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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group P. Resnick, Ed. 3 Internet-Draft Episteme 4 Obsoletes: 5322 (if approved) 11 December 2019 5 Updates: 4021 (if approved) 6 Intended status: Standards Track 7 Expires: 13 June 2020 9 Internet Message Format 10 draft-resnick-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 13 June 2020. 40 Copyright Notice 42 Copyright (c) 2019 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 . . . . . . . . . . . . . . . . . 8 81 2.3. Body . . . . . . . . . . . . . . . . . . . . . . . . . . 9 82 3. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 83 3.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 10 84 3.2. Lexical Tokens . . . . . . . . . . . . . . . . . . . . . 10 85 3.2.1. Quoted characters . . . . . . . . . . . . . . . . . . 10 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 . . . . . . . . . . . . . . . . 27 100 3.6.6. Resent Fields . . . . . . . . . . . . . . . . . . . . 28 101 3.6.7. Trace Fields . . . . . . . . . . . . . . . . . . . . 30 102 3.6.8. Optional Fields . . . . . . . . . . . . . . . . . . . 31 103 4. Obsolete Syntax . . . . . . . . . . . . . . . . . . . . . . . 31 104 4.1. Miscellaneous Obsolete Tokens . . . . . . . . . . . . . . 32 105 4.2. Obsolete Folding White Space . . . . . . . . . . . . . . 33 106 4.3. Obsolete Date and Time . . . . . . . . . . . . . . . . . 33 107 4.4. Obsolete Addressing . . . . . . . . . . . . . . . . . . . 35 108 4.5. Obsolete Header Fields . . . . . . . . . . . . . . . . . 36 109 4.5.1. Obsolete Origination Date Field . . . . . . . . . . . 37 110 4.5.2. Obsolete Originator Fields . . . . . . . . . . . . . 37 111 4.5.3. Obsolete Destination Address Fields . . . . . . . . . 37 112 4.5.4. Obsolete Identification Fields . . . . . . . . . . . 38 113 4.5.5. Obsolete Informational Fields . . . . . . . . . . . . 38 114 4.5.6. Obsolete Resent Fields . . . . . . . . . . . . . . . 39 115 4.5.7. Obsolete Trace Fields . . . . . . . . . . . . . . . . 39 116 4.5.8. Obsolete optional fields . . . . . . . . . . . . . . 39 117 5. Security Considerations . . . . . . . . . . . . . . . . . . . 39 118 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 40 119 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 43 120 7.1. Normative References . . . . . . . . . . . . . . . . . . 43 121 7.2. Informative References . . . . . . . . . . . . . . . . . 44 122 Appendix A. Example Messages . . . . . . . . . . . . . . . . . . 46 123 A.1. Addressing Examples . . . . . . . . . . . . . . . . . . . 46 124 A.1.1. A Message from One Person to Another with Simple 125 Addressing . . . . . . . . . . . . . . . . . . . . . 46 126 A.1.2. Different Types of Mailboxes . . . . . . . . . . . . 47 127 A.1.3. Group Addresses . . . . . . . . . . . . . . . . . . . 47 128 A.2. Reply Messages . . . . . . . . . . . . . . . . . . . . . 48 129 A.3. Resent Messages . . . . . . . . . . . . . . . . . . . . . 49 130 A.4. Messages with Trace Fields . . . . . . . . . . . . . . . 49 131 A.5. White Space, Comments, and Other Oddities . . . . . . . . 50 132 A.6. Obsoleted Forms . . . . . . . . . . . . . . . . . . . . . 51 133 A.6.1. Obsolete Addressing . . . . . . . . . . . . . . . . . 51 134 A.6.2. Obsolete Dates . . . . . . . . . . . . . . . . . . . 51 135 A.6.3. Obsolete White Space and Comments . . . . . . . . . . 51 136 Appendix B. Differences from Earlier Specifications . . . . . . 52 137 Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 54 138 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 54 140 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 [RFC5321] for a 164 discussion of the envelope.) The contents comprise the object to be 165 delivered to the recipient. This specification applies only to the 166 format and some of the semantics of message contents. It contains no 167 specification of the information in the envelope. 169 However, some message systems may use information from the contents 170 to create the envelope. It is intended that this specification 171 facilitate the acquisition of such information by programs. 173 This specification is intended as a definition of what message 174 content format is to be passed between systems. Though some message 175 systems locally store messages in this format (which eliminates the 176 need for translation between formats) and others use formats that 177 differ from the one specified in this specification, local storage is 178 outside of the scope of this specification. 180 | Note: This specification is not intended to dictate the 181 | internal formats used by sites, the specific message system 182 | features that they are expected to support, or any of the 183 | characteristics of user interface programs that create or read 184 | messages. In addition, this document does not specify an 185 | encoding of the characters for either transport or storage; 186 | that is, it does not specify the number of bits used or how 187 | those bits are specifically transferred over the wire or stored 188 | on disk. 190 1.2. Notational Conventions 192 1.2.1. Requirements Notation 194 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 195 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 196 "OPTIONAL" in this document are to be interpreted as described in 197 [BCP14] [RFC2119] [RFC8174] when, and only when, they appear in all 198 capitals, as shown here. 200 1.2.2. Syntactic Notation 202 This specification uses the Augmented Backus-Naur Form (ABNF) 203 [RFC5234] notation for the formal definitions of the syntax of 204 messages. Characters will be specified either by a decimal value 205 (e.g., the value %d65 for uppercase A and %d97 for lowercase A) or by 206 a case-insensitive literal value enclosed in quotation marks (e.g., 207 "A" for either uppercase or lowercase A). 209 1.2.3. Structure of This Document 211 This document is divided into several sections. 213 This section, section 1, is a short introduction to the document. 215 Section 2 lays out the general description of a message and its 216 constituent parts. This is an overview to help the reader understand 217 some of the general principles used in the later portions of this 218 document. Any examples in this section MUST NOT be taken as 219 specification of the formal syntax of any part of a message. 221 Section 3 specifies formal ABNF rules for the structure of each part 222 of a message (the syntax) and describes the relationship between 223 those parts and their meaning in the context of a message (the 224 semantics). That is, it lays out the actual rules for the structure 225 of each part of a message (the syntax) as well as a description of 226 the parts and instructions for their interpretation (the semantics). 227 This includes analysis of the syntax and semantics of subparts of 228 messages that have specific structure. The syntax included in 229 section 3 represents messages as they MUST be created. There are 230 also notes in section 3 to indicate if any of the options specified 231 in the syntax SHOULD be used over any of the others. 233 Both sections 2 and 3 describe messages that are legal to generate 234 for purposes of this specification. 236 Section 4 of this document specifies an "obsolete" syntax. There are 237 references in section 3 to these obsolete syntactic elements. The 238 rules of the obsolete syntax are elements that have appeared in 239 earlier versions of this specification or have previously been widely 240 used in Internet messages. As such, these elements MUST be 241 interpreted by parsers of messages in order to be conformant to this 242 specification. However, since items in this syntax have been 243 determined to be non-interoperable or to cause significant problems 244 for recipients of messages, they MUST NOT be generated by creators of 245 conformant messages. 247 Section 5 details security considerations to take into account when 248 implementing this specification. 250 Appendix A lists examples of different sorts of messages. These 251 examples are not exhaustive of the types of messages that appear on 252 the Internet, but give a broad overview of certain syntactic forms. 254 Appendix B lists the differences between this specification and 255 earlier specifications for Internet messages. 257 Appendix C contains acknowledgements. 259 2. Lexical Analysis of Messages 261 2.1. General Description 263 At the most basic level, a message is a series of characters. A 264 message that is conformant with this specification is composed of 265 characters with values in the range of 1 through 127 and interpreted 266 as US-ASCII [ANSI.X3-4.1986] characters. For brevity, this document 267 sometimes refers to this range of characters as simply "US-ASCII 268 characters". 270 | Note: This document specifies that messages are made up of 271 | characters in the US-ASCII range of 1 through 127. There are 272 | other documents, specifically the MIME document series 273 | ([RFC2045], [RFC2046], [RFC2047], [RFC2049], [BCP13]) and the 274 | Internationalized Email Headers specification ([RFC6532]), that 275 | extend this specification to allow for values outside of that 276 | range. Discussion of those mechanisms is not within the scope 277 | of this specification. 279 Messages are divided into lines of characters. A line is a series of 280 characters that is delimited with the two characters carriage-return 281 and line-feed; that is, the carriage return (CR) character (ASCII 282 value 13) followed immediately by the line feed (LF) character (ASCII 283 value 10). (The carriage return/line feed pair is usually written in 284 this document as "CRLF".) 285 A message consists of header fields (collectively called "the header 286 section of the message") followed, optionally, by a body. The header 287 section is a sequence of lines of characters with special syntax as 288 defined in this specification. The body is simply a sequence of 289 characters that follows the header section and is separated from the 290 header section by an empty line (i.e., a line with nothing preceding 291 the CRLF). 293 | Note: Common parlance and earlier versions of this 294 | specification use the term "header" to either refer to the 295 | entire header section or to refer to an individual header 296 | field. To avoid ambiguity, this document does not use the 297 | terms "header" or "headers" in isolation, but instead always 298 | uses "header field" to refer to the individual field and 299 | "header section" to refer to the entire collection. 301 2.1.1. Line Length Limits 303 There are two limits that this specification places on the number of 304 characters in a line. Each line of characters MUST be no more than 305 998 characters, and SHOULD be no more than 78 characters, excluding 306 the CRLF. 308 The 998 character limit is due to limitations in many implementations 309 that send, receive, or store IMF messages which simply cannot handle 310 more than 998 characters on a line. Receiving implementations would 311 do well to handle an arbitrarily large number of characters in a line 312 for robustness sake. However, there are so many implementations that 313 (in compliance with the transport requirements of [RFC5321]) do not 314 accept messages containing more than 1000 characters including the CR 315 and LF per line, it is important for implementations not to create 316 such messages. 318 The more conservative 78 character recommendation is to accommodate 319 the many implementations of user interfaces that display these 320 messages which may truncate, or disastrously wrap, the display of 321 more than 78 characters per line, in spite of the fact that such 322 implementations are non-conformant to the intent of this 323 specification (and that of [RFC5321] if they actually cause 324 information to be lost). Again, even though this limitation is put 325 on messages, it is incumbent upon implementations that display 326 messages to handle an arbitrarily large number of characters in a 327 line (certainly at least up to the 998 character limit) for the sake 328 of robustness. 330 2.2. Header Fields 332 Header fields are lines beginning with a field name, followed by a 333 colon (":"), followed by a field body, and terminated by CRLF. A 334 field name MUST be composed of visible US-ASCII characters (i.e., 335 characters that have values between 33 and 126, inclusive), except 336 colon. A field body may be composed of visible US-ASCII characters 337 as well as the space (SP, ASCII value 32) and horizontal tab (HTAB, 338 ASCII value 9) characters (together known as the white space 339 characters, WSP). A field body MUST NOT include CR and LF except 340 when used in "folding" and "unfolding", as described in section 341 2.2.3. All field bodies MUST conform to the syntax described in 342 sections 3 and 4 of this specification. 344 2.2.1. Unstructured Header Field Bodies 346 Some field bodies in this specification are defined simply as 347 "unstructured" (which is specified in section 3.2.5 as any visible 348 US-ASCII characters plus white space characters) with no further 349 restrictions. These are referred to as unstructured field bodies. 350 Semantically, unstructured field bodies are simply to be treated as a 351 single line of characters with no further processing (except for 352 "folding" and "unfolding" as described in section 2.2.3). 354 2.2.2. Structured Header Field Bodies 356 Some field bodies in this specification have a syntax that is more 357 restrictive than the unstructured field bodies described above. 358 These are referred to as "structured" field bodies. Structured field 359 bodies are sequences of specific lexical tokens as described in 360 sections 3 and 4 of this specification. Many of these tokens are 361 allowed (according to their syntax) to be introduced or end with 362 comments (as described in section 3.2.2) as well as the white space 363 characters, and those white space characters are subject to "folding" 364 and "unfolding" as described in section 2.2.3. Semantic analysis of 365 structured field bodies is given along with their syntax. 367 2.2.3. Long Header Fields 369 Each header field is logically a single line of characters comprising 370 the field name, the colon, and the field body. For convenience 371 however, and to deal with the 998/78 character limitations per line, 372 the field body portion of a header field can be split into a 373 multiple-line representation; this is called "folding". The general 374 rule is that wherever this specification allows for folding white 375 space (not simply WSP characters), a CRLF may be inserted before any 376 WSP. 378 For example, the header field: 380 Subject: This is a test 382 can be represented as: 384 Subject: This 385 is a test 387 | Note: Though structured field bodies are defined in such a way 388 | that folding can take place between many of the lexical tokens 389 | (and even within some of the lexical tokens), folding SHOULD be 390 | limited to placing the CRLF at higher-level syntactic breaks. 391 | For instance, if a field body is defined as comma-separated 392 | values, it is recommended that folding occur after the comma 393 | separating the structured items in preference to other places 394 | where the field could be folded, even if it is allowed 395 | elsewhere. 397 The process of moving from this folded multiple-line representation 398 of a header field to its single line representation is called 399 "unfolding". Unfolding is accomplished by simply removing any CRLF 400 that is immediately followed by WSP. Each header field should be 401 treated in its unfolded form for further syntactic and semantic 402 evaluation. An unfolded header field has no length restriction and 403 therefore may be indeterminately long. 405 2.3. Body 407 The body of a message is simply lines of US-ASCII characters. The 408 only two limitations on the body are as follows: 410 * CR and LF MUST only occur together as CRLF; they MUST NOT appear 411 independently in the body. 413 * Lines of characters in the body MUST be limited to 998 characters, 414 and SHOULD be limited to 78 characters, excluding the CRLF. 416 | Note: As was stated earlier, there are other documents, 417 | specifically the MIME documents ([RFC2045], [RFC2046], 418 | [RFC2049], [BCP13]), that extend (and limit) this specification 419 | to allow for different sorts of message bodies. Again, these 420 | mechanisms are beyond the scope of this document. 422 3. Syntax 423 3.1. Introduction 425 The syntax as given in this section defines the legal syntax of 426 Internet messages. Messages that are conformant to this 427 specification MUST conform to the syntax in this section. If there 428 are options in this section where one option SHOULD be generated, 429 that is indicated either in the prose or in a comment next to the 430 syntax. 432 For the defined expressions, a short description of the syntax and 433 use is given, followed by the syntax in ABNF, followed by a semantic 434 analysis. The following primitive tokens that are used but otherwise 435 unspecified are taken from the "Core Rules" of [RFC5234], 436 Appendix B.1: CR, LF, CRLF, HTAB, SP, WSP, DQUOTE, DIGIT, ALPHA, and 437 VCHAR. 439 In some of the definitions, there will be non-terminals whose names 440 start with "obs-". These "obs-" elements refer to tokens defined in 441 the obsolete syntax in section 4. In all cases, these productions 442 are to be ignored for the purposes of generating legal Internet 443 messages and MUST NOT be used as part of such a message. However, 444 when interpreting messages, these tokens MUST be honored as part of 445 the legal syntax. In this sense, section 3 defines a grammar for the 446 generation of messages, with "obs-" elements that are to be ignored, 447 while section 4 adds grammar for the interpretation of messages. 449 3.2. Lexical Tokens 451 The following rules are used to define an underlying lexical 452 analyzer, which feeds tokens to the higher-level parsers. This 453 section defines the tokens used in structured header field bodies. 455 | Note: Readers of this specification need to pay special 456 | attention to how these lexical tokens are used in both the 457 | lower-level and higher-level syntax later in the document. 458 | Particularly, the white space tokens and the comment tokens 459 | defined in section 3.2.2 get used in the lower-level tokens 460 | defined here, and those lower-level tokens are in turn used as 461 | parts of the higher-level tokens defined later. Therefore, 462 | white space and comments may be allowed in the higher-level 463 | tokens even though they may not explicitly appear in a 464 | particular definition. 466 3.2.1. Quoted characters 468 Some characters are reserved for special interpretation, such as 469 delimiting lexical tokens. To permit use of these characters as 470 uninterpreted data, a quoting mechanism is provided. 472 quoted-pair = ("\" (VCHAR / WSP)) / obs-qp 474 Where any quoted-pair appears, it is to be interpreted as the 475 character alone. That is to say, the "\" character that appears as 476 part of a quoted-pair is semantically "invisible". 478 | Note: The "\" character may appear in a message where it is not 479 | part of a quoted-pair. A "\" character that does not appear in 480 | a quoted-pair is not semantically invisible. The only places 481 | in this specification where quoted-pair currently appears are 482 | ccontent, qcontent, and in obs-dtext in section 4. 484 3.2.2. Folding White Space and Comments 486 White space characters, including white space used in folding 487 (described in section 2.2.3), may appear between many elements in 488 header field bodies. Also, strings of characters that are treated as 489 comments may be included in structured field bodies as characters 490 enclosed in parentheses. The following defines the folding white 491 space (FWS) and comment constructs. 493 Strings of characters enclosed in parentheses are considered comments 494 so long as they do not appear within a "quoted-string", as defined in 495 section 3.2.4. Comments may nest. 497 There are several places in this specification where comments and FWS 498 may be freely inserted. To accommodate that syntax, an additional 499 token for "CFWS" is defined for places where comments and/or FWS can 500 occur. However, where CFWS occurs in this specification, it MUST NOT 501 be inserted in such a way that any line of a folded header field is 502 made up entirely of WSP characters and nothing else. 504 FWS = ([*WSP CRLF] 1*WSP) / obs-FWS 505 ; Folding white space 507 ctext = %d33-39 / ; Visible US-ASCII 508 %d42-91 / ; characters not including 509 %d93-126 / ; "(", ")", or "\" 510 obs-ctext 512 ccontent = ctext / quoted-pair / comment 514 comment = "(" *([FWS] ccontent) [FWS] ")" 516 CFWS = (1*([FWS] comment) [FWS]) / FWS 518 Throughout this specification, where FWS (the folding white space 519 token) appears, it indicates a place where folding, as discussed in 520 section 2.2.3, may take place. Wherever folding appears in a message 521 (that is, a header field body containing a CRLF followed by any WSP), 522 unfolding (removal of the CRLF) is performed before any further 523 semantic analysis is performed on that header field according to this 524 specification. That is to say, any CRLF that appears in FWS is 525 semantically "invisible". 527 A comment is normally used in a structured field body to provide some 528 human-readable informational text. Since a comment is allowed to 529 contain FWS, folding is permitted within the comment. Also note that 530 since quoted-pair is allowed in a comment, the parentheses and 531 backslash characters may appear in a comment, so long as they appear 532 as a quoted-pair. Semantically, the enclosing parentheses are not 533 part of the comment; the comment is what is contained between the two 534 parentheses. As stated earlier, the "\" in any quoted-pair and the 535 CRLF in any FWS that appears within the comment are semantically 536 "invisible" and therefore not part of the comment either. 538 Runs of FWS, comment, or CFWS that occur between lexical tokens in a 539 structured header field are semantically interpreted as a single 540 space character. 542 3.2.3. Atom 544 Several productions in structured header field bodies are simply 545 strings of certain basic characters. Such productions are called 546 atoms. 548 Some of the structured header field bodies also allow the period 549 character (".", ASCII value 46) within runs of atext. An additional 550 "dot-atom" token is defined for those purposes. 552 | Note: The "specials" token does not appear anywhere else in 553 | this specification. It is simply the visible (i.e., non- 554 | control, non-white space) characters that do not appear in 555 | atext. It is provided only because it is useful for 556 | implementers who use tools that lexically analyze messages. 557 | Each of the characters in specials can be used to indicate a 558 | tokenization point in lexical analysis. 560 atext = ALPHA / DIGIT / ; Visible US-ASCII 561 "!" / "#" / ; characters not including 562 "$" / "%" / ; specials. Used for atoms. 563 "&" / "'" / 564 "*" / "+" / 565 "-" / "/" / 566 "=" / "?" / 567 "^" / "_" / 568 "`" / "{" / 569 "|" / "}" / 570 "~" 572 atom = [CFWS] 1*atext [CFWS] 574 dot-atom-text = 1*atext *("." 1*atext) 576 dot-atom = [CFWS] dot-atom-text [CFWS] 578 specials = "(" / ")" / ; Special characters that do 579 "<" / ">" / ; not appear in atext 580 "[" / "]" / 581 ":" / ";" / 582 "@" / "\" / 583 "," / "." / 584 DQUOTE 586 Both atom and dot-atom are interpreted as a single unit, comprising 587 the string of characters that make it up. Semantically, the optional 588 comments and FWS surrounding the rest of the characters are not part 589 of the atom; the atom is only the run of atext characters in an atom, 590 or the atext and "." characters in a dot-atom. 592 3.2.4. Quoted Strings 594 Strings of characters that include characters other than those 595 allowed in atoms can be represented in a quoted string format, where 596 the characters are surrounded by quote (DQUOTE, ASCII value 34) 597 characters. 599 qtext = %d33 / ; Visible US-ASCII 600 %d35-91 / ; characters not including 601 %d93-126 / ; "\" or the quote character 602 obs-qtext 604 qcontent = qtext / quoted-pair 606 quoted-string = [CFWS] 607 DQUOTE *([FWS] qcontent) [FWS] DQUOTE 608 [CFWS] 610 // Erratum 3135 (https://www.rfc-editor.org/errata/eid3135) wanted to 611 // disallow empty quoted strings. There doesn't appear to be // 612 consensus for that (e.g., see discussion starting at // 613 https://mailarchive.ietf.org/arch/msg/ietf- // 614 822/9NByCGWq7_dOLRNhrPkZR24074g) and therefore this erratum // 615 probably should have been rejected. 617 A quoted-string is treated as a unit. That is, quoted-string is 618 identical to atom, semantically. Since a quoted-string is allowed to 619 contain FWS, folding is permitted. Also note that since quoted-pair 620 is allowed in a quoted-string, the quote and backslash characters may 621 appear in a quoted-string so long as they appear as a quoted-pair. 623 Semantically, neither the optional CFWS outside of the quote 624 characters nor the quote characters themselves are part of the 625 quoted-string; the quoted-string is what is contained between the two 626 quote characters. As stated earlier, the "\" in any quoted-pair and 627 the CRLF in any FWS/CFWS that appears within the quoted-string are 628 semantically "invisible" and therefore not part of the quoted-string 629 either. 631 3.2.5. Miscellaneous Tokens 633 Three additional tokens are defined: word and phrase for combinations 634 of atoms and/or quoted-strings, and unstructured for use in 635 unstructured header fields and in some places within structured 636 header fields. 638 word = atom / quoted-string 640 phrase = 1*word / obs-phrase 642 unstructured = (*([FWS] VCHAR) *WSP) / obs-unstruct 644 3.3. Date and Time Specification 646 Date and time values occur in several header fields. This section 647 specifies the syntax for a full date and time specification. Though 648 folding white space is permitted throughout the date-time 649 specification, it is RECOMMENDED that a single space be used in each 650 place that FWS appears (whether it is required or optional); some 651 older implementations will not interpret longer sequences of folding 652 white space correctly. 654 date-time = [ day-of-week "," ] date time [CFWS] 656 day-of-week = ([FWS] day-name) / obs-day-of-week 658 day-name = "Mon" / "Tue" / "Wed" / "Thu" / 659 "Fri" / "Sat" / "Sun" 661 date = day month year 663 day = ([FWS] 1*2DIGIT FWS) / obs-day 665 month = "Jan" / "Feb" / "Mar" / "Apr" / 666 "May" / "Jun" / "Jul" / "Aug" / 667 "Sep" / "Oct" / "Nov" / "Dec" 669 year = (FWS 4*DIGIT FWS) / obs-year 671 time = time-of-day zone 673 time-of-day = hour ":" minute [ ":" second ] 675 hour = 2DIGIT / obs-hour 677 minute = 2DIGIT / obs-minute 679 second = 2DIGIT / obs-second 681 zone = (FWS ( "+" / "-" ) 4DIGIT) / obs-zone 683 The day is the numeric day of the month. The year is any numeric 684 year 1900 or later. 686 The time-of-day specifies the number of hours, minutes, and 687 optionally seconds since midnight of the date indicated (at the 688 offset specified by the zone). 690 The date and time-of-day SHOULD express local time. 692 The zone specifies the offset from Coordinated Universal Time (UTC) 693 that the date and time-of-day represent. The "+" or "-" indicates 694 whether the time-of-day is ahead of (i.e., east of) or behind (i.e., 695 west of) Universal Time. The first two digits indicate the number of 696 hours difference from Universal Time, and the last two digits 697 indicate the number of additional minutes difference from Universal 698 Time. (Hence, +hhmm means +(hh * 60 + mm) minutes, and -hhmm means 699 -(hh * 60 + mm) minutes). The form "+0000" SHOULD be used to 700 indicate a time zone at Universal Time. Though "-0000" also 701 indicates Universal Time, it is used to indicate that the time was 702 generated on a system that may be in a local time zone other than 703 Universal Time and that the date-time contains no information about 704 the local time zone. 706 A date-time specification MUST be semantically valid. That is, the 707 day-of-week (if included) MUST be the day implied by the date, the 708 numeric day-of-month MUST be between 1 and the number of days allowed 709 for the specified month (in the specified year), the time-of-day MUST 710 be in the range 00:00:00 through 23:59:60 (the number of seconds 711 allowing for a leap second; see [RFC1305]), and the last two digits 712 of the zone MUST be within the range 00 through 59. 714 3.4. Address Specification 716 Addresses occur in several message header fields to indicate senders 717 and recipients of messages. An address may either be an individual 718 mailbox, or a group of mailboxes. 720 address = mailbox / group 722 mailbox = name-addr / addr-spec 724 name-addr = [display-name] angle-addr 726 angle-addr = [CFWS] "<" addr-spec ">" [CFWS] / 727 obs-angle-addr 729 group = display-name ":" [group-list] ";" [CFWS] 731 display-name = phrase 733 mailbox-list = (mailbox *("," mailbox)) / obs-mbox-list 735 address-list = (address *("," address)) / obs-addr-list 737 group-list = mailbox-list / CFWS / obs-group-list 738 A mailbox receives mail. It is a conceptual entity that does not 739 necessarily pertain to file storage. For example, some sites may 740 choose to print mail on a printer and deliver the output to the 741 addressee's desk. 743 Normally, a mailbox is composed of two parts: (1) an optional display 744 name that indicates the name of the recipient (which can be a person 745 or a system) that could be displayed to the user of a mail 746 application, and (2) an addr-spec address enclosed in angle brackets 747 ("<" and ">"). There is an alternate simple form of a mailbox where 748 the addr-spec address appears alone, without the recipient's name or 749 the angle brackets. The Internet addr-spec address is described in 750 section 3.4.1. 752 | Note: Some legacy implementations used the simple form where 753 | the addr-spec appears without the angle brackets, but included 754 | the name of the recipient in parentheses as a comment following 755 | the addr-spec. Since the meaning of the information in a 756 | comment is unspecified, implementations SHOULD use the full 757 | name-addr form of the mailbox, instead of the legacy form, to 758 | specify the display name associated with a mailbox. Also, 759 | because some legacy implementations interpret the comment, 760 | comments generally SHOULD NOT be used in address fields to 761 | avoid confusing such implementations. 763 When it is desirable to treat several mailboxes as a single unit 764 (i.e., in a distribution list), the group construct can be used. The 765 group construct allows the sender to indicate a named group of 766 recipients. This is done by giving a display name for the group, 767 followed by a colon, followed by a comma-separated list of any number 768 of mailboxes (including zero and one), and ending with a semicolon. 769 Because the list of mailboxes can be empty, using the group construct 770 is also a simple way to communicate to recipients that the message 771 was sent to one or more named sets of recipients, without actually 772 providing the individual mailbox address for any of those recipients. 774 3.4.1. Addr-Spec Specification 776 An addr-spec is a specific Internet identifier that contains a 777 locally interpreted string followed by the at-sign character ("@", 778 ASCII value 64) followed by an Internet domain. The locally 779 interpreted string is either a quoted-string or a dot-atom. If the 780 string can be represented as a dot-atom (that is, it contains no 781 characters other than atext characters or one or more of "." 782 surrounded by atext characters), then the dot-atom form SHOULD be 783 used and the quoted-string form SHOULD NOT be used. Comments and 784 folding white space SHOULD NOT be used around the "@" in the addr- 785 spec. 787 | Note: A liberal syntax for the domain portion of addr-spec is 788 | given here. However, the domain portion contains addressing 789 | information specified by and used in other protocols (e.g., 790 | [STD13], [RFC1123], [RFC5321]). It is therefore incumbent upon 791 | implementations to conform to the syntax of addresses for the 792 | context in which they are used. 794 addr-spec = local-part "@" domain 796 local-part = dot-atom / quoted-string / obs-local-part 798 domain = dot-atom / domain-literal / obs-domain 800 domain-literal = [CFWS] "[" *([FWS] dtext) [FWS] "]" [CFWS] 802 dtext = %d33-90 / ; Visible US-ASCII 803 %d94-126 / ; characters not including 804 obs-dtext ; "[", "]", or "\" 806 The domain portion identifies the point to which the mail is 807 delivered. In the dot-atom form, this is interpreted as an Internet 808 domain name (either a host name or a mail exchanger name) as 809 described in [STD13] and [RFC1123]. In the domain-literal form, the 810 domain is interpreted as the literal Internet address of the 811 particular host. In both cases, how addressing is used and how 812 messages are transported to a particular host is covered in separate 813 documents, such as [RFC5321]. These mechanisms are outside of the 814 scope of this document. 816 The local-part portion is a domain-dependent string. In addresses, 817 it is simply interpreted on the particular host as a name of a 818 particular mailbox. 820 3.5. Overall Message Syntax 822 A message consists of header fields, optionally followed by a message 823 body. Lines in a message MUST be a maximum of 998 characters 824 excluding the CRLF, but it is RECOMMENDED that lines be limited to 78 825 characters excluding the CRLF. (See section 2.1.1 for explanation.) 826 In a message body, though all of the characters listed in the text 827 rule MAY be used, the use of US-ASCII control characters (values 1 828 through 8, 11, 12, and 14 through 31) is discouraged since their 829 interpretation by receivers for display is not guaranteed. 831 message = (fields / obs-fields) 832 [CRLF body] 834 body = (*(*998text CRLF) *998text) / obs-body 836 text = %d1-9 / ; Characters excluding CR 837 %d11 / ; and LF 838 %d12 / 839 %d14-127 841 The header fields carry most of the semantic information and are 842 defined in section 3.6. The body is simply a series of lines of text 843 that are uninterpreted for the purposes of this specification. 845 3.6. Field Definitions 847 The header fields of a message are defined here. All header fields 848 have the same general syntactic structure: a field name, followed by 849 a colon, followed by the field body. The specific syntax for each 850 header field is defined in the subsequent sections. 852 | Note: In the ABNF syntax for each field in subsequent sections, 853 | each field name is followed by the required colon. However, 854 | for brevity, sometimes the colon is not referred to in the 855 | textual description of the syntax. It is, nonetheless, 856 | required. 858 It is important to note that the header fields are not guaranteed to 859 be in a particular order. They may appear in any order, and they 860 have been known to be reordered occasionally when transported over 861 the Internet. However, for the purposes of this specification, 862 header fields SHOULD NOT be reordered when a message is transported 863 or transformed. More importantly, the trace header fields and resent 864 header fields MUST NOT be reordered, and SHOULD be kept in blocks 865 prepended to the message. See sections 3.6.6 and 3.6.7 for more 866 information. 868 The only required header fields are the origination date field and 869 the originator address field(s). All other header fields are 870 syntactically optional. More information is contained in the table 871 following this definition. 873 fields = *(trace 874 *optional-field / 875 *(resent-date / 876 resent-from / 877 resent-sender / 878 resent-to / 879 resent-cc / 880 resent-bcc / 881 resent-msg-id)) 882 *(orig-date / 883 from / 884 sender / 885 reply-to / 886 to / 887 cc / 888 bcc / 889 message-id / 890 in-reply-to / 891 references / 892 subject / 893 comments / 894 keywords / 895 optional-field) 897 // Should there be a 1 in front of the resent fields as per erratum 898 // 2950 (https://www.rfc-editor.org/errata/eid2950)? 900 The following table indicates limits on the number of times each 901 field may occur in the header section of a message as well as any 902 special limitations on the use of those fields. An asterisk ("*") 903 next to a value in the minimum or maximum column indicates that a 904 special restriction appears in the Notes column. 906 +----------------+--------+------------+--------------------------+ 907 | Field | Min | Max number | Notes | 908 | | number | | | 909 +================+========+============+==========================+ 910 | trace | 0 | unlimited | Block prepended - see | 911 | | | | 3.6.7 | 912 +----------------+--------+------------+--------------------------+ 913 | resent-date | 0* | unlimited* | One per block, required | 914 | | | | if other resent fields | 915 | | | | are present - see 3.6.6 | 916 +----------------+--------+------------+--------------------------+ 917 | resent-from | 0 | unlimited* | One per block - see | 918 | | | | 3.6.6 | 919 +----------------+--------+------------+--------------------------+ 920 | resent-sender | 0* | unlimited* | One per block, MUST | 921 | | | | occur with multi-address | 922 | | | | resent-from - see 3.6.6 | 923 +----------------+--------+------------+--------------------------+ 924 | resent-to | 0 | unlimited* | One per block - see | 925 | | | | 3.6.6 | 926 +----------------+--------+------------+--------------------------+ 927 | resent-cc | 0 | unlimited* | One per block - see | 928 | | | | 3.6.6 | 929 +----------------+--------+------------+--------------------------+ 930 | resent-bcc | 0 | unlimited* | One per block - see | 931 | | | | 3.6.6 | 932 +----------------+--------+------------+--------------------------+ 933 | resent-msg-id | 0 | unlimited* | One per block - see | 934 | | | | 3.6.6 | 935 +----------------+--------+------------+--------------------------+ 936 | orig-date | 1 | 1 | | 937 +----------------+--------+------------+--------------------------+ 938 | from | 1 | 1 | See sender and 3.6.2 | 939 +----------------+--------+------------+--------------------------+ 940 | sender | 0* | 1 | MUST occur with multi- | 941 | | | | address from - see 3.6.2 | 942 +----------------+--------+------------+--------------------------+ 943 | reply-to | 0 | 1 | | 944 +----------------+--------+------------+--------------------------+ 945 | to | 0 | 1 | | 946 +----------------+--------+------------+--------------------------+ 947 | cc | 0 | 1 | | 948 +----------------+--------+------------+--------------------------+ 949 | bcc | 0 | 1 | | 950 +----------------+--------+------------+--------------------------+ 951 | message-id | 0* | 1 | SHOULD be present - see | 952 | | | | 3.6.4 | 953 +----------------+--------+------------+--------------------------+ 954 | in-reply-to | 0* | 1 | SHOULD occur in some | 955 | | | | replies - see 3.6.4 | 956 +----------------+--------+------------+--------------------------+ 957 | references | 0* | 1 | SHOULD occur in some | 958 | | | | replies - see 3.6.4 | 959 +----------------+--------+------------+--------------------------+ 960 | subject | 0 | 1 | | 961 +----------------+--------+------------+--------------------------+ 962 | comments | 0 | unlimited | | 963 +----------------+--------+------------+--------------------------+ 964 | keywords | 0 | unlimited | | 965 +----------------+--------+------------+--------------------------+ 966 | optional-field | 0 | unlimited | | 967 +----------------+--------+------------+--------------------------+ 968 Table 1 970 The exact interpretation of each field is described in subsequent 971 sections. 973 3.6.1. The Origination Date Field 975 The origination date field consists of the field name "Date" followed 976 by a date-time specification. 978 orig-date = "Date:" date-time CRLF 980 The origination date specifies the date and time at which the creator 981 of the message indicated that the message was complete and ready to 982 enter the mail delivery system. For instance, this might be the time 983 that a user pushes the "send" or "submit" button in an application 984 program. In any case, it is specifically not intended to convey the 985 time that the message is actually transported, but rather the time at 986 which the human or other creator of the message has put the message 987 into its final form, ready for transport. (For example, a portable 988 computer user who is not connected to a network might queue a message 989 for delivery. The origination date is intended to contain the date 990 and time that the user queued the message, not the time when the user 991 connected to the network to send the message.) 993 3.6.2. Originator Fields 995 The originator fields of a message consist of the from field, the 996 sender field (when applicable), and optionally the reply-to field. 997 The from field consists of the field name "From" and a comma- 998 separated list of one or more mailbox specifications. If the from 999 field contains more than one mailbox specification in the mailbox- 1000 list, then the sender field, containing the field name "Sender" and a 1001 single mailbox specification, MUST appear in the message. In either 1002 case, an optional reply-to field MAY also be included, which contains 1003 the field name "Reply-To" and a comma-separated list of one or more 1004 addresses. 1006 from = "From:" mailbox-list CRLF 1008 sender = "Sender:" mailbox CRLF 1010 reply-to = "Reply-To:" address-list CRLF 1012 The originator fields indicate the mailbox(es) of the source of the 1013 message. The "From:" field specifies the author(s) of the message, 1014 that is, the mailbox(es) of the person(s) or system(s) responsible 1015 for the writing of the message. The "Sender:" field specifies the 1016 mailbox of the agent responsible for the actual transmission of the 1017 message. For example, if a secretary were to send a message for 1018 another person, the mailbox of the secretary would appear in the 1019 "Sender:" field and the mailbox of the actual author would appear in 1020 the "From:" field. If the originator of the message can be indicated 1021 by a single mailbox and the author and transmitter are identical, the 1022 "Sender:" field SHOULD NOT be used. Otherwise, both fields SHOULD 1023 appear. 1025 | Note: The transmitter information is always present. The 1026 | absence of the "Sender:" field is sometimes mistakenly taken to 1027 | mean that the agent responsible for transmission of the message 1028 | has not been specified. This absence merely means that the 1029 | transmitter is identical to the author and is therefore not 1030 | redundantly placed into the "Sender:" field. 1032 The originator fields also provide the information required when 1033 replying to a message. When the "Reply-To:" field is present, it 1034 indicates the address(es) to which the author of the message suggests 1035 that replies be sent. In the absence of the "Reply-To:" field, 1036 replies SHOULD by default be sent to the mailbox(es) specified in the 1037 "From:" field unless otherwise specified by the person composing the 1038 reply. 1040 In all cases, the "From:" field SHOULD NOT contain any mailbox that 1041 does not belong to the author(s) of the message. See also section 1042 3.6.3 for more information on forming the destination addresses for a 1043 reply. 1045 3.6.3. Destination Address Fields 1047 The destination fields of a message consist of three possible fields, 1048 each of the same form: the field name, which is either "To", "Cc", or 1049 "Bcc", followed by a comma-separated list of one or more addresses 1050 (either mailbox or group syntax). 1052 to = "To:" address-list CRLF 1054 cc = "Cc:" address-list CRLF 1056 bcc = "Bcc:" [address-list / CFWS] CRLF 1058 The destination fields specify the recipients of the message. Each 1059 destination field may have one or more addresses, and the addresses 1060 indicate the intended recipients of the message. The only difference 1061 between the three fields is how each is used. 1063 The "To:" field contains the address(es) of the primary recipient(s) 1064 of the message. 1066 The "Cc:" field (where the "Cc" means "Carbon Copy" in the sense of 1067 making a copy on a typewriter using carbon paper) contains the 1068 addresses of others who are to receive the message, though the 1069 content of the message may not be directed at them. 1071 The "Bcc:" field (where the "Bcc" means "Blind Carbon Copy") contains 1072 addresses of recipients of the message whose addresses are not to be 1073 revealed to other recipients of the message. There are three ways in 1074 which the "Bcc:" field is used. In the first case, when a message 1075 containing a "Bcc:" field is prepared to be sent, the "Bcc:" line is 1076 removed even though all of the recipients (including those specified 1077 in the "Bcc:" field) are sent a copy of the message. In the second 1078 case, recipients specified in the "To:" and "Cc:" lines each are sent 1079 a copy of the message with the "Bcc:" line removed as above, but the 1080 recipients on the "Bcc:" line get a separate copy of the message 1081 containing a "Bcc:" line. (When there are multiple recipient 1082 addresses in the "Bcc:" field, some implementations actually send a 1083 separate copy of the message to each recipient with a "Bcc:" 1084 containing only the address of that particular recipient.) Finally, 1085 since a "Bcc:" field may contain no addresses, a "Bcc:" field can be 1086 used without any addresses indicating to the recipients that blind 1087 copies were sent to someone. Which method to use with "Bcc:" fields 1088 is implementation dependent, but refer to the "Security 1089 Considerations" section of this document for a discussion of each. 1091 When a message is a reply to another message, the mailboxes of the 1092 authors of the original message (the mailboxes in the "From:" field) 1093 or mailboxes specified in the "Reply-To:" field (if it exists) MAY 1094 appear in the "To:" field of the reply since these would normally be 1095 the primary recipients of the reply. If a reply is sent to a message 1096 that has destination fields, it is often desirable to send a copy of 1097 the reply to all of the recipients of the message, in addition to the 1098 author. When such a reply is formed, addresses in the "To:" and 1099 "Cc:" fields of the original message MAY appear in the "Cc:" field of 1100 the reply, since these are normally secondary recipients of the 1101 reply. If a "Bcc:" field is present in the original message, 1102 addresses in that field MAY appear in the "Bcc:" field of the reply, 1103 but they SHOULD NOT appear in the "To:" or "Cc:" fields. 1105 | Note: Some mail applications have automatic reply commands that 1106 | include the destination addresses of the original message in 1107 | the destination addresses of the reply. How those reply 1108 | commands behave is implementation dependent and is beyond the 1109 | scope of this document. In particular, whether or not to 1110 | include the original destination addresses when the original 1111 | message had a "Reply-To:" field is not addressed here. 1113 3.6.4. Identification Fields 1115 Though listed as optional in the table (Table 1) in section 3.6, 1116 every message SHOULD have a "Message-ID:" field. Furthermore, reply 1117 messages SHOULD have "In-Reply-To:" and "References:" fields as 1118 appropriate and as described below. 1120 The "Message-ID:" field contains a single unique message identifier. 1121 The "References:" and "In-Reply-To:" fields each contain one or more 1122 unique message identifiers, optionally separated by CFWS. 1124 The message identifier (msg-id) syntax is a limited version of the 1125 addr-spec construct enclosed in the angle bracket characters, "<" and 1126 ">". Unlike addr-spec, this syntax only permits the dot-atom-text 1127 form on the left-hand side of the "@" and does not have internal CFWS 1128 anywhere in the message identifier. 1130 | Note: As with addr-spec, a liberal syntax is given for the 1131 | right-hand side of the "@" in a msg-id. However, later in this 1132 | section, the use of a domain for the right-hand side of the "@" 1133 | is RECOMMENDED. Again, the syntax of domain constructs is 1134 | specified by and used in other protocols (e.g., [STD13], 1135 | [RFC1123], [RFC5321]). It is therefore incumbent upon 1136 | implementations to conform to the syntax of addresses for the 1137 | context in which they are used. 1139 message-id = "Message-ID:" msg-id CRLF 1141 in-reply-to = "In-Reply-To:" 1*msg-id CRLF 1143 references = "References:" 1*msg-id CRLF 1145 msg-id = [CFWS] "<" msg-id-internal ">" [CFWS] 1147 msg-id-internal = id-left "@" id-right 1149 id-left = dot-atom-text / obs-id-left 1151 id-right = dot-atom-text / no-fold-literal / obs-id-right 1153 no-fold-literal = "[" *dtext "]" 1155 The "Message-ID:" field provides a unique message identifier that 1156 refers to a particular version of a particular message. The 1157 uniqueness of the message identifier is guaranteed by the host that 1158 generates it (see below). This message identifier is intended to be 1159 machine readable and not necessarily meaningful to humans. A message 1160 identifier pertains to exactly one version of a particular message; 1161 subsequent revisions to the message each receive new message 1162 identifiers. 1164 | Note: There are many instances when messages are "changed", but 1165 | those changes do not constitute a new instantiation of that 1166 | message, and therefore the message would not get a new message 1167 | identifier. For example, when messages are introduced into the 1168 | transport system, they are often prepended with additional 1169 | header fields such as trace fields (described in section 3.6.7) 1170 | and resent fields (described in section 3.6.6). The addition 1171 | of such header fields does not change the identity of the 1172 | message and therefore the original "Message-ID:" field is 1173 | retained. In all cases, it is the meaning that the sender of 1174 | the message wishes to convey (i.e., whether this is the same 1175 | message or a different message) that determines whether or not 1176 | the "Message-ID:" field changes, not any particular syntactic 1177 | difference that appears (or does not appear) in the message. 1179 The "In-Reply-To:" and "References:" fields are used when creating a 1180 reply to a message. They hold the message identifier of the original 1181 message and the message identifiers of other messages (for example, 1182 in the case of a reply to a message that was itself a reply). The 1183 "In-Reply-To:" field may be used to identify the message (or 1184 messages) to which the new message is a reply, while the 1185 "References:" field may be used to identify a "thread" of 1186 conversation. 1188 When creating a reply to a message, the "In-Reply-To:" and 1189 "References:" fields of the resultant message are constructed as 1190 follows: 1192 The "In-Reply-To:" field will contain the contents of the "Message- 1193 ID:" field of the message to which this one is a reply (the "parent 1194 message"). If there is more than one parent message, then the "In- 1195 Reply-To:" field will contain the contents of all of the parents' 1196 "Message-ID:" fields. If there is no "Message-ID:" field in any of 1197 the parent messages, then the new message will have no "In-Reply-To:" 1198 field. 1200 The "References:" field will contain the contents of the parent's 1201 "References:" field (if any) followed by the contents of the parent's 1202 "Message-ID:" field (if any). If the parent message does not contain 1203 a "References:" field but does have an "In-Reply-To:" field 1204 containing a single message identifier, then the "References:" field 1205 will contain the contents of the parent's "In-Reply-To:" field 1206 followed by the contents of the parent's "Message-ID:" field (if 1207 any). If the parent has none of the "References:", "In-Reply-To:", 1208 or "Message-ID:" fields, then the new message will have no 1209 "References:" field. 1211 | Note: Some implementations parse the "References:" field to 1212 | display the "thread of the discussion". These implementations 1213 | assume that each new message is a reply to a single parent and 1214 | hence that they can walk backwards through the "References:" 1215 | field to find the parent of each message listed there. 1216 | Therefore, trying to form a "References:" field for a reply 1217 | that has multiple parents is discouraged; how to do so is not 1218 | defined in this document. 1220 The message identifier (msg-id) itself MUST be a globally unique 1221 identifier for a message. The generator of the message identifier 1222 MUST guarantee that the msg-id is unique. There are several 1223 algorithms that can be used to accomplish this. Since the msg-id has 1224 a similar syntax to addr-spec (identical except that quoted strings, 1225 comments, and folding white space are not allowed), a good method is 1226 to put the domain name (or a domain literal IP address) of the host 1227 on which the message identifier was created on the right-hand side of 1228 the "@" (since domain names and IP addresses are normally unique), 1229 and put a combination of the current absolute date and time along 1230 with some other currently unique (perhaps sequential) identifier 1231 available on the system (for example, a process id number) on the 1232 left-hand side. Though other algorithms will work, it is RECOMMENDED 1233 that the right-hand side contain some domain identifier (either of 1234 the host itself or otherwise) such that the generator of the message 1235 identifier can guarantee the uniqueness of the left-hand side within 1236 the scope of that domain. 1238 Semantically, the angle bracket characters are not part of the msg- 1239 id; the msg-id is what is contained between the two angle bracket 1240 characters. 1242 3.6.5. Informational Fields 1244 The informational fields are all optional. The "Subject:" and 1245 "Comments:" fields are unstructured fields as defined in section 1246 2.2.1, and therefore may contain text or folding white space. The 1247 "Keywords:" field contains a comma-separated list of one or more 1248 words or quoted-strings. 1250 subject = "Subject:" unstructured CRLF 1252 comments = "Comments:" unstructured CRLF 1254 keywords = "Keywords:" phrase *("," phrase) CRLF 1256 These three fields are intended to have only human-readable content 1257 with information about the message. The "Subject:" field is the most 1258 common and contains a short string identifying the topic of the 1259 message. When used in a reply, the field body MAY start with the 1260 string "Re: " (an abbreviation of the Latin "in re", meaning "in the 1261 matter of") followed by the contents of the "Subject:" field body of 1262 the original message. If this is done, only one instance of the 1263 literal string "Re: " ought to be used since use of other strings or 1264 more than one instance can lead to undesirable consequences. The 1265 "Comments:" field contains any additional comments on the text of the 1266 body of the message. The "Keywords:" field contains a comma- 1267 separated list of important words and phrases that might be useful 1268 for the recipient. 1270 3.6.6. Resent Fields 1272 Resent fields SHOULD be added to any message that is reintroduced by 1273 a user into the transport system. A separate set of resent fields 1274 SHOULD be added each time this is done. All of the resent fields 1275 corresponding to a particular resending of the message SHOULD be 1276 grouped together. Each new set of resent fields is prepended to the 1277 message; that is, the most recent set of resent fields appears 1278 earlier in the message. No other fields in the message are changed 1279 when resent fields are added. 1281 Each of the resent fields corresponds to a particular field elsewhere 1282 in the syntax. For instance, the "Resent-Date:" field corresponds to 1283 the "Date:" field and the "Resent-To:" field corresponds to the "To:" 1284 field. In each case, the syntax for the field body is identical to 1285 the syntax given previously for the corresponding field. 1287 When resent fields are used, the "Resent-From:" and "Resent-Date:" 1288 fields MUST be present. The "Resent-Message-ID:" field SHOULD be 1289 present. "Resent-Sender:" SHOULD NOT be used if "Resent-Sender:" 1290 would be identical to "Resent-From:". 1292 resent-date = "Resent-Date:" date-time CRLF 1294 resent-from = "Resent-From:" mailbox-list CRLF 1296 resent-sender = "Resent-Sender:" mailbox CRLF 1298 resent-to = "Resent-To:" address-list CRLF 1300 resent-cc = "Resent-Cc:" address-list CRLF 1302 resent-bcc = "Resent-Bcc:" [address-list / CFWS] CRLF 1304 resent-msg-id = "Resent-Message-ID:" msg-id CRLF 1306 Resent fields are used to identify a message as having been 1307 reintroduced into the transport system by a user. The purpose of 1308 using resent fields is to have the message appear to the final 1309 recipient as if it were sent directly by the original sender, with 1310 all of the original fields remaining the same. Each set of resent 1311 fields correspond to a particular resending event. That is, if a 1312 message is resent multiple times, each set of resent fields gives 1313 identifying information for each individual time. Resent fields are 1314 strictly informational. They MUST NOT be used in the normal 1315 processing of replies or other such automatic actions on messages. 1317 | Note: Reintroducing a message into the transport system and 1318 | using resent fields is a different operation from "forwarding". 1319 | "Forwarding" has two meanings: One sense of forwarding is that 1320 | a mail reading program can be told by a user to forward a copy 1321 | of a message to another person, making the forwarded message 1322 | the body of the new message. A forwarded message in this sense 1323 | does not appear to have come from the original sender, but is 1324 | an entirely new message from the forwarder of the message. 1325 | Forwarding may also mean that a mail transport program gets a 1326 | message and forwards it on to a different destination for final 1327 | delivery. Resent header fields are not intended for use with 1328 | either type of forwarding. 1330 The resent originator fields indicate the mailbox of the person(s) or 1331 system(s) that resent the message. As with the regular originator 1332 fields, there are two forms: a simple "Resent-From:" form, which 1333 contains the mailbox of the individual doing the resending, and the 1334 more complex form, when one individual (identified in the "Resent- 1335 Sender:" field) resends a message on behalf of one or more others 1336 (identified in the "Resent-From:" field). 1338 | Note: When replying to a resent message, replies behave just as 1339 | they would with any other message, using the original "From:", 1340 | "Reply-To:", "Message-ID:", and other fields. The resent 1341 | fields are only informational and MUST NOT be used in the 1342 | normal processing of replies. 1344 The "Resent-Date:" indicates the date and time at which the resent 1345 message is dispatched by the resender of the message. Like the 1346 "Date:" field, it is not the date and time that the message was 1347 actually transported. 1349 The "Resent-To:", "Resent-Cc:", and "Resent-Bcc:" fields function 1350 identically to the "To:", "Cc:", and "Bcc:" fields, respectively, 1351 except that they indicate the recipients of the resent message, not 1352 the recipients of the original message. 1354 The "Resent-Message-ID:" field provides a unique identifier for the 1355 resent message. 1357 3.6.7. Trace Fields 1359 The trace fields are a group of header fields consisting of an 1360 optional "Return-Path:" field, and one or more "Received:" fields. 1361 The "Return-Path:" header field contains a pair of angle brackets 1362 that enclose an optional addr-spec. The "Received:" field contains a 1363 (possibly empty) list of tokens followed by a semicolon and a date- 1364 time specification. Each token must be a word, angle-addr, addr- 1365 spec, or a domain. Further restrictions are applied to the syntax of 1366 the trace fields by specifications that provide for their use, such 1367 as [RFC5321]. 1369 trace = [return] 1370 1*received 1372 return = "Return-Path:" path CRLF 1374 path = angle-addr / ([CFWS] "<" [CFWS] ">" [CFWS]) 1376 received = "Received:" 1377 [1*received-token / CFWS] ";" date-time CRLF 1379 received-token = word / angle-addr / addr-spec / domain 1381 A full discussion of the Internet mail use of trace fields is 1382 contained in [RFC5321]. For the purposes of this specification, the 1383 trace fields are strictly informational, and any formal 1384 interpretation of them is outside of the scope of this document. 1386 3.6.8. Optional Fields 1388 Fields may appear in messages that are otherwise unspecified in this 1389 document. They MUST conform to the syntax of an optional-field. 1390 This is a field name, made up of the visible US-ASCII characters 1391 except colon, followed by a colon, followed by any text that conforms 1392 to the unstructured syntax. 1394 The field names of any optional field MUST NOT be identical to any 1395 field name specified elsewhere in this document. 1397 optional-field = field-name ":" unstructured CRLF 1399 field-name = 1*ftext 1401 ftext = %d33-57 / ; Visible US-ASCII 1402 %d59-126 ; characters not including 1403 ; ":". 1405 // Erratum 5918 (https://www.rfc-editor.org/errata/eid5918) basically 1406 // suggests changing field-name to 1*77ftext (leaving room for the // 1407 colon and folding white space). That's probably what was // 1408 intended, but it probably also requires an obs-field-name, and // 1409 there's no indication that the current text has ever caused a // 1410 problem. The author is ambivalent. 1412 For the purposes of this specification, any optional field is 1413 uninterpreted. 1415 4. Obsolete Syntax 1417 Earlier versions of this specification allowed for different (usually 1418 more liberal) syntax than is allowed in this version. Also, there 1419 have been syntactic elements used in messages on the Internet whose 1420 interpretations have never been documented. Though these syntactic 1421 forms MUST NOT be generated according to the grammar in section 3, 1422 they MUST be accepted and parsed by a conformant receiver. This 1423 section documents many of these syntactic elements. Taking the 1424 grammar in section 3 and adding the definitions presented in this 1425 section will result in the grammar to use for the interpretation of 1426 messages. 1428 | Note: This section identifies syntactic forms that any 1429 | implementation MUST reasonably interpret. However, there are 1430 | certainly Internet messages that do not conform to even the 1431 | additional syntax given in this section. The fact that a 1432 | particular form does not appear in any section of this document 1433 | is not justification for computer programs to crash or for 1434 | malformed data to be irretrievably lost by any implementation. 1435 | It is up to the implementation to deal with messages robustly. 1437 One important difference between the obsolete (interpreting) and the 1438 current (generating) syntax is that in structured header field bodies 1439 (i.e., between the colon and the CRLF of any structured header 1440 field), white space characters, including folding white space, and 1441 comments could be freely inserted between any syntactic tokens. This 1442 allowed many complex forms that have proven difficult for some 1443 implementations to parse. 1445 Another key difference between the obsolete and the current syntax is 1446 that the rule in section 3.2.2 regarding lines composed entirely of 1447 white space in comments and folding white space does not apply. See 1448 the discussion of folding white space in section 4.2 below. 1450 Finally, certain characters that were formerly allowed in messages 1451 appear in this section. The NUL character (ASCII value 0) was once 1452 allowed, but is no longer for compatibility reasons. Similarly, US- 1453 ASCII control characters other than CR, LF, SP, and HTAB (ASCII 1454 values 1 through 8, 11, 12, 14 through 31, and 127) were allowed to 1455 appear in header field bodies. CR and LF were allowed to appear in 1456 messages other than as CRLF; this use is also shown here. 1458 Other differences in syntax and semantics are noted in the following 1459 sections. 1461 4.1. Miscellaneous Obsolete Tokens 1463 These syntactic elements are used elsewhere in the obsolete syntax or 1464 in the main syntax. Bare CR, bare LF, and NUL are added to obs-qp, 1465 obs-body, and obs-unstruct. US-ASCII control characters are added to 1466 obs-qp, obs-unstruct, obs-ctext, and obs-qtext. The period character 1467 is added to obs-phrase. The obs-phrase-list provides for a 1468 (potentially empty) comma-separated list of phrases that may include 1469 "null" elements. That is, there could be two or more commas in such 1470 a list with nothing in between them, or commas at the beginning or 1471 end of the list. 1473 | Note: The "period" (or "full stop") character (".") in obs- 1474 | phrase is not a form that was allowed in earlier versions of 1475 | this or any other specification. Period (nor any other 1476 | character from specials) was not allowed in phrase because it 1477 | introduced a parsing difficulty distinguishing between phrases 1478 | and portions of an addr-spec (see section 4.4). It appears 1479 | here because the period character is currently used in many 1480 | messages in the display-name portion of addresses, especially 1481 | for initials in names, and therefore must be interpreted 1482 | properly. 1484 obs-NO-WS-CTL = %d1-8 / ; US-ASCII control 1485 %d11 / ; characters that do not 1486 %d12 / ; include the carriage 1487 %d14-31 / ; return, line feed, and 1488 %d127 ; white space characters 1490 obs-ctext = obs-NO-WS-CTL 1492 obs-qtext = obs-NO-WS-CTL 1494 obs-utext = %d0 / obs-NO-WS-CTL / VCHAR 1496 obs-qp = "\" (%d0 / obs-NO-WS-CTL / LF / CR) 1498 obs-body = *(%d0 / LF / CR / text) 1500 obs-unstruct = *((*CR 1*(obs-utext / FWS)) / 1*LF) *CR 1502 obs-phrase = word *(word / "." / CFWS) 1504 obs-phrase-list = [phrase / CFWS] *("," [phrase / CFWS]) 1506 Bare CR and bare LF appear in messages with two different meanings. 1507 In many cases, bare CR or bare LF are used improperly instead of CRLF 1508 to indicate line separators. In other cases, bare CR and bare LF are 1509 used simply as US-ASCII control characters with their traditional 1510 ASCII meanings. 1512 4.2. Obsolete Folding White Space 1514 In the obsolete syntax, any amount of folding white space MAY be 1515 inserted where the obs-FWS rule is allowed. This creates the 1516 possibility of having two consecutive "folds" in a line, and 1517 therefore the possibility that a line which makes up a folded header 1518 field could be composed entirely of white space. 1520 obs-FWS = 1*([CRLF] WSP) 1522 4.3. Obsolete Date and Time 1524 The syntax for the obsolete date format allows a 2 digit year in the 1525 date field and allows for a list of alphabetic time zone specifiers 1526 that were used in earlier versions of this specification. It also 1527 permits comments and folding white space between many of the tokens. 1529 obs-day-of-week = [CFWS] day-name [CFWS] 1531 obs-day = [CFWS] 1*2DIGIT [CFWS] 1533 obs-year = [CFWS] 2*DIGIT [CFWS] 1535 obs-hour = [CFWS] 2DIGIT [CFWS] 1537 obs-minute = [CFWS] 2DIGIT [CFWS] 1539 obs-second = [CFWS] 2DIGIT [CFWS] 1541 obs-zone = "UT" / "GMT" / ; Universal Time 1542 ; North American UT 1543 ; offsets 1544 "EST" / "EDT" / ; Eastern: - 5/ - 4 1545 "CST" / "CDT" / ; Central: - 6/ - 5 1546 "MST" / "MDT" / ; Mountain: - 7/ - 6 1547 "PST" / "PDT" / ; Pacific: - 8/ - 7 1548 ; 1549 %d65-73 / ; Military zones - "A" 1550 %d75-90 / ; through "I" and "K" 1551 %d97-105 / ; through "Z", both 1552 %d107-122 ; upper and lower case 1554 Where a two or three digit year occurs in a date, the year is to be 1555 interpreted as follows: If a two digit year is encountered whose 1556 value is between 00 and 49, the year is interpreted by adding 2000, 1557 ending up with a value between 2000 and 2049. If a two digit year is 1558 encountered with a value between 50 and 99, or any three digit year 1559 is encountered, the year is interpreted by adding 1900. 1561 In the obsolete time zone, "UT" and "GMT" are indications of 1562 "Universal Time" and "Greenwich Mean Time", respectively, and are 1563 both semantically identical to "+0000". 1565 The remaining three character zones are the US time zones. The first 1566 letter, "E", "C", "M", or "P" stands for "Eastern", "Central", 1567 "Mountain", and "Pacific". The second letter is either "S" for 1568 "Standard" time, or "D" for "Daylight Savings" (or summer) time. 1569 Their interpretations are as follows: 1571 EDT is semantically equivalent to -0400 1572 EST is semantically equivalent to -0500 1573 CDT is semantically equivalent to -0500 1574 CST is semantically equivalent to -0600 1575 MDT is semantically equivalent to -0600 1576 MST is semantically equivalent to -0700 1577 PDT is semantically equivalent to -0700 1578 PST is semantically equivalent to -0800 1580 The 1 character military time zones were defined in a non-standard 1581 way in [RFC0822] and are therefore unpredictable in their meaning. 1582 The original definitions of the military zones "A" through "I" are 1583 equivalent to "+0100" through "+0900", respectively; "K", "L", and 1584 "M" are equivalent to "+1000", "+1100", and "+1200", respectively; 1585 "N" through "Y" are equivalent to "-0100" through "-1200". 1586 respectively; and "Z" is equivalent to "+0000". However, because of 1587 the error in [RFC0822], they SHOULD all be considered equivalent to 1588 "-0000" unless there is out-of-band information confirming their 1589 meaning. 1591 Other multi-character (usually between 3 and 5) alphabetic time zones 1592 have been used in Internet messages. Any such time zone whose 1593 meaning is not known SHOULD be considered equivalent to "-0000" 1594 unless there is out-of-band information confirming their meaning. 1596 4.4. Obsolete Addressing 1598 There are four primary differences in addressing. First, mailbox 1599 addresses were allowed to have a route portion before the addr-spec 1600 when enclosed in "<" and ">". The route is simply a comma-separated 1601 list of domain names, each preceded by "@", and the list terminated 1602 by a colon. Second, CFWS were allowed between the period-separated 1603 elements of local-part and domain (i.e., dot-atom was not used). In 1604 addition, local-part is allowed to contain quoted-string in addition 1605 to just atom. Third, mailbox-list and address-list were allowed to 1606 have "null" members. That is, there could be two or more commas in 1607 such a list with nothing in between them, or commas at the beginning 1608 or end of the list. Finally, US-ASCII control characters and quoted- 1609 pairs were allowed in domain literals and are added here. 1611 obs-angle-addr = [CFWS] "<" obs-route addr-spec ">" [CFWS] 1613 obs-route = obs-domain-list ":" 1615 obs-domain-list = *(CFWS / ",") "@" domain 1616 *("," [CFWS] ["@" domain]) 1618 obs-mbox-list = *([CFWS] ",") mailbox *("," [mailbox / CFWS]) 1620 obs-addr-list = *([CFWS] ",") address *("," [address / CFWS]) 1622 obs-group-list = 1*([CFWS] ",") [CFWS] 1624 obs-local-part = word *("." word) 1626 obs-domain = atom *("." atom) 1628 obs-dtext = obs-NO-WS-CTL / quoted-pair 1630 When interpreting addresses, the route portion SHOULD be ignored. 1632 4.5. Obsolete Header Fields 1634 Syntactically, the primary difference in the obsolete field syntax is 1635 that it allows multiple occurrences of any of the fields and they may 1636 occur in any order. Also, any amount of white space is allowed 1637 before the ":" at the end of the field name. 1639 obs-fields = *(obs-return / 1640 obs-received / 1641 obs-orig-date / 1642 obs-from / 1643 obs-sender / 1644 obs-reply-to / 1645 obs-to / 1646 obs-cc / 1647 obs-bcc / 1648 obs-message-id / 1649 obs-in-reply-to / 1650 obs-references / 1651 obs-subject / 1652 obs-comments / 1653 obs-keywords / 1654 obs-resent-date / 1655 obs-resent-from / 1656 obs-resent-send / 1657 obs-resent-rply / 1658 obs-resent-to / 1659 obs-resent-cc / 1660 obs-resent-bcc / 1661 obs-resent-mid / 1662 obs-optional) 1664 Except for destination address fields (described in section 4.5.3), 1665 the interpretation of multiple occurrences of fields is unspecified. 1666 Also, the interpretation of trace fields and resent fields that do 1667 not occur in blocks prepended to the message is unspecified as well. 1668 Unless otherwise noted in the following sections, interpretation of 1669 other fields is identical to the interpretation of their non-obsolete 1670 counterparts in section 3. 1672 4.5.1. Obsolete Origination Date Field 1674 obs-orig-date = "Date" *WSP ":" date-time CRLF 1676 4.5.2. Obsolete Originator Fields 1678 obs-from = "From" *WSP ":" mailbox-list CRLF 1680 obs-sender = "Sender" *WSP ":" mailbox CRLF 1682 obs-reply-to = "Reply-To" *WSP ":" address-list CRLF 1684 4.5.3. Obsolete Destination Address Fields 1685 obs-to = "To" *WSP ":" address-list CRLF 1687 obs-cc = "Cc" *WSP ":" address-list CRLF 1689 obs-bcc = "Bcc" *WSP ":" 1690 (address-list / (*([CFWS] ",") [CFWS])) CRLF 1692 When multiple occurrences of destination address fields occur in a 1693 message, they SHOULD be treated as if the address list in the first 1694 occurrence of the field is combined with the address lists of the 1695 subsequent occurrences by adding a comma and concatenating. 1697 4.5.4. Obsolete Identification Fields 1699 The obsolete "In-Reply-To:" and "References:" fields differ from the 1700 current syntax in that they allow phrase (words or quoted strings) to 1701 appear. The obsolete forms of the left and right sides of msg-id 1702 allow interspersed CFWS, making them syntactically identical to 1703 local-part and domain, respectively. 1705 obs-message-id = "Message-ID" *WSP ":" msg-id CRLF 1707 obs-in-reply-to = "In-Reply-To" *WSP ":" *(phrase / msg-id) CRLF 1709 obs-references = "References" *WSP ":" *(phrase / msg-id) CRLF 1711 obs-id-left = local-part 1713 obs-id-right = domain 1715 For purposes of interpretation, the phrases in the "In-Reply-To:" and 1716 "References:" fields are ignored. 1718 Semantically, none of the optional CFWS in the local-part and the 1719 domain is part of the obs-id-left and obs-id-right, respectively. 1721 4.5.5. Obsolete Informational Fields 1723 obs-subject = "Subject" *WSP ":" unstructured CRLF 1725 obs-comments = "Comments" *WSP ":" unstructured CRLF 1727 obs-keywords = "Keywords" *WSP ":" obs-phrase-list CRLF 1729 4.5.6. Obsolete Resent Fields 1731 The obsolete syntax adds a "Resent-Reply-To:" field, which consists 1732 of the field name, the optional comments and folding white space, the 1733 colon, and a comma separated list of addresses. 1735 obs-resent-from = "Resent-From" *WSP ":" mailbox-list CRLF 1737 obs-resent-send = "Resent-Sender" *WSP ":" mailbox CRLF 1739 obs-resent-date = "Resent-Date" *WSP ":" date-time CRLF 1741 obs-resent-to = "Resent-To" *WSP ":" address-list CRLF 1743 obs-resent-cc = "Resent-Cc" *WSP ":" address-list CRLF 1745 obs-resent-bcc = "Resent-Bcc" *WSP ":" 1746 (address-list / (*([CFWS] ",") [CFWS])) CRLF 1748 obs-resent-mid = "Resent-Message-ID" *WSP ":" msg-id CRLF 1750 obs-resent-rply = "Resent-Reply-To" *WSP ":" address-list CRLF 1752 As with other resent fields, the "Resent-Reply-To:" field is to be 1753 treated as trace information only. 1755 4.5.7. Obsolete Trace Fields 1757 The obs-return and obs-received are again given here as template 1758 definitions, just as return and received are in section 3. Their 1759 full syntax is given in [RFC5321]. 1761 obs-return = "Return-Path" *WSP ":" path CRLF 1763 obs-received = "Received" *WSP ":" 1764 [1*received-token / CFWS] [ ";" date-time CRLF ] 1766 4.5.8. Obsolete optional fields 1768 obs-optional = field-name *WSP ":" unstructured CRLF 1770 5. Security Considerations 1772 Care needs to be taken when displaying messages on a terminal or 1773 terminal emulator. Powerful terminals may act on escape sequences 1774 and other combinations of US-ASCII control characters with a variety 1775 of consequences. They can remap the keyboard or permit other 1776 modifications to the terminal that could lead to denial of service or 1777 even damaged data. They can trigger (sometimes programmable) 1778 answerback messages that can allow a message to cause commands to be 1779 issued on the recipient's behalf. They can also affect the operation 1780 of terminal attached devices such as printers. Message viewers may 1781 wish to strip potentially dangerous terminal escape sequences from 1782 the message prior to display. However, other escape sequences appear 1783 in messages for useful purposes (cf. [ISO.2022.1994], [RFC2045], 1784 [RFC2046], [RFC2047], [RFC2049], [BCP13]) and therefore should not be 1785 stripped indiscriminately. 1787 Transmission of non-text objects in messages raises additional 1788 security issues. These issues are discussed in [RFC2045], [RFC2046], 1789 [RFC2047], [RFC2049], [BCP13]. 1791 Many implementations use the "Bcc:" (blind carbon copy) field, 1792 described in section 3.6.3, to facilitate sending messages to 1793 recipients without revealing the addresses of one or more of the 1794 addressees to the other recipients. Mishandling this use of "Bcc:" 1795 may disclose confidential information that could eventually lead to 1796 security problems through knowledge of even the existence of a 1797 particular mail address. For example, if using the first method 1798 described in section 3.6.3, where the "Bcc:" line is removed from the 1799 message, blind recipients have no explicit indication that they have 1800 been sent a blind copy, except insofar as their address does not 1801 appear in the header section of a message. Because of this, one of 1802 the blind addressees could potentially send a reply to all of the 1803 shown recipients and accidentally reveal that the message went to the 1804 blind recipient. When the second method from section 3.6.3 is used, 1805 the blind recipient's address appears in the "Bcc:" field of a 1806 separate copy of the message. If the "Bcc:" field contains all of 1807 the blind addressees, all of the "Bcc:" recipients will be seen by 1808 each "Bcc:" recipient. Even if a separate message is sent to each 1809 "Bcc:" recipient with only the individual's address, implementations 1810 still need to be careful to process replies to the message as per 1811 section 3.6.3 so as not to accidentally reveal the blind recipient to 1812 other recipients. 1814 6. IANA Considerations 1816 This document updates the registrations that appeared in [RFC4021] 1817 that referred to the definitions in [RFC5322]. IANA is requested to 1818 update the Permanent Message Header Field Repository with the 1819 following header fields, in accordance with the procedures set out in 1820 [RFC3864]. 1822 Header field name Date 1823 Applicable protocol Mail 1824 Status standard 1825 Author/Change controller IETF 1826 Specification document(s) This document (section 3.6.1) 1828 Header field name From 1829 Applicable protocol Mail 1830 Status standard 1831 Author/Change controller IETF 1832 Specification document(s) This document (section 3.6.2) 1834 Header field name Sender 1835 Applicable protocol Mail 1836 Status standard 1837 Author/Change controller IETF 1838 Specification document(s) This document (section 3.6.2) 1840 Header field name Reply-To 1841 Applicable protocol Mail 1842 Status standard 1843 Author/Change controller IETF 1844 Specification document(s) This document (section 3.6.2) 1846 Header field name To 1847 Applicable protocol Mail 1848 Status standard 1849 Author/Change controller IETF 1850 Specification document(s) This document (section 3.6.3) 1852 Header field name Cc 1853 Applicable protocol Mail 1854 Status standard 1855 Author/Change controller IETF 1856 Specification document(s) This document (section 3.6.3) 1858 Header field name Bcc 1859 Applicable protocol Mail 1860 Status standard 1861 Author/Change controller IETF 1862 Specification document(s) This document (section 3.6.3) 1864 Header field name Message-ID 1865 Applicable protocol Mail 1866 Status standard 1867 Author/Change controller IETF 1868 Specification document(s) This document (section 3.6.4) 1870 Header field name In-Reply-To 1871 Applicable protocol Mail 1872 Status standard 1873 Author/Change controller IETF 1874 Specification document(s) This document (section 3.6.4) 1876 Header field name References 1877 Applicable protocol Mail 1878 Status standard 1879 Author/Change controller IETF 1880 Specification document(s) This document (section 3.6.4) 1882 Header field name Subject 1883 Applicable protocol Mail 1884 Status standard 1885 Author/Change controller IETF 1886 Specification document(s) This document (section 3.6.5) 1888 Header field name Comments 1889 Applicable protocol Mail 1890 Status standard 1891 Author/Change controller IETF 1892 Specification document(s) This document (section 3.6.5) 1894 Header field name Keywords 1895 Applicable protocol Mail 1896 Status standard 1897 Author/Change controller IETF 1898 Specification document(s) This document (section 3.6.5) 1900 Header field name Resent-Date 1901 Applicable protocol Mail 1902 Status standard 1903 Author/Change controller IETF 1904 Specification document(s) This document (section 3.6.6) 1906 Header field name Resent-From 1907 Applicable protocol Mail 1908 Status standard 1909 Author/Change controller IETF 1910 Specification document(s) This document (section 3.6.6) 1912 Header field name Resent-Sender 1913 Applicable protocol Mail 1914 Status standard 1915 Author/Change controller IETF 1916 Specification document(s) This document (section 3.6.6) 1918 Header field name Resent-To 1919 Applicable protocol Mail 1920 Status standard 1921 Author/Change controller IETF 1922 Specification document(s) This document (section 3.6.6) 1924 Header field name Resent-Cc 1925 Applicable protocol Mail 1926 Status standard 1927 Author/Change controller IETF 1928 Specification document(s) This document (section 3.6.6) 1930 Header field name Resent-Bcc 1931 Applicable protocol Mail 1932 Status standard 1933 Author/Change controller IETF 1934 Specification document(s) This document (section 3.6.6) 1936 Header field name Resent-Reply-To 1937 Applicable protocol Mail 1938 Status obsolete 1939 Author/Change controller IETF 1940 Specification document(s) This document (section 4.5.6) 1942 Header field name Resent-Message-ID 1943 Applicable protocol Mail 1944 Status standard 1945 Author/Change controller IETF 1946 Specification document(s) This document (section 3.6.6) 1948 Header field name Return-Path 1949 Applicable protocol Mail 1950 Status standard 1951 Author/Change controller IETF 1952 Specification document(s) This document (section 3.6.7) 1954 Header field name Received 1955 Applicable protocol Mail 1956 Status standard 1957 Author/Change controller IETF 1958 Specification document(s) This document (section 3.6.7) 1959 Related information [RFC5321] 1961 7. References 1963 7.1. Normative References 1965 [ANSI.X3-4.1986] 1966 American National Standards Institute, "Coded Character 1967 Set - 7-bit American Standard Code for Information 1968 Interchange", ANSI X3.4, 1986. 1970 [BCP14] Bradner, S., "Key words for use in RFCs to Indicate 1971 Requirement Levels", BCP 14, RFC 2119, March 1997. 1973 Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1974 2119 Key Words", BCP 14, RFC 8174, May 2017. 1976 1978 [RFC1123] Braden, R., Ed., "Requirements for Internet Hosts - 1979 Application and Support", STD 3, RFC 1123, 1980 DOI 10.17487/RFC1123, October 1989, 1981 . 1983 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 1984 Specifications: ABNF", STD 68, RFC 5234, 1985 DOI 10.17487/RFC5234, January 2008, 1986 . 1988 [STD13] Mockapetris, P., "Domain names - concepts and facilities", 1989 STD 13, RFC 1034, November 1987. 1991 Mockapetris, P., "Domain names - implementation and 1992 specification", STD 13, RFC 1035, November 1987. 1994 1996 7.2. Informative References 1998 [BCP13] Freed, N. and J. Klensin, "Multipurpose Internet Mail 1999 Extensions (MIME) Part Four: Registration Procedures", 2000 BCP 13, RFC 4289, December 2005. 2002 Freed, N., Klensin, J., and T. Hansen, "Media Type 2003 Specifications and Registration Procedures", BCP 13, 2004 RFC 6838, January 2013. 2006 2008 [ISO.2022.1994] 2009 International Organization for Standardization, 2010 "Information technology - Character code structure and 2011 extension techniques", ISO Standard 2022, 1994. 2013 [RFC0822] Crocker, D., "STANDARD FOR THE FORMAT OF ARPA INTERNET 2014 TEXT MESSAGES", STD 11, RFC 822, DOI 10.17487/RFC0822, 2015 August 1982, . 2017 [RFC1305] Mills, D., "Network Time Protocol (Version 3) 2018 Specification, Implementation and Analysis", RFC 1305, 2019 DOI 10.17487/RFC1305, March 1992, 2020 . 2022 [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 2023 Extensions (MIME) Part One: Format of Internet Message 2024 Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996, 2025 . 2027 [RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 2028 Extensions (MIME) Part Two: Media Types", RFC 2046, 2029 DOI 10.17487/RFC2046, November 1996, 2030 . 2032 [RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions) 2033 Part Three: Message Header Extensions for Non-ASCII Text", 2034 RFC 2047, DOI 10.17487/RFC2047, November 1996, 2035 . 2037 [RFC2049] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 2038 Extensions (MIME) Part Five: Conformance Criteria and 2039 Examples", RFC 2049, DOI 10.17487/RFC2049, November 1996, 2040 . 2042 [RFC2822] Resnick, P., Ed., "Internet Message Format", RFC 2822, 2043 DOI 10.17487/RFC2822, April 2001, 2044 . 2046 [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration 2047 Procedures for Message Header Fields", BCP 90, RFC 3864, 2048 DOI 10.17487/RFC3864, September 2004, 2049 . 2051 [RFC4021] Klyne, G. and J. Palme, "Registration of Mail and MIME 2052 Header Fields", RFC 4021, DOI 10.17487/RFC4021, March 2053 2005, . 2055 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 2056 DOI 10.17487/RFC5321, October 2008, 2057 . 2059 [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, 2060 DOI 10.17487/RFC5322, October 2008, 2061 . 2063 [RFC6532] Yang, A., Steele, S., and N. Freed, "Internationalized 2064 Email Headers", RFC 6532, DOI 10.17487/RFC6532, February 2065 2012, . 2067 Appendix A. Example Messages 2069 This section presents a selection of messages. These are intended to 2070 assist in the implementation of this specification, but should not be 2071 taken as normative; that is to say, although the examples in this 2072 section were carefully reviewed, if there happens to be a conflict 2073 between these examples and the syntax described in sections 3 and 4 2074 of this document, the syntax in those sections is to be taken as 2075 correct. 2077 In the text version of this document, messages in this section are 2078 delimited between lines of "----". The "----" lines are not part of 2079 the message itself. 2081 A.1. Addressing Examples 2083 The following are examples of messages that might be sent between two 2084 individuals. 2086 A.1.1. A Message from One Person to Another with Simple Addressing 2088 This could be called a canonical message. It has a single author, 2089 John Doe, a single recipient, Mary Smith, a subject, the date, a 2090 message identifier, and a textual message in the body. 2092 From: John Doe 2093 To: Mary Smith 2094 Subject: Saying Hello 2095 Date: Fri, 21 Nov 1997 09:55:06 -0600 2096 Message-ID: <1234@local.machine.example> 2098 This is a message just to say hello. 2099 So, "Hello". 2101 If John's secretary Michael actually sent the message, even though 2102 John was the author and replies to this message should go back to 2103 him, the sender field would be used: 2105 From: John Doe 2106 Sender: Michael Jones 2107 To: Mary Smith 2108 Subject: Saying Hello 2109 Date: Fri, 21 Nov 1997 09:55:06 -0600 2110 Message-ID: <1234@local.machine.example> 2112 This is a message just to say hello. 2113 So, "Hello". 2115 A.1.2. Different Types of Mailboxes 2117 This message includes multiple addresses in the destination fields 2118 and also uses several different forms of addresses. 2120 From: "Joe Q. Public" 2121 To: Mary Smith , jdoe@example.org, Who? 2122 Cc: , "Giant; \"Big\" Box" 2123 Date: Tue, 1 Jul 2003 10:52:37 +0200 2124 Message-ID: <5678.21-Nov-1997@example.com> 2126 Hi everyone. 2128 Note that the display names for Joe Q. Public and Giant; "Big" Box 2129 needed to be enclosed in double-quotes because the former contains 2130 the period and the latter contains both semicolon and double-quote 2131 characters (the double-quote characters appearing as quoted-pair 2132 constructs). Conversely, the display name for Who? could appear 2133 without them because the question mark is legal in an atom. Notice 2134 also that jdoe@example.org and boss@nil.test have no display names 2135 associated with them at all, and jdoe@example.org uses the simpler 2136 address form without the angle brackets. 2138 A.1.3. Group Addresses 2140 From: Pete 2141 To: A Group:Ed Jones ,joe@where.test,John ; 2142 Cc: Undisclosed recipients:; 2143 Date: Thu, 13 Feb 1969 23:32:54 -0330 2144 Message-ID: 2146 Testing. 2148 In this message, the "To:" field has a single group recipient named 2149 "A Group", which contains 3 addresses, and a "Cc:" field with an 2150 empty group recipient named Undisclosed recipients. 2152 A.2. Reply Messages 2154 The following is a series of three messages that make up a 2155 conversation thread between John and Mary. John first sends a 2156 message to Mary, Mary then replies to John's message, and then John 2157 replies to Mary's reply message. 2159 Note especially the "Message-ID:", "References:", and "In-Reply-To:" 2160 fields in each message. 2162 From: John Doe 2163 To: Mary Smith 2164 Subject: Saying Hello 2165 Date: Fri, 21 Nov 1997 09:55:06 -0600 2166 Message-ID: <1234@local.machine.example> 2168 This is a message just to say hello. 2169 So, "Hello". 2171 When sending replies, the Subject field is often retained, though 2172 prepended with "Re: " as described in section 3.6.5. 2174 From: Mary Smith 2175 To: John Doe 2176 Reply-To: "Mary Smith: Personal Account" 2177 Subject: Re: Saying Hello 2178 Date: Fri, 21 Nov 1997 10:01:10 -0600 2179 Message-ID: <3456@example.net> 2180 In-Reply-To: <1234@local.machine.example> 2181 References: <1234@local.machine.example> 2183 This is a reply to your hello. 2185 Note the "Reply-To:" field in the above message. When John replies 2186 to Mary's message above, the reply should go to the address in the 2187 "Reply-To:" field instead of the address in the "From:" field. 2189 To: "Mary Smith: Personal Account" 2190 From: John Doe 2191 Subject: Re: Saying Hello 2192 Date: Fri, 21 Nov 1997 11:00:00 -0600 2193 Message-ID: 2194 In-Reply-To: <3456@example.net> 2195 References: <1234@local.machine.example> <3456@example.net> 2197 This is a reply to your reply. 2199 A.3. Resent Messages 2201 Start with the message that has been used as an example several 2202 times: 2204 From: John Doe 2205 To: Mary Smith 2206 Subject: Saying Hello 2207 Date: Fri, 21 Nov 1997 09:55:06 -0600 2208 Message-ID: <1234@local.machine.example> 2210 This is a message just to say hello. 2211 So, "Hello". 2213 Say that Mary, upon receiving this message, wishes to send a copy of 2214 the message to Jane such that (a) the message would appear to have 2215 come straight from John; (b) if Jane replies to the message, the 2216 reply should go back to John; and (c) all of the original 2217 information, like the date the message was originally sent to Mary, 2218 the message identifier, and the original addressee, is preserved. In 2219 this case, resent fields are prepended to the message: 2221 Resent-From: Mary Smith 2222 Resent-To: Jane Brown 2223 Resent-Date: Mon, 24 Nov 1997 14:22:01 -0800 2224 Resent-Message-ID: <78910@example.net> 2225 From: John Doe 2226 To: Mary Smith 2227 Subject: Saying Hello 2228 Date: Fri, 21 Nov 1997 09:55:06 -0600 2229 Message-ID: <1234@local.machine.example> 2231 This is a message just to say hello. 2232 So, "Hello". 2234 If Jane, in turn, wished to resend this message to another person, 2235 she would prepend her own set of resent header fields to the above 2236 and send that. (Note that for brevity, trace fields are not shown.) 2238 A.4. Messages with Trace Fields 2240 As messages are sent through the transport system as described in 2241 [RFC5321], trace fields are prepended to the message. The following 2242 is an example of what those trace fields might look like. Note that 2243 there is some folding white space in the first one since these lines 2244 can be long. 2246 Received: from x.y.test 2247 by example.net 2248 via TCP 2249 with ESMTP 2250 id ABC12345 2251 for ; 21 Nov 1997 10:05:43 -0600 2252 Received: from node.example by x.y.test; 21 Nov 1997 10:01:22 -0600 2253 From: John Doe 2254 To: Mary Smith 2255 Subject: Saying Hello 2256 Date: Fri, 21 Nov 1997 09:55:06 -0600 2257 Message-ID: <1234@local.node.example> 2259 This is a message just to say hello. 2260 So, "Hello". 2262 A.5. White Space, Comments, and Other Oddities 2264 White space, including folding white space, and comments can be 2265 inserted between many of the tokens of fields. Taking the example 2266 from A.1.3, white space and comments can be inserted into all of the 2267 fields. 2269 From: Pete(A nice \) chap) 2270 To:A Group(Some people) 2271 :Chris Jones , 2272 joe@example.org, 2273 John (my dear friend); (the end of the group) 2274 Cc:(Empty list)(start)Hidden recipients :(nobody(that I know)) ; 2275 Date: Thu, 2276 13 2277 Feb 2278 1969 2279 23:32 2280 -0330 (Newfoundland Time) 2281 Message-ID: 2283 Testing. 2285 The above example is aesthetically displeasing, but perfectly legal. 2286 Note particularly (1) the comments in the "From:" field (including 2287 one that has a ")" character appearing as part of a quoted-pair); (2) 2288 the white space absent after the ":" in the "To:" field as well as 2289 the comment and folding white space after the group name, the special 2290 character (".") in the comment in Chris Jones's address, and the 2291 folding white space before and after "joe@example.org,"; (3) the 2292 multiple and nested comments in the "Cc:" field as well as the 2293 comment immediately following the ":" after "Cc"; (4) the folding 2294 white space (but no comments except at the end) and the missing 2295 seconds in the time of the date field; and (5) the white space before 2296 (but not within) the identifier in the "Message-ID:" field. 2298 A.6. Obsoleted Forms 2300 The following are examples of obsolete (that is, the "MUST NOT 2301 generate") syntactic elements described in section 4 of this 2302 document. 2304 A.6.1. Obsolete Addressing 2306 Note in the example below the lack of quotes around Joe Q. Public, 2307 the route that appears in the address for Mary Smith, the two commas 2308 that appear in the "To:" field, and the spaces that appear around the 2309 "." in the jdoe address. 2311 From: Joe Q. Public 2312 To: Mary Smith <@node.test:mary@example.net>, , jdoe@test . example 2313 Date: Tue, 1 Jul 2003 10:52:37 +0200 2314 Message-ID: <5678.21-Nov-1997@example.com> 2316 Hi everyone. 2318 A.6.2. Obsolete Dates 2320 The following message uses an obsolete date format, including a non- 2321 numeric time zone and a two digit year. Note that although the day- 2322 of-week is missing, that is not specific to the obsolete syntax; it 2323 is optional in the current syntax as well. 2325 From: John Doe 2326 To: Mary Smith 2327 Subject: Saying Hello 2328 Date: 21 Nov 97 09:55:06 GMT 2329 Message-ID: <1234@local.machine.example> 2331 This is a message just to say hello. 2332 So, "Hello". 2334 A.6.3. Obsolete White Space and Comments 2336 White space and comments can appear between many more elements than 2337 in the current syntax. Also, folding lines that are made up entirely 2338 of white space are legal. 2340 From : John Doe 2341 To : Mary Smith 2342 __ 2343 2344 Subject : Saying Hello 2345 Date : Fri, 21 Nov 1997 09(comment): 55 : 06 -0600 2346 Message-ID : <1234 @ local(blah) .machine .example> 2348 This is a message just to say hello. 2349 So, "Hello". 2351 Note especially the second line of the "To:" field. It starts with 2352 two space characters. (Note that "__" represent blank spaces.) 2353 Therefore, it is considered part of the folding, as described in 2354 section 4.2. Also, the comments and white space throughout 2355 addresses, dates, and message identifiers are all part of the 2356 obsolete syntax. 2358 Appendix B. Differences from Earlier Specifications 2360 This appendix contains a list of changes that have been made in the 2361 Internet Message Format from earlier specifications, specifically 2362 [RFC0822], [RFC1123], [RFC2822], and [RFC5322]. Items marked with an 2363 asterisk (*) below are items which appear in section 4 of this 2364 document and therefore can no longer be generated. 2366 The following are the changes made from [RFC0822] and [RFC1123] to 2367 [RFC2822] that remain in this document: 2369 1. Period allowed in obsolete form of phrase. 2370 2. ABNF moved out of document, now in [RFC5234]. 2371 3. Four or more digits allowed for year. 2372 4. Header field ordering (and lack thereof) made explicit. 2373 5. Encrypted header field removed. 2374 6. Specifically allow and give meaning to "-0000" time zone. 2375 7. Folding white space is not allowed between every token. 2376 8. Requirement for destinations removed. 2377 9. Forwarding and resending redefined. 2378 10. Extension header fields no longer specifically called out. 2379 11. ASCII 0 (null) removed.* 2380 12. Folding continuation lines cannot contain only white space.* 2381 13. Free insertion of comments not allowed in date.* 2382 14. Non-numeric time zones not allowed.* 2383 15. Two digit years not allowed.* 2384 16. Three digit years interpreted, but not allowed for generation.* 2385 17. Routes in addresses not allowed.* 2386 18. CFWS within local-parts and domains not allowed.* 2387 19. Empty members of address lists not allowed.* 2388 20. Folding white space between field name and colon not allowed.* 2389 21. Comments between field name and colon not allowed. 2390 22. Tightened syntax of in-reply-to and references.* 2391 23. CFWS within msg-id not allowed.* 2392 24. Tightened semantics of resent fields as informational only. 2393 25. Resent-Reply-To not allowed.* 2394 26. No multiple occurrences of fields (except resent and received).* 2395 27. Free CR and LF not allowed.* 2396 28. Line length limits specified. 2397 29. Bcc more clearly specified. 2399 The following are changes from [RFC2822]. 2401 1. Assorted typographical/grammatical errors fixed and 2402 clarifications made. 2403 2. Changed "standard" to "document" or "specification" throughout. 2404 3. Made distinction between "header field" and "header section". 2405 4. Removed NO-WS-CTL from ctext, qtext, dtext, and unstructured.* 2406 5. Moved discussion of specials to the "Atom" section. Moved text 2407 to "Overall message syntax" section. 2408 6. Simplified CFWS syntax. 2409 7. Fixed unstructured syntax. 2410 8. Changed date and time syntax to deal with white space in 2411 obsolete date syntax. 2412 9. Removed quoted-pair from domain literals and message 2413 identifiers.* 2414 10. Clarified that other specifications limit domain syntax. 2415 11. Simplified "Bcc:" and "Resent-Bcc:" syntax. 2416 12. Allowed optional-field to appear within trace information. 2417 13. Removed no-fold-quote from msg-id. Clarified syntax 2418 limitations. 2419 14. Generalized "Received:" syntax to fix bugs and move definition 2420 out of this document. 2421 15. Simplified obs-qp. Fixed and simplified obs-utext (which now 2422 only appears in the obsolete syntax). Removed obs-text and obs- 2423 char, adding obs-body. 2424 16. Fixed obsolete date syntax to allow for more (or less) comments 2425 and white space. 2426 17. Fixed all obsolete list syntax (obs-domain-list, obs-mbox-list, 2427 obs-addr-list, obs-phrase-list, and the newly added obs-group- 2428 list). 2429 18. Fixed obs-reply-to syntax. 2430 19. Fixed obs-bcc and obs-resent-bcc to allow empty lists. 2431 20. Removed obs-path. 2433 The following are changes from [RFC5322]. 2435 1. Clarified addr-spec description (erratum 1766 (https://www.rfc- 2436 editor.org/errata/eid1766)). 2437 2. Fixed obs-unstruct to be more limited (errata 1905).* 2438 3. Simplified obs-body (erratum 1906 (https://www.rfc- 2439 editor.org/errata/eid1906)).* 2440 4. Fixed obs-FWS to allow for a leading CRLF (erratum 1908 2441 (https://www.rfc-editor.org/errata/eid1908)).* 2442 5. Fixed comments within addresses in A.5 (errata 2515 2443 (https://www.rfc-editor.org/errata/eid2515) and 2579 2444 (https://www.rfc-editor.org/errata/eid2579)). 2445 6. Fixed time zone description (erratum 2726 (https://www.rfc- 2446 editor.org/errata/eid2726)). 2447 7. Removed inappropriate uses of "sent" in 3.6.3, 3.6.6, and 5 2448 (erratum 3048 (https://www.rfc-editor.org/errata/eid3048)). 2449 8. Allow for CFWS in otherwise empty list of "Received:" field 2450 tokens (erratum 3979 (https://www.rfc-editor.org/errata/ 2451 eid3979)). 2452 9. Changed "printable" to "visible" to clarify that it doesn't 2453 include the space character (erratum 4692 (https://www.rfc- 2454 editor.org/errata/eid4692)). 2455 10. Clarify midnight in time-of-day (erratum 5905 (https://www.rfc- 2456 editor.org/errata/eid5905)). 2457 11. Allow for date-time in obs-received (erratum 5867 2458 (https://www.rfc-editor.org/errata/eid5867)). 2459 12. Separated out "msg-id-internal" in "msg-id". 2460 13. Updated references to STD 13, BCP 13, and BCP 14. 2462 There are also 3 errata that were "Held For Document Update" that 2463 have not been addressed. See the following document sections: 2465 1. Section 3.6 2466 2. Section 3.2.4 2467 3. Section 3.6.8 2469 Appendix C. Acknowledgements 2471 Many people contributed to this document. They included folks who 2472 participated in the Detailed Revision and Update of Messaging 2473 Standards (DRUMS) Working Group of the Internet Engineering Task 2474 Force (IETF), the chair of DRUMS, the Area Directors of the IETF, 2475 reporters of errata on earlier versions of this document, and people 2476 who simply sent their comments in via email. The editor is deeply 2477 indebted to them all and thanks them sincerely. (The list of these 2478 people has been temporarily removed to try to bring it up to date.) 2480 Author's Address 2481 Peter W. Resnick (editor) 2482 Episteme Technology Consulting LLC 2483 503 West Indiana Avenue 2484 Urbana, IL 61801-4941 2485 United States of America 2487 Phone: +1 217 337 1905 2488 Email: resnick@episteme.net 2489 URI: https://www.episteme.net/