | SHOULD occur in some | 960 | | | | replies - see 3.6.4 | 961 +----------------+--------+------------+--------------------------+ 962 | subject | 0 | 1 | | 963 +----------------+--------+------------+--------------------------+ 964 | comments | 0 | unlimited | | 965 +----------------+--------+------------+--------------------------+ 966 | keywords | 0 | unlimited | | 967 +----------------+--------+------------+--------------------------+ 968 | optional-field | 0 | unlimited | | 969 +----------------+--------+------------+--------------------------+ 970 Table 1 972 The exact interpretation of each field is described in subsequent 973 sections. 975 3.6.1. The Origination Date Field 977 The origination date field consists of the field name "Date" followed 978 by a date-time specification. 980 orig-date = "Date:" date-time CRLF 982 The origination date specifies the date and time at which the creator 983 of the message indicated that the message was complete and ready to 984 enter the mail delivery system. For instance, this might be the time 985 that a user pushes the "send" or "submit" button in an application 986 program. In any case, it is specifically not intended to convey the 987 time that the message is actually transported, but rather the time at 988 which the human or other creator of the message has put the message 989 into its final form, ready for transport. (For example, a portable 990 computer user who is not connected to a network might queue a message 991 for delivery. The origination date is intended to contain the date 992 and time that the user queued the message, not the time when the user 993 connected to the network to send the message.) 995 3.6.2. Originator Fields 997 The originator fields of a message consist of the from field, the 998 sender field (when applicable), and optionally the reply-to field. 999 The from field consists of the field name "From" and a comma- 1000 separated list of one or more mailbox specifications. If the from 1001 field contains more than one mailbox specification in the mailbox- 1002 list, then the sender field, containing the field name "Sender" and a 1003 single mailbox specification, MUST appear in the message. In either 1004 case, an optional reply-to field MAY also be included, which contains 1005 the field name "Reply-To" and a comma-separated list of one or more 1006 addresses. 1008 from = "From:" mailbox-list CRLF 1010 sender = "Sender:" mailbox CRLF 1012 reply-to = "Reply-To:" address-list CRLF 1014 The originator fields indicate the mailbox(es) of the source of the 1015 message. The "From:" field specifies the author(s) of the message, 1016 that is, the mailbox(es) of the person(s) or system(s) responsible 1017 for the writing of the message. The "Sender:" field specifies the 1018 mailbox of the agent responsible for the actual transmission of the 1019 message. For example, if a secretary were to send a message for 1020 another person, the mailbox of the secretary would appear in the 1021 "Sender:" field and the mailbox of the actual author would appear in 1022 the "From:" field. If the originator of the message can be indicated 1023 by a single mailbox and the author and transmitter are identical, the 1024 "Sender:" field SHOULD NOT be used. Otherwise, both fields SHOULD 1025 appear. 1027 | Note: The transmitter information is always present. The 1028 | absence of the "Sender:" field is sometimes mistakenly taken to 1029 | mean that the agent responsible for transmission of the message 1030 | has not been specified. This absence merely means that the 1031 | transmitter is identical to the author and is therefore not 1032 | redundantly placed into the "Sender:" field. 1034 The originator fields also provide the information required when 1035 replying to a message. When the "Reply-To:" field is present, it 1036 indicates the address(es) to which the author of the message suggests 1037 that replies be sent. In the absence of the "Reply-To:" field, 1038 replies SHOULD by default be sent to the mailbox(es) specified in the 1039 "From:" field unless otherwise specified by the person composing the 1040 reply. 1042 In all cases, the "From:" field SHOULD NOT contain any mailbox that 1043 does not belong to the author(s) of the message. See also section 1044 3.6.3 for more information on forming the destination addresses for a 1045 reply. 1047 3.6.3. Destination Address Fields 1049 The destination fields of a message consist of three possible fields, 1050 each of the same form: the field name, which is either "To", "Cc", or 1051 "Bcc", followed by a comma-separated list of one or more addresses 1052 (either mailbox or group syntax). 1054 to = "To:" address-list CRLF 1056 cc = "Cc:" address-list CRLF 1058 bcc = "Bcc:" [address-list / CFWS] CRLF 1060 The destination fields specify the recipients of the message. Each 1061 destination field may have one or more addresses, and the addresses 1062 indicate the intended recipients of the message. The only difference 1063 between the three fields is how each is used. 1065 The "To:" field contains the address(es) of the primary recipient(s) 1066 of the message. 1068 The "Cc:" field (where the "Cc" means "Carbon Copy" in the sense of 1069 making a copy on a typewriter using carbon paper) contains the 1070 addresses of others who are to receive the message, though the 1071 content of the message may not be directed at them. 1073 The "Bcc:" field (where the "Bcc" means "Blind Carbon Copy") contains 1074 addresses of recipients of the message whose addresses are not to be 1075 revealed to other recipients of the message. There are three ways in 1076 which the "Bcc:" field is used. In the first case, when a message 1077 containing a "Bcc:" field is prepared to be sent, the "Bcc:" line is 1078 removed even though all of the recipients (including those specified 1079 in the "Bcc:" field) are sent a copy of the message. In the second 1080 case, recipients specified in the "To:" and "Cc:" lines each are sent 1081 a copy of the message with the "Bcc:" line removed as above, but the 1082 recipients on the "Bcc:" line get a separate copy of the message 1083 containing a "Bcc:" line. (When there are multiple recipient 1084 addresses in the "Bcc:" field, some implementations actually send a 1085 separate copy of the message to each recipient with a "Bcc:" 1086 containing only the address of that particular recipient.) Finally, 1087 since a "Bcc:" field may contain no addresses, a "Bcc:" field can be 1088 used without any addresses indicating to the recipients that blind 1089 copies were sent to someone. Which method to use with "Bcc:" fields 1090 is implementation dependent, but refer to the "Security 1091 Considerations" section of this document for a discussion of each. 1093 When a message is a reply to another message, the mailboxes of the 1094 authors of the original message (the mailboxes in the "From:" field) 1095 or mailboxes specified in the "Reply-To:" field (if it exists) MAY 1096 appear in the "To:" field of the reply since these would normally be 1097 the primary recipients of the reply. If a reply is sent to a message 1098 that has destination fields, it is often desirable to send a copy of 1099 the reply to all of the recipients of the message, in addition to the 1100 author. When such a reply is formed, addresses in the "To:" and 1101 "Cc:" fields of the original message MAY appear in the "Cc:" field of 1102 the reply, since these are normally secondary recipients of the 1103 reply. If a "Bcc:" field is present in the original message, 1104 addresses in that field MAY appear in the "Bcc:" field of the reply, 1105 but they SHOULD NOT appear in the "To:" or "Cc:" fields. 1107 | Note: Some mail applications have automatic reply commands that 1108 | include the destination addresses of the original message in 1109 | the destination addresses of the reply. How those reply 1110 | commands behave is implementation dependent and is beyond the 1111 | scope of this document. In particular, whether or not to 1112 | include the original destination addresses when the original 1113 | message had a "Reply-To:" field is not addressed here. 1115 3.6.4. Identification Fields 1117 Though listed as optional in the table (Table 1) in section 3.6, 1118 every message SHOULD have a "Message-ID:" field. Furthermore, reply 1119 messages SHOULD have "In-Reply-To:" and "References:" fields as 1120 appropriate and as described below. 1122 The "Message-ID:" field contains a single unique message identifier. 1123 The "References:" and "In-Reply-To:" fields each contain one or more 1124 unique message identifiers, optionally separated by CFWS. 1126 The message identifier (msg-id) syntax is a limited version of the 1127 addr-spec construct enclosed in the angle bracket characters, "<" and 1128 ">". Unlike addr-spec, this syntax only permits the dot-atom-text 1129 form on the left-hand side of the "@" and does not have internal CFWS 1130 anywhere in the message identifier. 1132 | Note: As with addr-spec, a liberal syntax is given for the 1133 | right-hand side of the "@" in a msg-id. However, later in this 1134 | section, the use of a domain for the right-hand side of the "@" 1135 | is RECOMMENDED. Again, the syntax of domain constructs is 1136 | specified by and used in other protocols (e.g., [STD13], 1137 | [RFC1123], [I-D.klensin-rfc5321bis]). It is therefore 1138 | incumbent upon implementations to conform to the syntax of 1139 | addresses for the context in which they are used. 1141 message-id = "Message-ID:" msg-id CRLF 1143 in-reply-to = "In-Reply-To:" 1*msg-id CRLF 1145 references = "References:" 1*msg-id CRLF 1147 msg-id = [CFWS] "<" msg-id-internal ">" [CFWS] 1149 msg-id-internal = id-left "@" id-right 1151 id-left = dot-atom-text / obs-id-left 1153 id-right = dot-atom-text / no-fold-literal / obs-id-right 1155 no-fold-literal = "[" *dtext "]" 1157 The "Message-ID:" field provides a unique message identifier that 1158 refers to a particular version of a particular message. The 1159 uniqueness of the message identifier is guaranteed by the host that 1160 generates it (see below). This message identifier is intended to be 1161 machine readable and not necessarily meaningful to humans. A message 1162 identifier pertains to exactly one version of a particular message; 1163 subsequent revisions to the message each receive new message 1164 identifiers. 1166 | Note: There are many instances when messages are "changed", but 1167 | those changes do not constitute a new instantiation of that 1168 | message, and therefore the message would not get a new message 1169 | identifier. For example, when messages are introduced into the 1170 | transport system, they are often prepended with additional 1171 | header fields such as trace fields (described in section 3.6.7) 1172 | and resent fields (described in section 3.6.6). The addition 1173 | of such header fields does not change the identity of the 1174 | message and therefore the original "Message-ID:" field is 1175 | retained. In all cases, it is the meaning that the sender of 1176 | the message wishes to convey (i.e., whether this is the same 1177 | message or a different message) that determines whether or not 1178 | the "Message-ID:" field changes, not any particular syntactic 1179 | difference that appears (or does not appear) in the message. 1181 The "In-Reply-To:" and "References:" fields are used when creating a 1182 reply to a message. They hold the message identifier of the original 1183 message and the message identifiers of other messages (for example, 1184 in the case of a reply to a message that was itself a reply). The 1185 "In-Reply-To:" field may be used to identify the message (or 1186 messages) to which the new message is a reply, while the 1187 "References:" field may be used to identify a "thread" of 1188 conversation. 1190 When creating a reply to a message, the "In-Reply-To:" and 1191 "References:" fields of the resultant message are constructed as 1192 follows: 1194 The "In-Reply-To:" field will contain the contents of the "Message- 1195 ID:" field of the message to which this one is a reply (the "parent 1196 message"). If there is more than one parent message, then the "In- 1197 Reply-To:" field will contain the contents of all of the parents' 1198 "Message-ID:" fields. If there is no "Message-ID:" field in any of 1199 the parent messages, then the new message will have no "In-Reply-To:" 1200 field. 1202 The "References:" field will contain the contents of the parent's 1203 "References:" field (if any) followed by the contents of the parent's 1204 "Message-ID:" field (if any). If the parent message does not contain 1205 a "References:" field but does have an "In-Reply-To:" field 1206 containing a single message identifier, then the "References:" field 1207 will contain the contents of the parent's "In-Reply-To:" field 1208 followed by the contents of the parent's "Message-ID:" field (if 1209 any). If the parent has none of the "References:", "In-Reply-To:", 1210 or "Message-ID:" fields, then the new message will have no 1211 "References:" field. 1213 | Note: Some implementations parse the "References:" field to 1214 | display the "thread of the discussion". These implementations 1215 | assume that each new message is a reply to a single parent and 1216 | hence that they can walk backwards through the "References:" 1217 | field to find the parent of each message listed there. 1218 | Therefore, trying to form a "References:" field for a reply 1219 | that has multiple parents is discouraged; how to do so is not 1220 | defined in this document. 1222 The message identifier (msg-id) itself MUST be a globally unique 1223 identifier for a message. The generator of the message identifier 1224 MUST guarantee that the msg-id is unique. There are several 1225 algorithms that can be used to accomplish this. Since the msg-id has 1226 a similar syntax to addr-spec (identical except that quoted strings, 1227 comments, and folding white space are not allowed), a good method is 1228 to put the domain name (or a domain literal IP address) of the host 1229 on which the message identifier was created on the right-hand side of 1230 the "@" (since domain names and IP addresses are normally unique), 1231 and put a combination of the current absolute date and time along 1232 with some other currently unique (perhaps sequential) identifier 1233 available on the system (for example, a process id number) on the 1234 left-hand side. Though other algorithms will work, it is RECOMMENDED 1235 that the right-hand side contain some domain identifier (either of 1236 the host itself or otherwise) such that the generator of the message 1237 identifier can guarantee the uniqueness of the left-hand side within 1238 the scope of that domain. 1240 Semantically, the angle bracket characters are not part of the msg- 1241 id; the msg-id is what is contained between the two angle bracket 1242 characters. 1244 3.6.5. Informational Fields 1246 The informational fields are all optional. The "Subject:" and 1247 "Comments:" fields are unstructured fields as defined in section 1248 2.2.1, and therefore may contain text or folding white space. The 1249 "Keywords:" field contains a comma-separated list of one or more 1250 words or quoted-strings. 1252 subject = "Subject:" unstructured CRLF 1254 comments = "Comments:" unstructured CRLF 1256 keywords = "Keywords:" phrase *("," phrase) CRLF 1257 These three fields are intended to have only human-readable content 1258 with information about the message. The "Subject:" field is the most 1259 common and contains a short string identifying the topic of the 1260 message. When used in a reply, the field body MAY start with the 1261 string "Re: " (an abbreviation of the Latin "in re", meaning "in the 1262 matter of") followed by the contents of the "Subject:" field body of 1263 the original message. If this is done, only one instance of the 1264 literal string "Re: " ought to be used since use of other strings or 1265 more than one instance can lead to undesirable consequences. The 1266 "Comments:" field contains any additional comments on the text of the 1267 body of the message. The "Keywords:" field contains a comma- 1268 separated list of important words and phrases that might be useful 1269 for the recipient. 1271 3.6.6. Resent Fields 1273 Resent fields SHOULD be added to any message that is reintroduced by 1274 a user into the transport system. A separate set of resent fields 1275 SHOULD be added each time this is done. All of the resent fields 1276 corresponding to a particular resending of the message SHOULD be 1277 grouped together. Each new set of resent fields is prepended to the 1278 message; that is, the most recent set of resent fields appears 1279 earlier in the message. No other fields in the message are changed 1280 when resent fields are added. 1282 Each of the resent fields corresponds to a particular field elsewhere 1283 in the syntax. For instance, the "Resent-Date:" field corresponds to 1284 the "Date:" field and the "Resent-To:" field corresponds to the "To:" 1285 field. In each case, the syntax for the field body is identical to 1286 the syntax given previously for the corresponding field. 1288 When resent fields are used, the "Resent-From:" and "Resent-Date:" 1289 fields MUST be present. The "Resent-Message-ID:" field SHOULD be 1290 present. "Resent-Sender:" SHOULD NOT be used if "Resent-Sender:" 1291 would be identical to "Resent-From:". 1293 resent-date = "Resent-Date:" date-time CRLF 1295 resent-from = "Resent-From:" mailbox-list CRLF 1297 resent-sender = "Resent-Sender:" mailbox CRLF 1299 resent-to = "Resent-To:" address-list CRLF 1301 resent-cc = "Resent-Cc:" address-list CRLF 1303 resent-bcc = "Resent-Bcc:" [address-list / CFWS] CRLF 1305 resent-msg-id = "Resent-Message-ID:" msg-id CRLF 1307 Resent fields are used to identify a message as having been 1308 reintroduced into the transport system by a user. The purpose of 1309 using resent fields is to have the message appear to the final 1310 recipient as if it were sent directly by the original sender, with 1311 all of the original fields remaining the same. Each set of resent 1312 fields correspond to a particular resending event. That is, if a 1313 message is resent multiple times, each set of resent fields gives 1314 identifying information for each individual time. Resent fields are 1315 strictly informational. They MUST NOT be used in the normal 1316 processing of replies or other such automatic actions on messages. 1318 | Note: Reintroducing a message into the transport system and 1319 | using resent fields is a different operation from "forwarding". 1320 | "Forwarding" has two meanings: One sense of forwarding is that 1321 | a mail reading program can be told by a user to forward a copy 1322 | of a message to another person, making the forwarded message 1323 | the body of the new message. A forwarded message in this sense 1324 | does not appear to have come from the original sender, but is 1325 | an entirely new message from the forwarder of the message. 1326 | Forwarding may also mean that a mail transport program gets a 1327 | message and forwards it on to a different destination for final 1328 | delivery. Resent header fields are not intended for use with 1329 | either type of forwarding. 1331 The resent originator fields indicate the mailbox of the person(s) or 1332 system(s) that resent the message. As with the regular originator 1333 fields, there are two forms: a simple "Resent-From:" form, which 1334 contains the mailbox of the individual doing the resending, and the 1335 more complex form, when one individual (identified in the "Resent- 1336 Sender:" field) resends a message on behalf of one or more others 1337 (identified in the "Resent-From:" field). 1339 | Note: When replying to a resent message, replies behave just as 1340 | they would with any other message, using the original "From:", 1341 | "Reply-To:", "Message-ID:", and other fields. The resent 1342 | fields are only informational and MUST NOT be used in the 1343 | normal processing of replies. 1345 The "Resent-Date:" indicates the date and time at which the resent 1346 message is dispatched by the resender of the message. Like the 1347 "Date:" field, it is not the date and time that the message was 1348 actually transported. 1350 The "Resent-To:", "Resent-Cc:", and "Resent-Bcc:" fields function 1351 identically to the "To:", "Cc:", and "Bcc:" fields, respectively, 1352 except that they indicate the recipients of the resent message, not 1353 the recipients of the original message. 1355 The "Resent-Message-ID:" field provides a unique identifier for the 1356 resent message. 1358 3.6.7. Trace Fields 1360 The trace fields are a group of header fields consisting of an 1361 optional "Return-Path:" field, and one or more "Received:" fields. 1362 The "Return-Path:" header field contains a pair of angle brackets 1363 that enclose an optional addr-spec. The "Received:" field contains a 1364 (possibly empty) list of tokens followed by a semicolon and a date- 1365 time specification. Each token must be a word, angle-addr, addr- 1366 spec, or a domain. Further restrictions are applied to the syntax of 1367 the trace fields by specifications that provide for their use, such 1368 as [I-D.klensin-rfc5321bis]. 1370 trace = [return] 1371 1*received 1373 return = "Return-Path:" path CRLF 1375 path = angle-addr / ([CFWS] "<" [CFWS] ">" [CFWS]) 1377 received = "Received:" 1378 [1*received-token / CFWS] ";" date-time CRLF 1380 received-token = word / angle-addr / addr-spec / domain 1382 A full discussion of the Internet mail use of trace fields is 1383 contained in [I-D.klensin-rfc5321bis]. For the purposes of this 1384 specification, the trace fields are strictly informational, and any 1385 formal interpretation of them is outside of the scope of this 1386 document. 1388 3.6.8. Optional Fields 1390 Fields may appear in messages that are otherwise unspecified in this 1391 document. They MUST conform to the syntax of an optional-field. 1392 This is a field name, made up of the visible US-ASCII characters 1393 except colon, followed by a colon, followed by any text that conforms 1394 to the unstructured syntax. 1396 The field names of any optional field MUST NOT be identical to any 1397 field name specified elsewhere in this document. 1399 optional-field = field-name ":" unstructured CRLF 1401 field-name = 1*ftext 1403 ftext = %d33-57 / ; Visible US-ASCII 1404 %d59-126 ; characters not including 1405 ; ":". 1407 // Erratum 5918 (https://www.rfc-editor.org/errata/eid5918) basically 1408 // suggests changing field-name to 1*77ftext (leaving room for the 1409 // colon and folding white space). That's probably what was 1410 // intended, but it probably also requires an obs-field-name, and 1411 // there's no indication that the current text has ever caused a 1412 // problem. The editor is ambivalent. 1414 For the purposes of this specification, any optional field is 1415 uninterpreted. 1417 4. Obsolete Syntax 1419 Earlier versions of this specification allowed for different (usually 1420 more liberal) syntax than is allowed in this version. Also, there 1421 have been syntactic elements used in messages on the Internet whose 1422 interpretations have never been documented. Though these syntactic 1423 forms MUST NOT be generated according to the grammar in section 3, 1424 they MUST be accepted and parsed by a conformant receiver. This 1425 section documents many of these syntactic elements. Taking the 1426 grammar in section 3 and adding the definitions presented in this 1427 section will result in the grammar to use for the interpretation of 1428 messages. 1430 | Note: This section identifies syntactic forms that any 1431 | implementation MUST reasonably interpret. However, there are 1432 | certainly Internet messages that do not conform to even the 1433 | additional syntax given in this section. The fact that a 1434 | particular form does not appear in any section of this document 1435 | is not justification for computer programs to crash or for 1436 | malformed data to be irretrievably lost by any implementation. 1437 | It is up to the implementation to deal with messages robustly. 1439 One important difference between the obsolete (interpreting) and the 1440 current (generating) syntax is that in structured header field bodies 1441 (i.e., between the colon and the CRLF of any structured header 1442 field), white space characters, including folding white space, and 1443 comments could be freely inserted between any syntactic tokens. This 1444 allowed many complex forms that have proven difficult for some 1445 implementations to parse. 1447 Another key difference between the obsolete and the current syntax is 1448 that the rule in section 3.2.2 regarding lines composed entirely of 1449 white space in comments and folding white space does not apply. See 1450 the discussion of folding white space in section 4.2 below. 1452 Finally, certain characters that were formerly allowed in messages 1453 appear in this section. The NUL character (ASCII value 0) was once 1454 allowed, but is no longer for compatibility reasons. Similarly, US- 1455 ASCII control characters other than CR, LF, SP, and HTAB (ASCII 1456 values 1 through 8, 11, 12, 14 through 31, and 127) were allowed to 1457 appear in header field bodies. CR and LF were allowed to appear in 1458 messages other than as CRLF; this use is also shown here. 1460 Other differences in syntax and semantics are noted in the following 1461 sections. 1463 4.1. Miscellaneous Obsolete Tokens 1465 These syntactic elements are used elsewhere in the obsolete syntax or 1466 in the main syntax. Bare CR, bare LF, and NUL are added to obs-qp, 1467 obs-body, and obs-unstruct. US-ASCII control characters are added to 1468 obs-qp, obs-unstruct, obs-ctext, and obs-qtext. The period character 1469 is added to obs-phrase. The obs-phrase-list provides for a 1470 (potentially empty) comma-separated list of phrases that may include 1471 "null" elements. That is, there could be two or more commas in such 1472 a list with nothing in between them, or commas at the beginning or 1473 end of the list. 1475 | Note: The "period" (or "full stop") character (".") in obs- 1476 | phrase is not a form that was allowed in earlier versions of 1477 | this or any other specification. Period (nor any other 1478 | character from specials) was not allowed in phrase because it 1479 | introduced a parsing difficulty distinguishing between phrases 1480 | and portions of an addr-spec (see section 4.4). It appears 1481 | here because the period character is currently used in many 1482 | messages in the display-name portion of addresses, especially 1483 | for initials in names, and therefore must be interpreted 1484 | properly. 1486 obs-NO-WS-CTL = %d1-8 / ; US-ASCII control 1487 %d11 / ; characters that do not 1488 %d12 / ; include the carriage 1489 %d14-31 / ; return, line feed, and 1490 %d127 ; white space characters 1492 obs-ctext = obs-NO-WS-CTL 1494 obs-qtext = obs-NO-WS-CTL 1496 obs-utext = %d0 / obs-NO-WS-CTL / VCHAR 1498 obs-qp = "\" (%d0 / obs-NO-WS-CTL / LF / CR) 1500 obs-body = *(%d0 / LF / CR / text) 1502 obs-unstruct = *((*CR 1*(obs-utext / FWS)) / 1*LF) *CR 1504 obs-phrase = word *(word / "." / CFWS) 1506 obs-phrase-list = [phrase / CFWS] *("," [phrase / CFWS]) 1508 Bare CR and bare LF appear in messages with two different meanings. 1509 In many cases, bare CR or bare LF are used improperly instead of CRLF 1510 to indicate line separators. In other cases, bare CR and bare LF are 1511 used simply as US-ASCII control characters with their traditional 1512 ASCII meanings. 1514 4.2. Obsolete Folding White Space 1516 In the obsolete syntax, any amount of folding white space MAY be 1517 inserted where the obs-FWS rule is allowed. This creates the 1518 possibility of having two consecutive "folds" in a line, and 1519 therefore the possibility that a line which makes up a folded header 1520 field could be composed entirely of white space. 1522 obs-FWS = 1*([CRLF] WSP) 1524 4.3. Obsolete Date and Time 1526 The syntax for the obsolete date format allows a 2 digit year in the 1527 date field and allows for a list of alphabetic time zone specifiers 1528 that were used in earlier versions of this specification. It also 1529 permits comments and folding white space between many of the tokens. 1531 obs-day-of-week = [CFWS] day-name [CFWS] 1533 obs-day = [CFWS] 1*2DIGIT [CFWS] 1535 obs-year = [CFWS] 2*DIGIT [CFWS] 1537 obs-hour = [CFWS] 2DIGIT [CFWS] 1539 obs-minute = [CFWS] 2DIGIT [CFWS] 1541 obs-second = [CFWS] 2DIGIT [CFWS] 1543 obs-zone = "UT" / "GMT" / ; Universal Time 1544 ; North American UT 1545 ; offsets 1546 "EST" / "EDT" / ; Eastern: - 5/ - 4 1547 "CST" / "CDT" / ; Central: - 6/ - 5 1548 "MST" / "MDT" / ; Mountain: - 7/ - 6 1549 "PST" / "PDT" / ; Pacific: - 8/ - 7 1550 ; 1551 %d65-73 / ; Military zones - "A" 1552 %d75-90 / ; through "I" and "K" 1553 %d97-105 / ; through "Z", both 1554 %d107-122 ; upper and lower case 1556 Where a two or three digit year occurs in a date, the year is to be 1557 interpreted as follows: If a two digit year is encountered whose 1558 value is between 00 and 49, the year is interpreted by adding 2000, 1559 ending up with a value between 2000 and 2049. If a two digit year is 1560 encountered with a value between 50 and 99, or any three digit year 1561 is encountered, the year is interpreted by adding 1900. 1563 In the obsolete time zone, "UT" and "GMT" are indications of 1564 "Universal Time" and "Greenwich Mean Time", respectively, and are 1565 both semantically identical to "+0000". 1567 The remaining three character zones are the US time zones. The first 1568 letter, "E", "C", "M", or "P" stands for "Eastern", "Central", 1569 "Mountain", and "Pacific". The second letter is either "S" for 1570 "Standard" time, or "D" for "Daylight" (daylight saving or summer) 1571 time. Their interpretations are as follows: 1573 EDT is semantically equivalent to -0400 1574 EST is semantically equivalent to -0500 1575 CDT is semantically equivalent to -0500 1576 CST is semantically equivalent to -0600 1577 MDT is semantically equivalent to -0600 1578 MST is semantically equivalent to -0700 1579 PDT is semantically equivalent to -0700 1580 PST is semantically equivalent to -0800 1582 The 1 character military time zones were defined in a non-standard 1583 way in [RFC0822] and are therefore unpredictable in their meaning. 1584 The original definitions of the military zones "A" through "I" are 1585 equivalent to "+0100" through "+0900", respectively; "K", "L", and 1586 "M" are equivalent to "+1000", "+1100", and "+1200", respectively; 1587 "N" through "Y" are equivalent to "-0100" through "-1200". 1588 respectively; and "Z" is equivalent to "+0000". However, because of 1589 the error in [RFC0822], they SHOULD all be considered equivalent to 1590 "-0000" unless there is out-of-band information confirming their 1591 meaning. 1593 Other multi-character (usually between 3 and 5) alphabetic time zones 1594 have been used in Internet messages. Any such time zone whose 1595 meaning is not known SHOULD be considered equivalent to "-0000" 1596 unless there is out-of-band information confirming their meaning. 1598 4.4. Obsolete Addressing 1600 There are four primary differences in addressing. First, mailbox 1601 addresses were allowed to have a route portion before the addr-spec 1602 when enclosed in "<" and ">". The route is simply a comma-separated 1603 list of domain names, each preceded by "@", and the list terminated 1604 by a colon. Second, CFWS were allowed between the period-separated 1605 elements of local-part and domain (i.e., dot-atom was not used). In 1606 addition, local-part is allowed to contain quoted-string in addition 1607 to just atom. Third, mailbox-list and address-list were allowed to 1608 have "null" members. That is, there could be two or more commas in 1609 such a list with nothing in between them, or commas at the beginning 1610 or end of the list. Finally, US-ASCII control characters and quoted- 1611 pairs were allowed in domain literals and are added here. 1613 obs-angle-addr = [CFWS] "<" obs-route addr-spec ">" [CFWS] 1615 obs-route = obs-domain-list ":" 1617 obs-domain-list = *(CFWS / ",") "@" domain 1618 *("," [CFWS] ["@" domain]) 1620 obs-mbox-list = *([CFWS] ",") mailbox *("," [mailbox / CFWS]) 1622 obs-addr-list = *([CFWS] ",") address *("," [address / CFWS]) 1624 obs-group-list = 1*([CFWS] ",") [CFWS] 1626 obs-local-part = word *("." word) 1628 obs-domain = atom *("." atom) 1630 obs-dtext = obs-NO-WS-CTL / quoted-pair 1632 When interpreting addresses, the route portion SHOULD be ignored. 1634 4.5. Obsolete Header Fields 1636 Syntactically, the primary difference in the obsolete field syntax is 1637 that it allows multiple occurrences of any of the fields and they may 1638 occur in any order. Also, any amount of white space is allowed 1639 before the ":" at the end of the field name. 1641 obs-fields = *(obs-return / 1642 obs-received / 1643 obs-orig-date / 1644 obs-from / 1645 obs-sender / 1646 obs-reply-to / 1647 obs-to / 1648 obs-cc / 1649 obs-bcc / 1650 obs-message-id / 1651 obs-in-reply-to / 1652 obs-references / 1653 obs-subject / 1654 obs-comments / 1655 obs-keywords / 1656 obs-resent-date / 1657 obs-resent-from / 1658 obs-resent-send / 1659 obs-resent-rply / 1660 obs-resent-to / 1661 obs-resent-cc / 1662 obs-resent-bcc / 1663 obs-resent-mid / 1664 obs-optional) 1666 Except for destination address fields (described in section 4.5.3), 1667 the interpretation of multiple occurrences of fields is unspecified. 1668 Also, the interpretation of trace fields and resent fields that do 1669 not occur in blocks prepended to the message is unspecified as well. 1670 Unless otherwise noted in the following sections, interpretation of 1671 other fields is identical to the interpretation of their non-obsolete 1672 counterparts in section 3. 1674 4.5.1. Obsolete Origination Date Field 1676 obs-orig-date = "Date" *WSP ":" date-time CRLF 1678 4.5.2. Obsolete Originator Fields 1680 obs-from = "From" *WSP ":" mailbox-list CRLF 1682 obs-sender = "Sender" *WSP ":" mailbox CRLF 1684 obs-reply-to = "Reply-To" *WSP ":" address-list CRLF 1686 4.5.3. Obsolete Destination Address Fields 1687 obs-to = "To" *WSP ":" address-list CRLF 1689 obs-cc = "Cc" *WSP ":" address-list CRLF 1691 obs-bcc = "Bcc" *WSP ":" 1692 (address-list / (*([CFWS] ",") [CFWS])) CRLF 1694 When multiple occurrences of destination address fields occur in a 1695 message, they SHOULD be treated as if the address list in the first 1696 occurrence of the field is combined with the address lists of the 1697 subsequent occurrences by adding a comma and concatenating. 1699 4.5.4. Obsolete Identification Fields 1701 The obsolete "In-Reply-To:" and "References:" fields differ from the 1702 current syntax in that they allow phrase (words or quoted strings) to 1703 appear. The obsolete forms of the left and right sides of msg-id 1704 allow interspersed CFWS, making them syntactically identical to 1705 local-part and domain, respectively. 1707 obs-message-id = "Message-ID" *WSP ":" msg-id CRLF 1709 obs-in-reply-to = "In-Reply-To" *WSP ":" *(phrase / msg-id) CRLF 1711 obs-references = "References" *WSP ":" *(phrase / msg-id) CRLF 1713 obs-id-left = local-part 1715 obs-id-right = domain 1717 For purposes of interpretation, the phrases in the "In-Reply-To:" and 1718 "References:" fields are ignored. 1720 Semantically, none of the optional CFWS in the local-part and the 1721 domain is part of the obs-id-left and obs-id-right, respectively. 1723 4.5.5. Obsolete Informational Fields 1725 obs-subject = "Subject" *WSP ":" unstructured CRLF 1727 obs-comments = "Comments" *WSP ":" unstructured CRLF 1729 obs-keywords = "Keywords" *WSP ":" obs-phrase-list CRLF 1731 4.5.6. Obsolete Resent Fields 1733 The obsolete syntax adds a "Resent-Reply-To:" field, which consists 1734 of the field name, the optional comments and folding white space, the 1735 colon, and a comma separated list of addresses. 1737 obs-resent-from = "Resent-From" *WSP ":" mailbox-list CRLF 1739 obs-resent-send = "Resent-Sender" *WSP ":" mailbox CRLF 1741 obs-resent-date = "Resent-Date" *WSP ":" date-time CRLF 1743 obs-resent-to = "Resent-To" *WSP ":" address-list CRLF 1745 obs-resent-cc = "Resent-Cc" *WSP ":" address-list CRLF 1747 obs-resent-bcc = "Resent-Bcc" *WSP ":" 1748 (address-list / (*([CFWS] ",") [CFWS])) CRLF 1750 obs-resent-mid = "Resent-Message-ID" *WSP ":" msg-id CRLF 1752 obs-resent-rply = "Resent-Reply-To" *WSP ":" address-list CRLF 1754 As with other resent fields, the "Resent-Reply-To:" field is to be 1755 treated as trace information only. 1757 4.5.7. Obsolete Trace Fields 1759 The obs-return and obs-received are again given here as template 1760 definitions, just as return and received are in section 3. Their 1761 full syntax is given in [I-D.klensin-rfc5321bis]. 1763 obs-return = "Return-Path" *WSP ":" path CRLF 1765 obs-received = "Received" *WSP ":" 1766 [1*received-token / CFWS] [ ";" date-time CRLF ] 1768 4.5.8. Obsolete optional fields 1770 obs-optional = field-name *WSP ":" unstructured CRLF 1772 5. Security Considerations 1774 Care needs to be taken when displaying messages on a terminal or 1775 terminal emulator. Powerful terminals may act on escape sequences 1776 and other combinations of US-ASCII control characters with a variety 1777 of consequences. They can remap the keyboard or permit other 1778 modifications to the terminal that could lead to denial of service or 1779 even damaged data. They can trigger (sometimes programmable) 1780 answerback messages that can allow a message to cause commands to be 1781 issued on the recipient's behalf. They can also affect the operation 1782 of terminal attached devices such as printers. Message viewers may 1783 wish to strip potentially dangerous terminal escape sequences from 1784 the message prior to display. However, other escape sequences appear 1785 in messages for useful purposes (cf. [ISO.2022.1994], [RFC2045], 1786 [RFC2046], [RFC2047], [RFC2049], [BCP13]) and therefore should not be 1787 stripped indiscriminately. 1789 Transmission of non-text objects in messages raises additional 1790 security issues. These issues are discussed in [RFC2045], [RFC2046], 1791 [RFC2047], [RFC2049], [BCP13]. 1793 Many implementations use the "Bcc:" (blind carbon copy) field, 1794 described in section 3.6.3, to facilitate sending messages to 1795 recipients without revealing the addresses of one or more of the 1796 addressees to the other recipients. Mishandling this use of "Bcc:" 1797 may disclose confidential information that could eventually lead to 1798 security problems through knowledge of even the existence of a 1799 particular mail address. For example, if using the first method 1800 described in section 3.6.3, where the "Bcc:" line is removed from the 1801 message, blind recipients have no explicit indication that they have 1802 been sent a blind copy, except insofar as their address does not 1803 appear in the header section of a message. Because of this, one of 1804 the blind addressees could potentially send a reply to all of the 1805 shown recipients and accidentally reveal that the message went to the 1806 blind recipient. When the second method from section 3.6.3 is used, 1807 the blind recipient's address appears in the "Bcc:" field of a 1808 separate copy of the message. If the "Bcc:" field contains all of 1809 the blind addressees, all of the "Bcc:" recipients will be seen by 1810 each "Bcc:" recipient. Even if a separate message is sent to each 1811 "Bcc:" recipient with only the individual's address, implementations 1812 still need to be careful to process replies to the message as per 1813 section 3.6.3 so as not to accidentally reveal the blind recipient to 1814 other recipients. 1816 6. IANA Considerations 1818 This document updates the registrations that appeared in [RFC4021] 1819 that referred to the definitions in [RFC2822]. IANA is requested to 1820 update the Permanent Message Header Field Repository with the 1821 following header fields, in accordance with the procedures set out in 1822 [RFC3864]. 1824 Header field name Date 1825 Applicable protocol Mail 1826 Status standard 1827 Author/Change controller IETF 1828 Specification document(s) This document (section 3.6.1) 1830 Header field name From 1831 Applicable protocol Mail 1832 Status standard 1833 Author/Change controller IETF 1834 Specification document(s) This document (section 3.6.2) 1836 Header field name Sender 1837 Applicable protocol Mail 1838 Status standard 1839 Author/Change controller IETF 1840 Specification document(s) This document (section 3.6.2) 1842 Header field name Reply-To 1843 Applicable protocol Mail 1844 Status standard 1845 Author/Change controller IETF 1846 Specification document(s) This document (section 3.6.2) 1848 Header field name To 1849 Applicable protocol Mail 1850 Status standard 1851 Author/Change controller IETF 1852 Specification document(s) This document (section 3.6.3) 1854 Header field name Cc 1855 Applicable protocol Mail 1856 Status standard 1857 Author/Change controller IETF 1858 Specification document(s) This document (section 3.6.3) 1860 Header field name Bcc 1861 Applicable protocol Mail 1862 Status standard 1863 Author/Change controller IETF 1864 Specification document(s) This document (section 3.6.3) 1866 Header field name Message-ID 1867 Applicable protocol Mail 1868 Status standard 1869 Author/Change controller IETF 1870 Specification document(s) This document (section 3.6.4) 1872 Header field name In-Reply-To 1873 Applicable protocol Mail 1874 Status standard 1875 Author/Change controller IETF 1876 Specification document(s) This document (section 3.6.4) 1878 Header field name References 1879 Applicable protocol Mail 1880 Status standard 1881 Author/Change controller IETF 1882 Specification document(s) This document (section 3.6.4) 1884 Header field name Subject 1885 Applicable protocol Mail 1886 Status standard 1887 Author/Change controller IETF 1888 Specification document(s) This document (section 3.6.5) 1890 Header field name Comments 1891 Applicable protocol Mail 1892 Status standard 1893 Author/Change controller IETF 1894 Specification document(s) This document (section 3.6.5) 1896 Header field name Keywords 1897 Applicable protocol Mail 1898 Status standard 1899 Author/Change controller IETF 1900 Specification document(s) This document (section 3.6.5) 1902 Header field name Resent-Date 1903 Applicable protocol Mail 1904 Status standard 1905 Author/Change controller IETF 1906 Specification document(s) This document (section 3.6.6) 1908 Header field name Resent-From 1909 Applicable protocol Mail 1910 Status standard 1911 Author/Change controller IETF 1912 Specification document(s) This document (section 3.6.6) 1914 Header field name Resent-Sender 1915 Applicable protocol Mail 1916 Status standard 1917 Author/Change controller IETF 1918 Specification document(s) This document (section 3.6.6) 1920 Header field name Resent-To 1921 Applicable protocol Mail 1922 Status standard 1923 Author/Change controller IETF 1924 Specification document(s) This document (section 3.6.6) 1926 Header field name Resent-Cc 1927 Applicable protocol Mail 1928 Status standard 1929 Author/Change controller IETF 1930 Specification document(s) This document (section 3.6.6) 1932 Header field name Resent-Bcc 1933 Applicable protocol Mail 1934 Status standard 1935 Author/Change controller IETF 1936 Specification document(s) This document (section 3.6.6) 1938 Header field name Resent-Reply-To 1939 Applicable protocol Mail 1940 Status obsolete 1941 Author/Change controller IETF 1942 Specification document(s) This document (section 4.5.6) 1944 Header field name Resent-Message-ID 1945 Applicable protocol Mail 1946 Status standard 1947 Author/Change controller IETF 1948 Specification document(s) This document (section 3.6.6) 1950 Header field name Return-Path 1951 Applicable protocol Mail 1952 Status standard 1953 Author/Change controller IETF 1954 Specification document(s) This document (section 3.6.7) 1956 Header field name Received 1957 Applicable protocol Mail 1958 Status standard 1959 Author/Change controller IETF 1960 Specification document(s) This document (section 3.6.7) 1961 Related information [I-D.klensin-rfc5321bis] 1963 7. References 1965 7.1. Normative References 1967 [ANSI.X3-4.1986] 1968 American National Standards Institute, "Coded Character 1969 Set - 7-bit American Standard Code for Information 1970 Interchange", ANSI X3.4, 1986. 1972 [BCP14] Bradner, S., "Key words for use in RFCs to Indicate 1973 Requirement Levels", BCP 14, RFC 2119, March 1997. 1975 Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1976 2119 Key Words", BCP 14, RFC 8174, May 2017. 1978 1980 [RFC1123] Braden, R., Ed., "Requirements for Internet Hosts - 1981 Application and Support", STD 3, RFC 1123, 1982 DOI 10.17487/RFC1123, October 1989, 1983 . 1985 [STD13] Mockapetris, P., "Domain names - concepts and facilities", 1986 STD 13, RFC 1034, November 1987. 1988 Mockapetris, P., "Domain names - implementation and 1989 specification", STD 13, RFC 1035, November 1987. 1991 1993 [STD68] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 1994 Specifications: ABNF", STD 68, RFC 5234, January 2008. 1996 1998 7.2. Informative References 2000 [BCP13] Freed, N. and J. Klensin, "Multipurpose Internet Mail 2001 Extensions (MIME) Part Four: Registration Procedures", 2002 BCP 13, RFC 4289, December 2005. 2004 Freed, N., Klensin, J., and T. Hansen, "Media Type 2005 Specifications and Registration Procedures", BCP 13, 2006 RFC 6838, January 2013. 2008 2010 [ISO.2022.1994] 2011 International Organization for Standardization, 2012 "Information technology - Character code structure and 2013 extension techniques", ISO Standard 2022, 1994. 2015 [RFC0822] Crocker, D., "STANDARD FOR THE FORMAT OF ARPA INTERNET 2016 TEXT MESSAGES", STD 11, RFC 822, DOI 10.17487/RFC0822, 2017 August 1982, . 2019 [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 2020 Extensions (MIME) Part One: Format of Internet Message 2021 Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996, 2022 . 2024 [RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 2025 Extensions (MIME) Part Two: Media Types", RFC 2046, 2026 DOI 10.17487/RFC2046, November 1996, 2027 . 2029 [RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions) 2030 Part Three: Message Header Extensions for Non-ASCII Text", 2031 RFC 2047, DOI 10.17487/RFC2047, November 1996, 2032 . 2034 [RFC2049] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 2035 Extensions (MIME) Part Five: Conformance Criteria and 2036 Examples", RFC 2049, DOI 10.17487/RFC2049, November 1996, 2037 . 2039 [RFC2822] Resnick, P., Ed., "Internet Message Format", RFC 2822, 2040 DOI 10.17487/RFC2822, April 2001, 2041 . 2043 [RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet: 2044 Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002, 2045 . 2047 [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration 2048 Procedures for Message Header Fields", BCP 90, RFC 3864, 2049 DOI 10.17487/RFC3864, September 2004, 2050 . 2052 [RFC4021] Klyne, G. and J. Palme, "Registration of Mail and MIME 2053 Header Fields", RFC 4021, DOI 10.17487/RFC4021, March 2054 2005, . 2056 [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, 2057 DOI 10.17487/RFC5322, October 2008, 2058 . 2060 [RFC6532] Yang, A., Steele, S., and N. Freed, "Internationalized 2061 Email Headers", RFC 6532, DOI 10.17487/RFC6532, February 2062 2012, . 2064 [I-D.klensin-rfc5321bis] 2065 Klensin, J., "Simple Mail Transfer Protocol", Work in 2066 Progress, Internet-Draft, draft-klensin-rfc5321bis-02, 27 2067 December 2019, 2068 . 2070 Appendix A. Example Messages 2072 This section presents a selection of messages. These are intended to 2073 assist in the implementation of this specification, but should not be 2074 taken as normative; that is to say, although the examples in this 2075 section were carefully reviewed, if there happens to be a conflict 2076 between these examples and the syntax described in sections 3 and 4 2077 of this document, the syntax in those sections is to be taken as 2078 correct. 2080 In the text version of this document, messages in this section are 2081 delimited between lines of "----". The "----" lines are not part of 2082 the message itself. 2084 A.1. Addressing Examples 2086 The following are examples of messages that might be sent between two 2087 individuals. 2089 A.1.1. A Message from One Person to Another with Simple Addressing 2091 This could be called a canonical message. It has a single author, 2092 John Doe, a single recipient, Mary Smith, a subject, the date, a 2093 message identifier, and a textual message in the body. 2095 From: John Doe 2096 To: Mary Smith 2097 Subject: Saying Hello 2098 Date: Fri, 21 Nov 1997 09:55:06 -0600 2099 Message-ID: <1234@local.machine.example> 2101 This is a message just to say hello. 2102 So, "Hello". 2104 If John's secretary Michael actually sent the message, even though 2105 John was the author and replies to this message should go back to 2106 him, the sender field would be used: 2108 From: John Doe 2109 Sender: Michael Jones 2110 To: Mary Smith 2111 Subject: Saying Hello 2112 Date: Fri, 21 Nov 1997 09:55:06 -0600 2113 Message-ID: <1234@local.machine.example> 2115 This is a message just to say hello. 2116 So, "Hello". 2118 A.1.2. Different Types of Mailboxes 2120 This message includes multiple addresses in the destination fields 2121 and also uses several different forms of addresses. 2123 From: "Joe Q. Public" 2124 To: Mary Smith , jdoe@example.org, Who? 2125 Cc: , "Giant; \"Big\" Box" 2126 Date: Tue, 1 Jul 2003 10:52:37 +0200 2127 Message-ID: <5678.21-Nov-1997@example.com> 2129 Hi everyone. 2131 Note that the display names for Joe Q. Public and Giant; "Big" Box 2132 needed to be enclosed in double-quotes because the former contains 2133 the period and the latter contains both semicolon and double-quote 2134 characters (the double-quote characters appearing as quoted-pair 2135 constructs). Conversely, the display name for Who? could appear 2136 without them because the question mark is legal in an atom. Notice 2137 also that jdoe@example.org and boss@nil.test have no display names 2138 associated with them at all, and jdoe@example.org uses the simpler 2139 address form without the angle brackets. 2141 A.1.3. Group Addresses 2143 From: Pete 2144 To: A Group:Ed Jones ,joe@where.test,John ; 2145 Cc: Undisclosed recipients:; 2146 Date: Thu, 13 Feb 1969 23:32:54 -0330 2147 Message-ID: 2149 Testing. 2151 In this message, the "To:" field has a single group recipient named 2152 "A Group", which contains 3 addresses, and a "Cc:" field with an 2153 empty group recipient named Undisclosed recipients. 2155 A.2. Reply Messages 2157 The following is a series of three messages that make up a 2158 conversation thread between John and Mary. John first sends a 2159 message to Mary, Mary then replies to John's message, and then John 2160 replies to Mary's reply message. 2162 Note especially the "Message-ID:", "References:", and "In-Reply-To:" 2163 fields in each message. 2165 From: John Doe 2166 To: Mary Smith 2167 Subject: Saying Hello 2168 Date: Fri, 21 Nov 1997 09:55:06 -0600 2169 Message-ID: <1234@local.machine.example> 2171 This is a message just to say hello. 2172 So, "Hello". 2174 When sending replies, the Subject field is often retained, though 2175 prepended with "Re: " as described in section 3.6.5. 2177 From: Mary Smith 2178 To: John Doe 2179 Reply-To: "Mary Smith: Personal Account" 2180 Subject: Re: Saying Hello 2181 Date: Fri, 21 Nov 1997 10:01:10 -0600 2182 Message-ID: <3456@example.net> 2183 In-Reply-To: <1234@local.machine.example> 2184 References: <1234@local.machine.example> 2186 This is a reply to your hello. 2188 Note the "Reply-To:" field in the above message. When John replies 2189 to Mary's message above, the reply should go to the address in the 2190 "Reply-To:" field instead of the address in the "From:" field. 2192 To: "Mary Smith: Personal Account" 2193 From: John Doe 2194 Subject: Re: Saying Hello 2195 Date: Fri, 21 Nov 1997 11:00:00 -0600 2196 Message-ID: 2197 In-Reply-To: <3456@example.net> 2198 References: <1234@local.machine.example> <3456@example.net> 2200 This is a reply to your reply. 2202 A.3. Resent Messages 2204 Start with the message that has been used as an example several 2205 times: 2207 From: John Doe 2208 To: Mary Smith 2209 Subject: Saying Hello 2210 Date: Fri, 21 Nov 1997 09:55:06 -0600 2211 Message-ID: <1234@local.machine.example> 2213 This is a message just to say hello. 2214 So, "Hello". 2216 Say that Mary, upon receiving this message, wishes to send a copy of 2217 the message to Jane such that (a) the message would appear to have 2218 come straight from John; (b) if Jane replies to the message, the 2219 reply should go back to John; and (c) all of the original 2220 information, like the date the message was originally sent to Mary, 2221 the message identifier, and the original addressee, is preserved. In 2222 this case, resent fields are prepended to the message: 2224 Resent-From: Mary Smith 2225 Resent-To: Jane Brown 2226 Resent-Date: Mon, 24 Nov 1997 14:22:01 -0800 2227 Resent-Message-ID: <78910@example.net> 2228 From: John Doe 2229 To: Mary Smith 2230 Subject: Saying Hello 2231 Date: Fri, 21 Nov 1997 09:55:06 -0600 2232 Message-ID: <1234@local.machine.example> 2234 This is a message just to say hello. 2235 So, "Hello". 2237 If Jane, in turn, wished to resend this message to another person, 2238 she would prepend her own set of resent header fields to the above 2239 and send that. (Note that for brevity, trace fields are not shown.) 2241 A.4. Messages with Trace Fields 2243 As messages are sent through the transport system as described in 2244 [I-D.klensin-rfc5321bis], trace fields are prepended to the message. 2245 The following is an example of what those trace fields might look 2246 like. Note that there is some folding white space in the first one 2247 since these lines can be long. 2249 Received: from x.y.test 2250 by example.net 2251 via TCP 2252 with ESMTP 2253 id ABC12345 2254 for ; 21 Nov 1997 10:05:43 -0600 2255 Received: from node.example by x.y.test; 21 Nov 1997 10:01:22 -0600 2256 From: John Doe 2257 To: Mary Smith 2258 Subject: Saying Hello 2259 Date: Fri, 21 Nov 1997 09:55:06 -0600 2260 Message-ID: <1234@local.node.example> 2262 This is a message just to say hello. 2263 So, "Hello". 2265 A.5. White Space, Comments, and Other Oddities 2267 White space, including folding white space, and comments can be 2268 inserted between many of the tokens of fields. Taking the example 2269 from A.1.3, white space and comments can be inserted into all of the 2270 fields. 2272 From: Pete(A nice \) chap) 2273 To:A Group(Some people) 2274 :Chris Jones , 2275 joe@example.org, 2276 John (my dear friend); (the end of the group) 2277 Cc:(Empty list)(start)Hidden recipients :(nobody(that I know)) ; 2278 Date: Thu, 2279 13 2280 Feb 2281 1969 2282 23:32 2283 -0330 (Newfoundland Time) 2284 Message-ID: 2286 Testing. 2288 The above example is aesthetically displeasing, but perfectly legal. 2289 Note particularly (1) the comments in the "From:" field (including 2290 one that has a ")" character appearing as part of a quoted-pair); (2) 2291 the white space absent after the ":" in the "To:" field as well as 2292 the comment and folding white space after the group name, the special 2293 character (".") in the comment in Chris Jones's address, and the 2294 folding white space before and after "joe@example.org,"; (3) the 2295 multiple and nested comments in the "Cc:" field as well as the 2296 comment immediately following the ":" after "Cc"; (4) the folding 2297 white space (but no comments except at the end) and the missing 2298 seconds in the time of the date field; and (5) the white space before 2299 (but not within) the identifier in the "Message-ID:" field. 2301 A.6. Obsoleted Forms 2303 The following are examples of obsolete (that is, the "MUST NOT 2304 generate") syntactic elements described in section 4 of this 2305 document. 2307 A.6.1. Obsolete Addressing 2309 Note in the example below the lack of quotes around Joe Q. Public, 2310 the route that appears in the address for Mary Smith, the two commas 2311 that appear in the "To:" field, and the spaces that appear around the 2312 "." in the jdoe address. 2314 From: Joe Q. Public 2315 To: Mary Smith <@node.test:mary@example.net>, , jdoe@test . example 2316 Date: Tue, 1 Jul 2003 10:52:37 +0200 2317 Message-ID: <5678.21-Nov-1997@example.com> 2319 Hi everyone. 2321 A.6.2. Obsolete Dates 2323 The following message uses an obsolete date format, including a non- 2324 numeric time zone and a two digit year. Note that although the day- 2325 of-week is missing, that is not specific to the obsolete syntax; it 2326 is optional in the current syntax as well. 2328 From: John Doe 2329 To: Mary Smith 2330 Subject: Saying Hello 2331 Date: 21 Nov 97 09:55:06 GMT 2332 Message-ID: <1234@local.machine.example> 2334 This is a message just to say hello. 2335 So, "Hello". 2337 A.6.3. Obsolete White Space and Comments 2339 White space and comments can appear between many more elements than 2340 in the current syntax. Also, folding lines that are made up entirely 2341 of white space are legal. 2343 From : John Doe 2344 To : Mary Smith 2345 __ 2346 2347 Subject : Saying Hello 2348 Date : Fri, 21 Nov 1997 09(comment): 55 : 06 -0600 2349 Message-ID : <1234 @ local(blah) .machine .example> 2351 This is a message just to say hello. 2352 So, "Hello". 2354 Note especially the second line of the "To:" field. It starts with 2355 two space characters. (Note that "__" represent blank spaces.) 2356 Therefore, it is considered part of the folding, as described in 2357 section 4.2. Also, the comments and white space throughout 2358 addresses, dates, and message identifiers are all part of the 2359 obsolete syntax. 2361 Appendix B. Differences from Earlier Specifications 2363 This appendix contains a list of changes that have been made in the 2364 Internet Message Format from earlier specifications, specifically 2365 [RFC0822], [RFC1123], [RFC2822], and [RFC5322]. Items marked with an 2366 asterisk (*) below are items which appear in section 4 of this 2367 document and therefore can no longer be generated. 2369 The following are the changes made from [RFC0822] and [RFC1123] to 2370 [RFC2822] that remain in this document: 2372 1. Period allowed in obsolete form of phrase. 2373 2. ABNF moved out of document, now in [STD68]. 2374 3. Four or more digits allowed for year. 2375 4. Header field ordering (and lack thereof) made explicit. 2376 5. Encrypted header field removed. 2377 6. Specifically allow and give meaning to "-0000" time zone. 2378 7. Folding white space is not allowed between every token. 2379 8. Requirement for destinations removed. 2380 9. Forwarding and resending redefined. 2381 10. Extension header fields no longer specifically called out. 2382 11. ASCII 0 (null) removed.* 2383 12. Folding continuation lines cannot contain only white space.* 2384 13. Free insertion of comments not allowed in date.* 2385 14. Non-numeric time zones not allowed.* 2386 15. Two digit years not allowed.* 2387 16. Three digit years interpreted, but not allowed for generation.* 2388 17. Routes in addresses not allowed.* 2389 18. CFWS within local-parts and domains not allowed.* 2390 19. Empty members of address lists not allowed.* 2391 20. Folding white space between field name and colon not allowed.* 2392 21. Comments between field name and colon not allowed. 2393 22. Tightened syntax of in-reply-to and references.* 2394 23. CFWS within msg-id not allowed.* 2395 24. Tightened semantics of resent fields as informational only. 2396 25. Resent-Reply-To not allowed.* 2397 26. No multiple occurrences of fields (except resent and received).* 2398 27. Free CR and LF not allowed.* 2399 28. Line length limits specified. 2400 29. Bcc more clearly specified. 2402 The following are changes from [RFC2822]. 2404 1. Assorted typographical/grammatical errors fixed and 2405 clarifications made. 2406 2. Changed "standard" to "document" or "specification" throughout. 2407 3. Made distinction between "header field" and "header section". 2408 4. Removed NO-WS-CTL from ctext, qtext, dtext, and unstructured.* 2409 5. Moved discussion of specials to the "Atom" section. Moved text 2410 to "Overall message syntax" section. 2411 6. Simplified CFWS syntax. 2412 7. Fixed unstructured syntax (erratum 373 (https://www.rfc- 2413 editor.org/errata/eid373)). 2414 8. Changed date and time syntax to deal with white space in 2415 obsolete date syntax. 2416 9. Removed quoted-pair from domain literals and message 2417 identifiers.* 2418 10. Clarified that other specifications limit domain syntax. 2419 11. Simplified "Bcc:" and "Resent-Bcc:" syntax. 2420 12. Allowed optional-field to appear within trace information. 2421 13. Removed no-fold-quote from msg-id. Clarified syntax 2422 limitations. 2423 14. Generalized "Received:" syntax to fix bugs and move definition 2424 out of this document. 2425 15. Simplified obs-qp. Fixed and simplified obs-utext (which now 2426 only appears in the obsolete syntax). Removed obs-text and obs- 2427 char, adding obs-body. 2428 16. Fixed obsolete date syntax to allow for more (or less) comments 2429 and white space. 2430 17. Fixed all obsolete list syntax (obs-domain-list, obs-mbox-list, 2431 obs-addr-list, obs-phrase-list, and the newly added obs-group- 2432 list). 2434 18. Fixed obs-reply-to syntax. 2435 19. Fixed obs-bcc and obs-resent-bcc to allow empty lists. 2436 20. Removed obs-path. 2438 The following are changes from [RFC5322]. 2440 1. Clarified addr-spec description (erratum 1766 (https://www.rfc- 2441 editor.org/errata/eid1766)). 2442 2. Fixed obs-unstruct to be more limited (erratum 1905 2443 (https://www.rfc-editor.org/errata/eid1905)).* 2444 3. Simplified obs-body (erratum 1906 (https://www.rfc- 2445 editor.org/errata/eid1906)).* 2446 4. Fixed obs-FWS to allow for a leading CRLF (erratum 1908 2447 (https://www.rfc-editor.org/errata/eid1908)).* 2448 5. Fixed comments within addresses in A.5 (errata 2515 2449 (https://www.rfc-editor.org/errata/eid2515) and 2579 2450 (https://www.rfc-editor.org/errata/eid2579)). 2451 6. Fixed time zone description (erratum 2726 (https://www.rfc- 2452 editor.org/errata/eid2726)). 2453 7. Removed inappropriate uses of "sent" in 3.6.3, 3.6.6, and 5 2454 (erratum 3048 (https://www.rfc-editor.org/errata/eid3048)). 2455 8. Allow for CFWS in otherwise empty list of "Received:" field 2456 tokens (erratum 3979 (https://www.rfc-editor.org/errata/ 2457 eid3979)). 2458 9. Changed "printable" to "visible" to clarify that it doesn't 2459 include the space character (erratum 4692 (https://www.rfc- 2460 editor.org/errata/eid4692)). 2461 10. Clarify midnight in time-of-day (erratum 5905 (https://www.rfc- 2462 editor.org/errata/eid5905)). 2463 11. Allow for date-time in obs-received (erratum 5867 2464 (https://www.rfc-editor.org/errata/eid5867)).* 2465 12. Separated out "msg-id-internal" in "msg-id". 2466 13. Updated references to STD 13, STD 68, BCP 13, and BCP 14, and 2467 reference for leap seconds to RFC 3339. 2468 14. Fixed typo in daylight saving time in description of obs-zone.* 2470 There are also 3 errata that were "Held For Document Update" that 2471 have not been addressed. See the comments in the following document 2472 sections: 2474 1. Erratum 2950: Section 3.6 2475 2. Erratum 3135: Section 3.2.4 2476 3. Erratum 5918: Section 3.6.8 2478 Appendix C. Acknowledgements 2480 Many people contributed to this document. They included folks who 2481 participated in the Detailed Revision and Update of Messaging 2482 Standards (DRUMS) Working Group of the Internet Engineering Task 2483 Force (IETF), the chair of DRUMS, the Area Directors of the IETF, 2484 reporters of errata on earlier versions of this document, and people 2485 who simply sent their comments in via email. The editor is deeply 2486 indebted to them all and thanks them sincerely. (The list of these 2487 people has been temporarily removed to try to bring it up to date.) 2489 Author's Address 2491 Peter W. Resnick (editor) 2492 Episteme Technology Consulting LLC 2493 503 West Indiana Avenue 2494 Urbana, IL 61801-4941 2495 United States of America 2497 Phone: +1 217 337 1905 2498 Email: resnick@episteme.net 2499 URI: https://www.episteme.net/