Network Working Group P. Resnick, Editor Request for Comments: 2822 QUALCOMM Incorporated Obsoletes: 822 April 2001 Category: Standards Track Internet Message Format Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2001). All Rights Reserved.
AbstractThis standard specifies a syntax for text messages that are sent between computer users, within the framework of "electronic mail" messages. This standard supersedes the one specified in Request For Comments (RFC) 822, "Standard for the Format of ARPA Internet Text Messages", updating it to reflect current practice and incorporating incremental changes that were specified in other RFCs. 1. Introduction ............................................... 3 1.1. Scope .................................................... 3 1.2. Notational conventions ................................... 4 1.2.1. Requirements notation .................................. 4 1.2.2. Syntactic notation ..................................... 4 1.3. Structure of this document ............................... 4 2. Lexical Analysis of Messages ............................... 5 2.1. General Description ...................................... 5 2.1.1. Line Length Limits ..................................... 6 2.2. Header Fields ............................................ 7 2.2.1. Unstructured Header Field Bodies ....................... 7 2.2.2. Structured Header Field Bodies ......................... 7 2.2.3. Long Header Fields ..................................... 7 2.3. Body ..................................................... 8 3. Syntax ..................................................... 9 3.1. Introduction ............................................. 9 3.2. Lexical Tokens ........................................... 9
3.2.1. Primitive Tokens ....................................... 9 3.2.2. Quoted characters ......................................10 3.2.3. Folding white space and comments .......................11 3.2.4. Atom ...................................................12 3.2.5. Quoted strings .........................................13 3.2.6. Miscellaneous tokens ...................................13 3.3. Date and Time Specification ..............................14 3.4. Address Specification ....................................15 3.4.1. Addr-spec specification ................................16 3.5 Overall message syntax ....................................17 3.6. Field definitions ........................................18 3.6.1. The origination date field .............................20 3.6.2. Originator fields ......................................21 3.6.3. Destination address fields .............................22 3.6.4. Identification fields ..................................23 3.6.5. Informational fields ...................................26 3.6.6. Resent fields ..........................................26 3.6.7. Trace fields ...........................................28 3.6.8. Optional fields ........................................29 4. Obsolete Syntax ............................................29 4.1. Miscellaneous obsolete tokens ............................30 4.2. Obsolete folding white space .............................31 4.3. Obsolete Date and Time ...................................31 4.4. Obsolete Addressing ......................................33 4.5. Obsolete header fields ...................................33 4.5.1. Obsolete origination date field ........................34 4.5.2. Obsolete originator fields .............................34 4.5.3. Obsolete destination address fields ....................34 4.5.4. Obsolete identification fields .........................35 4.5.5. Obsolete informational fields ..........................35 4.5.6. Obsolete resent fields .................................35 4.5.7. Obsolete trace fields ..................................36 4.5.8. Obsolete optional fields ...............................36 5. Security Considerations ....................................36 6. Bibliography ...............................................37 7. Editor's Address ...........................................38 8. Acknowledgements ...........................................39 Appendix A. Example messages ..................................41 A.1. Addressing examples ......................................41 A.1.1. A message from one person to another with simple addressing .............................................41 A.1.2. Different types of mailboxes ...........................42 A.1.3. Group addresses ........................................43 A.2. Reply messages ...........................................43 A.3. Resent messages ..........................................44 A.4. Messages with trace fields ...............................46 A.5. White space, comments, and other oddities ................47 A.6. Obsoleted forms ..........................................47
A.6.1. Obsolete addressing ....................................48 A.6.2. Obsolete dates .........................................48 A.6.3. Obsolete white space and comments ......................48 Appendix B. Differences from earlier standards ................49 Appendix C. Notices ...........................................50 Full Copyright Statement ......................................51 RFC822], updating it to reflect current practice and incorporating incremental changes that were specified in other RFCs [STD3]. This standard specifies a syntax only for text messages. In particular, it makes no provision for the transmission of images, audio, or other sorts of structured data in electronic mail messages. There are several extensions published, such as the MIME document series [RFC2045, RFC2046, RFC2049], which describe mechanisms for the transmission of such data through electronic mail, either by extending the syntax provided here or by structuring such messages to conform to this syntax. Those mechanisms are outside of the scope of this standard. In the context of electronic mail, messages are viewed as having an envelope and contents. The envelope contains whatever information is needed to accomplish transmission and delivery. (See [RFC2821] for a discussion of the envelope.) The contents comprise the object to be delivered to the recipient. This standard applies only to the format and some of the semantics of message contents. It contains no specification of the information in the envelope. However, some message systems may use information from the contents to create the envelope. It is intended that this standard facilitate the acquisition of such information by programs. This specification is intended as a definition of what message content format is to be passed between systems. Though some message systems locally store messages in this format (which eliminates the need for translation between formats) and others use formats that differ from the one specified in this standard, local storage is outside of the scope of this standard.
Note: This standard is not intended to dictate the internal formats used by sites, the specific message system features that they are expected to support, or any of the characteristics of user interface programs that create or read messages. In addition, this standard does not specify an encoding of the characters for either transport or storage; that is, it does not specify the number of bits used or how those bits are specifically transferred over the wire or stored on disk. RFC2119]. RFC2234] for the formal definitions of the syntax of messages. Characters will be specified either by a decimal value (e.g., the value %d65 for uppercase A and %d97 for lowercase A) or by a case-insensitive literal value enclosed in quotation marks (e.g., "A" for either uppercase or lowercase A). See [RFC2234] for the full description of the notation. section 1, is a short introduction to the document. Section 2 lays out the general description of a message and its constituent parts. This is an overview to help the reader understand some of the general principles used in the later portions of this document. Any examples in this section MUST NOT be taken as specification of the formal syntax of any part of a message. Section 3 specifies formal ABNF rules for the structure of each part of a message (the syntax) and describes the relationship between those parts and their meaning in the context of a message (the semantics). That is, it describes the actual rules for the structure of each part of a message (the syntax) as well as a description of the parts and instructions on how they ought to be interpreted (the semantics). This includes analysis of the syntax and semantics of
subparts of messages that have specific structure. The syntax included in section 3 represents messages as they MUST be created. There are also notes in section 3 to indicate if any of the options specified in the syntax SHOULD be used over any of the others. Both sections 2 and 3 describe messages that are legal to generate for purposes of this standard. Section 4 of this document specifies an "obsolete" syntax. There are references in section 3 to these obsolete syntactic elements. The rules of the obsolete syntax are elements that have appeared in earlier revisions of this standard or have previously been widely used in Internet messages. As such, these elements MUST be interpreted by parsers of messages in order to be conformant to this standard. However, since items in this syntax have been determined to be non-interoperable or to cause significant problems for recipients of messages, they MUST NOT be generated by creators of conformant messages. Section 5 details security considerations to take into account when implementing this standard. Section 6 is a bibliography of references in this document. Section 7 contains the editor's address. Section 8 contains acknowledgements. Appendix A lists examples of different sorts of messages. These examples are not exhaustive of the types of messages that appear on the Internet, but give a broad overview of certain syntactic forms. Appendix B lists the differences between this standard and earlier standards for Internet messages. Appendix C has copyright and intellectual property notices. ASCII]. For brevity, this document sometimes refers to this range of characters as simply "US-ASCII characters".
Note: This standard specifies that messages are made up of characters in the US-ASCII range of 1 through 127. There are other documents, specifically the MIME document series [RFC2045, RFC2046, RFC2047, RFC2048, RFC2049], that extend this standard to allow for values outside of that range. Discussion of those mechanisms is not within the scope of this standard. Messages are divided into lines of characters. A line is a series of characters that is delimited with the two characters carriage-return and line-feed; that is, the carriage return (CR) character (ASCII value 13) followed immediately by the line feed (LF) character (ASCII value 10). (The carriage-return/line-feed pair is usually written in this document as "CRLF".) A message consists of header fields (collectively called "the header of the message") followed, optionally, by a body. The header is a sequence of lines of characters with special syntax as defined in this standard. The body is simply a sequence of characters that follows the header and is separated from the header by an empty line (i.e., a line with nothing preceding the CRLF). RFC2821]) do not accept messages containing more than 1000 character including the CR and LF per line, it is important for implementations not to create such messages. The more conservative 78 character recommendation is to accommodate the many implementations of user interfaces that display these messages which may truncate, or disastrously wrap, the display of more than 78 characters per line, in spite of the fact that such implementations are non-conformant to the intent of this specification (and that of [RFC2821] if they actually cause information to be lost). Again, even though this limitation is put on messages, it is encumbant upon implementations which display messages
to handle an arbitrarily large number of characters in a line (certainly at least up to the 998 character limit) for the sake of robustness. section 2.2.3. All field bodies MUST conform to the syntax described in sections 3 and 4 of this standard. section 2.2.3). sections 3 and 4 of this standard. Many of these tokens are allowed (according to their syntax) to be introduced or end with comments (as described in section 3.2.3) as well as the space (SP, ASCII value 32) and horizontal tab (HTAB, ASCII value 9) characters (together known as the white space characters, WSP), and those WSP characters are subject to header "folding" and "unfolding" as described in section 2.2.3. Semantic analysis of structured field bodies is given along with their syntax.
that wherever this standard allows for folding white space (not simply WSP characters), a CRLF may be inserted before any WSP. For example, the header field: Subject: This is a test can be represented as: Subject: This is a test Note: Though structured field bodies are defined in such a way that folding can take place between many of the lexical tokens (and even within some of the lexical tokens), folding SHOULD be limited to placing the CRLF at higher-level syntactic breaks. For instance, if a field body is defined as comma-separated values, it is recommended that folding occur after the comma separating the structured items in preference to other places where the field could be folded, even if it is allowed elsewhere. The process of moving from this folded multiple-line representation of a header field to its single line representation is called "unfolding". Unfolding is accomplished by simply removing any CRLF that is immediately followed by WSP. Each header field should be treated in its unfolded form for further syntactic and semantic evaluation. RFC2045, RFC2046, RFC2048, RFC2049] that extend this standard to allow for different sorts of message bodies. Again, these mechanisms are beyond the scope of this document.
RFC2234]. In some of the definitions, there will be nonterminals whose names start with "obs-". These "obs-" elements refer to tokens defined in the obsolete syntax in section 4. In all cases, these productions are to be ignored for the purposes of generating legal Internet messages and MUST NOT be used as part of such a message. However, when interpreting messages, these tokens MUST be honored as part of the legal syntax. In this sense, section 3 defines a grammar for generation of messages, with "obs-" elements that are to be ignored, while section 4 adds grammar for interpretation of messages. section 3.2.3 get used in the lower-level tokens defined here, and those lower-level tokens are in turn used as parts of the higher-level tokens defined later. Therefore, the white space and comments may be allowed in the higher-level tokens even though they may not explicitly appear in a particular definition. RFC2234]. Some of them will not appear anywhere else in the syntax, but they are convenient to refer to in other parts of this document.
Note: The "specials" below are just such an example. Though the specials token does not appear anywhere else in this standard, it is useful for implementers who use tools that lexically analyze messages. Each of the characters in specials can be used to indicate a tokenization point in lexical analysis. NO-WS-CTL = %d1-8 / ; US-ASCII control characters %d11 / ; that do not include the %d12 / ; carriage return, line feed, %d14-31 / ; and white space characters %d127 text = %d1-9 / ; Characters excluding CR and LF %d11 / %d12 / %d14-127 / obs-text specials = "(" / ")" / ; Special characters used in "<" / ">" / ; other parts of the syntax "[" / "]" / ":" / ";" / "@" / "\" / "," / "." / DQUOTE No special semantics are attached to these tokens. They are simply single characters.
section 2.2.3), may appear between many elements in header field bodies. Also, strings of characters that are treated as comments may be included in structured field bodies as characters enclosed in parentheses. The following defines the folding white space (FWS) and comment constructs. Strings of characters enclosed in parentheses are considered comments so long as they do not appear within a "quoted-string", as defined in section 3.2.5. Comments may nest. There are several places in this standard where comments and FWS may be freely inserted. To accommodate that syntax, an additional token for "CFWS" is defined for places where comments and/or FWS can occur. However, where CFWS occurs in this standard, it MUST NOT be inserted in such a way that any line of a folded header field is made up entirely of WSP characters and nothing else. FWS = ([*WSP CRLF] 1*WSP) / ; Folding white space obs-FWS ctext = NO-WS-CTL / ; Non white space controls %d33-39 / ; The rest of the US-ASCII %d42-91 / ; characters not including "(", %d93-126 ; ")", or "\" ccontent = ctext / quoted-pair / comment comment = "(" *([FWS] ccontent) [FWS] ")" CFWS = *([FWS] comment) (([FWS] comment) / FWS) Throughout this standard, where FWS (the folding white space token) appears, it indicates a place where header folding, as discussed in section 2.2.3, may take place. Wherever header folding appears in a message (that is, a header field body containing a CRLF followed by any WSP), header unfolding (removal of the CRLF) is performed before any further lexical analysis is performed on that header field according to this standard. That is to say, any CRLF that appears in FWS is semantically "invisible." A comment is normally used in a structured field body to provide some human readable informational text. Since a comment is allowed to contain FWS, folding is permitted within the comment. Also note that since quoted-pair is allowed in a comment, the parentheses and
backslash characters may appear in a comment so long as they appear as a quoted-pair. Semantically, the enclosing parentheses are not part of the comment; the comment is what is contained between the two parentheses. As stated earlier, the "\" in any quoted-pair and the CRLF in any FWS that appears within the comment are semantically "invisible" and therefore not part of the comment either. Runs of FWS, comment or CFWS that occur between lexical tokens in a structured field header are semantically interpreted as a single space character.
utext = NO-WS-CTL / ; Non white space controls %d33-126 / ; The rest of US-ASCII obs-utext unstructured = *([FWS] utext) [FWS]
The day is the numeric day of the month. The year is any numeric year 1900 or later. The time-of-day specifies the number of hours, minutes, and optionally seconds since midnight of the date indicated. The date and time-of-day SHOULD express local time. The zone specifies the offset from Coordinated Universal Time (UTC, formerly referred to as "Greenwich Mean Time") that the date and time-of-day represent. The "+" or "-" indicates whether the time-of-day is ahead of (i.e., east of) or behind (i.e., west of) Universal Time. The first two digits indicate the number of hours difference from Universal Time, and the last two digits indicate the number of minutes difference from Universal Time. (Hence, +hhmm means +(hh * 60 + mm) minutes, and -hhmm means -(hh * 60 + mm) minutes). The form "+0000" SHOULD be used to indicate a time zone at Universal Time. Though "-0000" also indicates Universal Time, it is used to indicate that the time was generated on a system that may be in a local time zone other than Universal Time and therefore indicates that the date-time contains no information about the local time zone. A date-time specification MUST be semantically valid. That is, the day-of-the-week (if included) MUST be the day implied by the date, the numeric day-of-month MUST be between 1 and the number of days allowed for the specified month (in the specified year), the time-of-day MUST be in the range 00:00:00 through 23:59:60 (the number of seconds allowing for a leap second; see [STD12]), and the zone MUST be within the range -9959 through +9959.
display-name = phrase mailbox-list = (mailbox *("," mailbox)) / obs-mbox-list address-list = (address *("," address)) / obs-addr-list A mailbox receives mail. It is a conceptual entity which does not necessarily pertain to file storage. For example, some sites may choose to print mail on a printer and deliver the output to the addressee's desk. Normally, a mailbox is comprised of two parts: (1) an optional display name that indicates the name of the recipient (which could be a person or a system) that could be displayed to the user of a mail application, and (2) an addr-spec address enclosed in angle brackets ("<" and ">"). There is also an alternate simple form of a mailbox where the addr-spec address appears alone, without the recipient's name or the angle brackets. The Internet addr-spec address is described in section 3.4.1. Note: Some legacy implementations used the simple form where the addr-spec appears without the angle brackets, but included the name of the recipient in parentheses as a comment following the addr-spec. Since the meaning of the information in a comment is unspecified, implementations SHOULD use the full name-addr form of the mailbox, instead of the legacy form, to specify the display name associated with a mailbox. Also, because some legacy implementations interpret the comment, comments generally SHOULD NOT be used in address fields to avoid confusing such implementations. When it is desirable to treat several mailboxes as a single unit (i.e., in a distribution list), the group construct can be used. The group construct allows the sender to indicate a named group of recipients. This is done by giving a display name for the group, followed by a colon, followed by a comma separated list of any number of mailboxes (including zero and one), and ending with a semicolon. Because the list of mailboxes can be empty, using the group construct is also a simple way to communicate to recipients that the message was sent to one or more named sets of recipients, without actually providing the individual mailbox address for each of those recipients.
characters), then the dot-atom form SHOULD be used and the quoted-string form SHOULD NOT be used. Comments and folding white space SHOULD NOT be used around the "@" in the addr-spec. addr-spec = local-part "@" domain local-part = dot-atom / quoted-string / obs-local-part domain = dot-atom / domain-literal / obs-domain domain-literal = [CFWS] "[" *([FWS] dcontent) [FWS] "]" [CFWS] dcontent = dtext / quoted-pair dtext = NO-WS-CTL / ; Non white space controls %d33-90 / ; The rest of the US-ASCII %d94-126 ; characters not including "[", ; "]", or "\" The domain portion identifies the point to which the mail is delivered. In the dot-atom form, this is interpreted as an Internet domain name (either a host name or a mail exchanger name) as described in [STD3, STD13, STD14]. In the domain-literal form, the domain is interpreted as the literal Internet address of the particular host. In both cases, how addressing is used and how messages are transported to a particular host is covered in the mail transport document [RFC2821]. These mechanisms are outside of the scope of this document. The local-part portion is a domain dependent string. In addresses, it is simply interpreted on the particular host as a name of a particular mailbox. section 2.1.1 for explanation.) In a message body, though all of the characters listed in the text rule MAY be used, the use of US-ASCII control characters (values 1 through 8, 11, 12, and 14 through 31) is discouraged since their interpretation by receivers for display is not guaranteed.
message = (fields / obs-fields) [CRLF body] body = *(*998text CRLF) *998text The header fields carry most of the semantic information and are defined in section 3.6. The body is simply a series of lines of text which are uninterpreted for the purposes of this standard. sections 3.6.6 and 3.6.7 for more information. The only required header fields are the origination date field and the originator address field(s). All other header fields are syntactically optional. More information is contained in the table following this definition. fields = *(trace *(resent-date / resent-from / resent-sender / resent-to / resent-cc / resent-bcc / resent-msg-id)) *(orig-date / from / sender / reply-to /
to / cc / bcc / message-id / in-reply-to / references / subject / comments / keywords / optional-field) The following table indicates limits on the number of times each field may occur in a message header as well as any special limitations on the use of those fields. An asterisk next to a value in the minimum or maximum column indicates that a special restriction appears in the Notes column. Field Min number Max number Notes trace 0 unlimited Block prepended - see 3.6.7 resent-date 0* unlimited* One per block, required if other resent fields present - see 3.6.6 resent-from 0 unlimited* One per block - see 3.6.6 resent-sender 0* unlimited* One per block, MUST occur with multi-address resent-from - see 3.6.6 resent-to 0 unlimited* One per block - see 3.6.6 resent-cc 0 unlimited* One per block - see 3.6.6 resent-bcc 0 unlimited* One per block - see 3.6.6 resent-msg-id 0 unlimited* One per block - see 3.6.6 orig-date 1 1 from 1 1 See sender and 3.6.2
sender 0* 1 MUST occur with multi- address from - see 3.6.2 reply-to 0 1 to 0 1 cc 0 1 bcc 0 1 message-id 0* 1 SHOULD be present - see 3.6.4 in-reply-to 0* 1 SHOULD occur in some replies - see 3.6.4 references 0* 1 SHOULD occur in some replies - see 3.6.4 subject 0 1 comments 0 unlimited keywords 0 unlimited optional-field 0 unlimited The exact interpretation of each field is described in subsequent sections.
for delivery. The origination date is intended to contain the date and time that the user queued the message, not the time when the user connected to the network to send the message.) section 3.6.3 for more information on forming the destination addresses for a reply.
When a message is a reply to another message, the mailboxes of the authors of the original message (the mailboxes in the "From:" field) or mailboxes specified in the "Reply-To:" field (if it exists) MAY appear in the "To:" field of the reply since these would normally be the primary recipients of the reply. If a reply is sent to a message that has destination fields, it is often desirable to send a copy of the reply to all of the recipients of the message, in addition to the author. When such a reply is formed, addresses in the "To:" and "Cc:" fields of the original message MAY appear in the "Cc:" field of the reply, since these are normally secondary recipients of the reply. If a "Bcc:" field is present in the original message, addresses in that field MAY appear in the "Bcc:" field of the reply, but SHOULD NOT appear in the "To:" or "Cc:" fields. Note: Some mail applications have automatic reply commands that include the destination addresses of the original message in the destination addresses of the reply. How those reply commands behave is implementation dependent and is beyond the scope of this document. In particular, whether or not to include the original destination addresses when the original message had a "Reply-To:" field is not addressed here.
no-fold-literal = "[" *(dtext / quoted-pair) "]" The "Message-ID:" field provides a unique message identifier that refers to a particular version of a particular message. The uniqueness of the message identifier is guaranteed by the host that generates it (see below). This message identifier is intended to be machine readable and not necessarily meaningful to humans. A message identifier pertains to exactly one instantiation of a particular message; subsequent revisions to the message each receive new message identifiers. Note: There are many instances when messages are "changed", but those changes do not constitute a new instantiation of that message, and therefore the message would not get a new message identifier. For example, when messages are introduced into the transport system, they are often prepended with additional header fields such as trace fields (described in section 3.6.7) and resent fields (described in section 3.6.6). The addition of such header fields does not change the identity of the message and therefore the original "Message-ID:" field is retained. In all cases, it is the meaning that the sender of the message wishes to convey (i.e., whether this is the same message or a different message) that determines whether or not the "Message-ID:" field changes, not any particular syntactic difference that appears (or does not appear) in the message. The "In-Reply-To:" and "References:" fields are used when creating a reply to a message. They hold the message identifier of the original message and the message identifiers of other messages (for example, in the case of a reply to a message which was itself a reply). The "In-Reply-To:" field may be used to identify the message (or messages) to which the new message is a reply, while the "References:" field may be used to identify a "thread" of conversation. When creating a reply to a message, the "In-Reply-To:" and "References:" fields of the resultant message are constructed as follows: The "In-Reply-To:" field will contain the contents of the "Message- ID:" field of the message to which this one is a reply (the "parent message"). If there is more than one parent message, then the "In- Reply-To:" field will contain the contents of all of the parents' "Message-ID:" fields. If there is no "Message-ID:" field in any of the parent messages, then the new message will have no "In-Reply-To:" field.
The "References:" field will contain the contents of the parent's "References:" field (if any) followed by the contents of the parent's "Message-ID:" field (if any). If the parent message does not contain a "References:" field but does have an "In-Reply-To:" field containing a single message identifier, then the "References:" field will contain the contents of the parent's "In-Reply-To:" field followed by the contents of the parent's "Message-ID:" field (if any). If the parent has none of the "References:", "In-Reply-To:", or "Message-ID:" fields, then the new message will have no "References:" field. Note: Some implementations parse the "References:" field to display the "thread of the discussion". These implementations assume that each new message is a reply to a single parent and hence that they can walk backwards through the "References:" field to find the parent of each message listed there. Therefore, trying to form a "References:" field for a reply that has multiple parents is discouraged and how to do so is not defined in this document. The message identifier (msg-id) itself MUST be a globally unique identifier for a message. The generator of the message identifier MUST guarantee that the msg-id is unique. There are several algorithms that can be used to accomplish this. Since the msg-id has a similar syntax to angle-addr (identical except that comments and folding white space are not allowed), a good method is to put the domain name (or a domain literal IP address) of the host on which the message identifier was created on the right hand side of the "@", and put a combination of the current absolute date and time along with some other currently unique (perhaps sequential) identifier available on the system (for example, a process id number) on the left hand side. Using a date on the left hand side and a domain name or domain literal on the right hand side makes it possible to guarantee uniqueness since no two hosts use the same domain name or IP address at the same time. Though other algorithms will work, it is RECOMMENDED that the right hand side contain some domain identifier (either of the host itself or otherwise) such that the generator of the message identifier can guarantee the uniqueness of the left hand side within the scope of that domain. Semantically, the angle bracket characters are not part of the msg-id; the msg-id is what is contained between the two angle bracket characters.
section 2.2.1, and therefore may contain text or folding white space. subject = "Subject:" unstructured CRLF comments = "Comments:" unstructured CRLF keywords = "Keywords:" phrase *("," phrase) CRLF These three fields are intended to have only human-readable content with information about the message. The "Subject:" field is the most common and contains a short string identifying the topic of the message. When used in a reply, the field body MAY start with the string "Re: " (from the Latin "res", in the matter of) followed by the contents of the "Subject:" field body of the original message. If this is done, only one instance of the literal string "Re: " ought to be used since use of other strings or more than one instance can lead to undesirable consequences. The "Comments:" field contains any additional comments on the text of the body of the message. The "Keywords:" field contains a comma-separated list of important words and phrases that might be useful for the recipient.
resent-date = "Resent-Date:" date-time CRLF resent-from = "Resent-From:" mailbox-list CRLF resent-sender = "Resent-Sender:" mailbox CRLF resent-to = "Resent-To:" address-list CRLF resent-cc = "Resent-Cc:" address-list CRLF resent-bcc = "Resent-Bcc:" (address-list / [CFWS]) CRLF resent-msg-id = "Resent-Message-ID:" msg-id CRLF Resent fields are used to identify a message as having been reintroduced into the transport system by a user. The purpose of using resent fields is to have the message appear to the final recipient as if it were sent directly by the original sender, with all of the original fields remaining the same. Each set of resent fields correspond to a particular resending event. That is, if a message is resent multiple times, each set of resent fields gives identifying information for each individual time. Resent fields are strictly informational. They MUST NOT be used in the normal processing of replies or other such automatic actions on messages. Note: Reintroducing a message into the transport system and using resent fields is a different operation from "forwarding". "Forwarding" has two meanings: One sense of forwarding is that a mail reading program can be told by a user to forward a copy of a message to another person, making the forwarded message the body of the new message. A forwarded message in this sense does not appear to have come from the original sender, but is an entirely new message from the forwarder of the message. On the other hand, forwarding is also used to mean when a mail transport program gets a message and forwards it on to a different destination for final delivery. Resent header fields are not intended for use with either type of forwarding. The resent originator fields indicate the mailbox of the person(s) or system(s) that resent the message. As with the regular originator fields, there are two forms: a simple "Resent-From:" form which contains the mailbox of the individual doing the resending, and the more complex form, when one individual (identified in the "Resent-Sender:" field) resends a message on behalf of one or more others (identified in the "Resent-From:" field). Note: When replying to a resent message, replies behave just as they would with any other message, using the original "From:",
"Reply-To:", "Message-ID:", and other fields. The resent fields are only informational and MUST NOT be used in the normal processing of replies. The "Resent-Date:" indicates the date and time at which the resent message is dispatched by the resender of the message. Like the "Date:" field, it is not the date and time that the message was actually transported. The "Resent-To:", "Resent-Cc:", and "Resent-Bcc:" fields function identically to the "To:", "Cc:", and "Bcc:" fields respectively, except that they indicate the recipients of the resent message, not the recipients of the original message. The "Resent-Message-ID:" field provides a unique identifier for the resent message. RFC2821]. trace = [return] 1*received return = "Return-Path:" path CRLF path = ([CFWS] "<" ([CFWS] / addr-spec) ">" [CFWS]) / obs-path received = "Received:" name-val-list ";" date-time CRLF name-val-list = [CFWS] [name-val-pair *(CFWS name-val-pair)] name-val-pair = item-name CFWS item-value item-name = ALPHA *(["-"] (ALPHA / DIGIT)) item-value = 1*angle-addr / addr-spec / atom / domain / msg-id
A full discussion of the Internet mail use of trace fields is contained in [RFC2821]. For the purposes of this standard, the trace fields are strictly informational, and any formal interpretation of them is outside of the scope of this document. section 3, they MUST be accepted and parsed by a conformant receiver. This section documents many of these syntactic elements. Taking the grammar in section 3 and adding the definitions presented in this section will result in the grammar to use for interpretation of messages. Note: This section identifies syntactic forms that any implementation MUST reasonably interpret. However, there are certainly Internet messages which do not conform to even the additional syntax given in this section. The fact that a particular form does not appear in any section of this document is not justification for computer programs to crash or for malformed data to be irretrievably lost by any implementation. To repeat an example, though this document requires lines in messages to be no longer than 998 characters, silently
discarding the 999th and subsequent characters in a line without warning would still be bad behavior for an implementation. It is up to the implementation to deal with messages robustly. One important difference between the obsolete (interpreting) and the current (generating) syntax is that in structured header field bodies (i.e., between the colon and the CRLF of any structured header field), white space characters, including folding white space, and comments can be freely inserted between any syntactic tokens. This allows many complex forms that have proven difficult for some implementations to parse. Another key difference between the obsolete and the current syntax is that the rule in section 3.2.3 regarding lines composed entirely of white space in comments and folding white space does not apply. See the discussion of folding white space in section 4.2 below. Finally, certain characters that were formerly allowed in messages appear in this section. The NUL character (ASCII value 0) was once allowed, but is no longer for compatibility reasons. CR and LF were allowed to appear in messages other than as CRLF; this use is also shown here. Other differences in syntax and semantics are noted in the following sections. section 4.4). It appears here because the period character is currently used in many messages in the display-name portion of addresses, especially for initials in names, and therefore must be interpreted properly. In the future, period may appear in the regular syntax of phrase. obs-qp = "\" (%d0-127) obs-text = *LF *CR *(obs-char *LF *CR)
obs-char = %d0-9 / %d11 / ; %d0-127 except CR and %d12 / %d14-127 ; LF obs-utext = obs-text obs-phrase = word *(word / "." / CFWS) obs-phrase-list = phrase / 1*([phrase] [CFWS] "," [CFWS]) [phrase] Bare CR and bare LF appear in messages with two different meanings. In many cases, bare CR or bare LF are used improperly instead of CRLF to indicate line separators. In other cases, bare CR and bare LF are used simply as ASCII control characters with their traditional ASCII meanings.
; North American UT ; offsets "EST" / "EDT" / ; Eastern: - 5/ - 4 "CST" / "CDT" / ; Central: - 6/ - 5 "MST" / "MDT" / ; Mountain: - 7/ - 6 "PST" / "PDT" / ; Pacific: - 8/ - 7 %d65-73 / ; Military zones - "A" %d75-90 / ; through "I" and "K" %d97-105 / ; through "Z", both %d107-122 ; upper and lower case Where a two or three digit year occurs in a date, the year is to be interpreted as follows: If a two digit year is encountered whose value is between 00 and 49, the year is interpreted by adding 2000, ending up with a value between 2000 and 2049. If a two digit year is encountered with a value between 50 and 99, or any three digit year is encountered, the year is interpreted by adding 1900. In the obsolete time zone, "UT" and "GMT" are indications of "Universal Time" and "Greenwich Mean Time" respectively and are both semantically identical to "+0000". The remaining three character zones are the US time zones. The first letter, "E", "C", "M", or "P" stands for "Eastern", "Central", "Mountain" and "Pacific". The second letter is either "S" for "Standard" time, or "D" for "Daylight" (or summer) time. Their interpretations are as follows: EDT is semantically equivalent to -0400 EST is semantically equivalent to -0500 CDT is semantically equivalent to -0500 CST is semantically equivalent to -0600 MDT is semantically equivalent to -0600 MST is semantically equivalent to -0700 PDT is semantically equivalent to -0700 PST is semantically equivalent to -0800 The 1 character military time zones were defined in a non-standard way in [RFC822] and are therefore unpredictable in their meaning. The original definitions of the military zones "A" through "I" are equivalent to "+0100" through "+0900" respectively; "K", "L", and "M" are equivalent to "+1000", "+1100", and "+1200" respectively; "N" through "Y" are equivalent to "-0100" through "-1200" respectively; and "Z" is equivalent to "+0000". However, because of the error in [RFC822], they SHOULD all be considered equivalent to "-0000" unless there is out-of-band information confirming their meaning.
Other multi-character (usually between 3 and 5) alphabetic time zones have been used in Internet messages. Any such time zone whose meaning is not known SHOULD be considered equivalent to "-0000" unless there is out-of-band information confirming their meaning.
obs-cc / obs-bcc / obs-message-id / obs-in-reply-to / obs-references / obs-subject / obs-comments / obs-keywords / obs-resent-date / obs-resent-from / obs-resent-send / obs-resent-rply / obs-resent-to / obs-resent-cc / obs-resent-bcc / obs-resent-mid / obs-optional) Except for destination address fields (described in section 4.5.3), the interpretation of multiple occurrences of fields is unspecified. Also, the interpretation of trace fields and resent fields which do not occur in blocks prepended to the message is unspecified as well. Unless otherwise noted in the following sections, interpretation of other fields is identical to the interpretation of their non-obsolete counterparts in section 3.
When multiple occurrences of destination address fields occur in a message, they SHOULD be treated as if the address-list in the first occurrence of the field is combined with the address lists of the subsequent occurrences by adding a comma and concatenating.
obs-resent-date = "Resent-Date" *WSP ":" date-time CRLF obs-resent-to = "Resent-To" *WSP ":" address-list CRLF obs-resent-cc = "Resent-Cc" *WSP ":" address-list CRLF obs-resent-bcc = "Resent-Bcc" *WSP ":" (address-list / [CFWS]) CRLF obs-resent-mid = "Resent-Message-ID" *WSP ":" msg-id CRLF obs-resent-rply = "Resent-Reply-To" *WSP ":" address-list CRLF As with other resent fields, the "Resent-Reply-To:" field is to be treated as trace information only. section 3. Their full syntax is given in [RFC2821]. obs-return = "Return-Path" *WSP ":" path CRLF obs-received = "Received" *WSP ":" name-val-list CRLF obs-path = obs-angle-addr RFC2045, RFC2046, RFC2047, RFC2048, RFC2049, ISO2022]) and therefore should not be stripped indiscriminately.
Transmission of non-text objects in messages raises additional security issues. These issues are discussed in [RFC2045, RFC2046, RFC2047, RFC2048, RFC2049]. Many implementations use the "Bcc:" (blind carbon copy) field described in section 3.6.3 to facilitate sending messages to recipients without revealing the addresses of one or more of the addressees to the other recipients. Mishandling this use of "Bcc:" has implications for confidential information that might be revealed, which could eventually lead to security problems through knowledge of even the existence of a particular mail address. For example, if using the first method described in section 3.6.3, where the "Bcc:" line is removed from the message, blind recipients have no explicit indication that they have been sent a blind copy, except insofar as their address does not appear in the message header. Because of this, one of the blind addressees could potentially send a reply to all of the shown recipients and accidentally reveal that the message went to the blind recipient. When the second method from section 3.6.3 is used, the blind recipient's address appears in the "Bcc:" field of a separate copy of the message. If the "Bcc:" field sent contains all of the blind addressees, all of the "Bcc:" recipients will be seen by each "Bcc:" recipient. Even if a separate message is sent to each "Bcc:" recipient with only the individual's address, implementations still need to be careful to process replies to the message as per section 3.6.3 so as not to accidentally reveal the blind recipient to other recipients. [ASCII] American National Standards Institute (ANSI), Coded Character Set - 7-Bit American National Standard Code for Information Interchange, ANSI X3.4, 1986. [ISO2022] International Organization for Standardization (ISO), Information processing - ISO 7-bit and 8-bit coded character sets - Code extension techniques, Third edition - 1986-05-01, ISO 2022, 1986. [RFC822] Crocker, D., "Standard for the Format of ARPA Internet Text Messages", RFC 822, August 1982. [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996. [RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types", RFC 2046, November 1996.
[RFC2047] Moore, K., "Multipurpose Internet Mail Extensions (MIME) Part Three: Message Header Extensions for Non-ASCII Text", RFC 2047, November 1996. [RFC2048] Freed, N., Klensin, J. and J. Postel, "Multipurpose Internet Mail Extensions (MIME) Part Four: Format of Internet Message Bodies", RFC 2048, November 1996. [RFC2049] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part Five: Conformance Criteria and Examples", RFC 2049, November 1996. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2234] Crocker, D., Editor, and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, November 1997. [RFC2821] Klensin, J., Editor, "Simple Mail Transfer Protocol", RFC 2821, March 2001. [STD3] Braden, R., "Host Requirements", STD 3, RFC 1122 and RFC 1123, October 1989. [STD12] Mills, D., "Network Time Protocol", STD 12, RFC 1119, September 1989. [STD13] Mockapetris, P., "Domain Name System", STD 13, RFC 1034 and RFC 1035, November 1987. [STD14] Partridge, C., "Mail Routing and the Domain System", STD 14, RFC 974, January 1986.
Steve Dorner Eliot Lear Dan Wing Harold A. Driscoll Barry Leiba Jack De Winter Michael Elkins Jay Levitt Gregory J. Woodhouse Robert Elz Lars-Johan Liman Greg A. Woods Johnny Eriksson Charles Lindsey Kazu Yamamoto Erik E. Fair Pete Loshin Alain Zahm Roger Fajman Simon Lyall Jamie Zawinski Patrik Faltstrom Bill Manning Timothy S. Zurcher Claus Andre Farber John Martin
sections 3 and 4 of this document, the syntax in those sections is to be taken as correct. Messages are delimited in this section between lines of "----". The "----" lines are not part of the message itself.
If John's secretary Michael actually sent the message, though John was the author and replies to this message should go back to him, the sender field would be used: ---- From: John Doe <email@example.com> Sender: Michael Jones <firstname.lastname@example.org> To: Mary Smith <email@example.com> Subject: Saying Hello Date: Fri, 21 Nov 1997 09:55:06 -0600 Message-ID: <firstname.lastname@example.org> This is a message just to say hello. So, "Hello". ----
When sending replies, the Subject field is often retained, though prepended with "Re: " as described in section 3.6.5. ---- From: Mary Smith <email@example.com> To: John Doe <firstname.lastname@example.org> Reply-To: "Mary Smith: Personal Account" <email@example.com> Subject: Re: Saying Hello Date: Fri, 21 Nov 1997 10:01:10 -0600 Message-ID: <firstname.lastname@example.org> In-Reply-To: <email@example.com> References: <firstname.lastname@example.org> This is a reply to your hello. ---- Note the "Reply-To:" field in the above message. When John replies to Mary's message above, the reply should go to the address in the "Reply-To:" field instead of the address in the "From:" field. ---- To: "Mary Smith: Personal Account" <email@example.com> From: John Doe <firstname.lastname@example.org> Subject: Re: Saying Hello Date: Fri, 21 Nov 1997 11:00:00 -0600 Message-ID: <email@example.com> In-Reply-To: <firstname.lastname@example.org> References: <email@example.com> <firstname.lastname@example.org> This is a reply to your reply. ----
Say that Mary, upon receiving this message, wishes to send a copy of the message to Jane such that (a) the message would appear to have come straight from John; (b) if Jane replies to the message, the reply should go back to John; and (c) all of the original information, like the date the message was originally sent to Mary, the message identifier, and the original addressee, is preserved. In this case, resent fields are prepended to the message: ---- Resent-From: Mary Smith <email@example.com> Resent-To: Jane Brown <firstname.lastname@example.org> Resent-Date: Mon, 24 Nov 1997 14:22:01 -0800 Resent-Message-ID: <email@example.com> From: John Doe <firstname.lastname@example.org> To: Mary Smith <email@example.com> Subject: Saying Hello Date: Fri, 21 Nov 1997 09:55:06 -0600 Message-ID: <firstname.lastname@example.org> This is a message just to say hello. So, "Hello". ---- If Jane, in turn, wished to resend this message to another person, she would prepend her own set of resent header fields to the above and send that.
RFC2821], trace fields are prepended to the message. The following is an example of what those trace fields might look like. Note that there is some folding white space in the first one since these lines can be long. ---- Received: from x.y.test by example.net via TCP with ESMTP id ABC12345 for <email@example.com>; 21 Nov 1997 10:05:43 -0600 Received: from machine.example by x.y.test; 21 Nov 1997 10:01:22 -0600 From: John Doe <firstname.lastname@example.org> To: Mary Smith <email@example.com> Subject: Saying Hello Date: Fri, 21 Nov 1997 09:55:06 -0600 Message-ID: <firstname.lastname@example.org> This is a message just to say hello. So, "Hello". ----
section 4 of this document.
---- From : John Doe <jdoe@machine(comment). example> To : Mary Smith __ <email@example.com> Subject : Saying Hello Date : Fri, 21 Nov 1997 09(comment): 55 : 06 -0600 Message-ID : <1234 @ local(blah) .machine .example> This is a message just to say hello. So, "Hello". ---- Note especially the second line of the "To:" field. It starts with two space characters. (Note that "__" represent blank spaces.) Therefore, it is considered part of the folding as described in section 4.2. Also, the comments and white space throughout addresses, dates, and message identifiers are all part of the obsolete syntax. RFC822] and [STD3]. Items marked with an asterisk (*) below are items which appear in section 4 of this document and therefore can no longer be generated. 1. Period allowed in obsolete form of phrase. 2. ABNF moved out of document to [RFC2234]. 3. Four or more digits allowed for year. 4. Header field ordering (and lack thereof) made explicit. 5. Encrypted header field removed. 6. Received syntax loosened to allow any token/value pair. 7. Specifically allow and give meaning to "-0000" time zone. 8. Folding white space is not allowed between every token. 9. Requirement for destinations removed. 10. Forwarding and resending redefined. 11. Extension header fields no longer specifically called out. 12. ASCII 0 (null) removed.* 13. Folding continuation lines cannot contain only white space.* 14. Free insertion of comments not allowed in date.* 15. Non-numeric time zones not allowed.* 16. Two digit years not allowed.* 17. Three digit years interpreted, but not allowed for generation. 18. Routes in addresses not allowed.* 19. CFWS within local-parts and domains not allowed.* 20. Empty members of address lists not allowed.*
21. Folding white space between field name and colon not allowed.* 22. Comments between field name and colon not allowed. 23. Tightened syntax of in-reply-to and references.* 24. CFWS within msg-id not allowed.* 25. Tightened semantics of resent fields as informational only. 26. Resent-Reply-To not allowed.* 27. No multiple occurrences of fields (except resent and received).* 28. Free CR and LF not allowed.* 29. Routes in return path not allowed.* 30. Line length limits specified. 31. Bcc more clearly specified.
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