Network Working Group                                    P. Resnick, Ed.
Request for Comments: 5322                         Qualcomm Incorporated
Obsoletes: 2822                                             October 2008
Updates: 4021
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.

Abstract

   This document specifies the Internet Message Format (IMF), a syntax
   for text messages that are sent between computer users, within the
   framework of "electronic mail" messages.  This specification is a
   revision of Request For Comments (RFC) 2822, which itself superseded
   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 . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Scope  . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Notational Conventions . . . . . . . . . . . . . . . . . .  5
       1.2.1.  Requirements Notation  . . . . . . . . . . . . . . . .  5
       1.2.2.  Syntactic Notation . . . . . . . . . . . . . . . . . .  5
       1.2.3.  Structure of This Document . . . . . . . . . . . . . .  5
   2.  Lexical Analysis of Messages . . . . . . . . . . . . . . . . .  6
     2.1.  General Description  . . . . . . . . . . . . . . . . . . .  6
       2.1.1.  Line Length Limits . . . . . . . . . . . . . . . . . .  7
     2.2.  Header Fields  . . . . . . . . . . . . . . . . . . . . . .  8
       2.2.1.  Unstructured Header Field Bodies . . . . . . . . . . .  8
       2.2.2.  Structured Header Field Bodies . . . . . . . . . . . .  8
       2.2.3.  Long Header Fields . . . . . . . . . . . . . . . . . .  8
     2.3.  Body . . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   3.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     3.1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . 10
     3.2.  Lexical Tokens . . . . . . . . . . . . . . . . . . . . . . 10
       3.2.1.  Quoted characters  . . . . . . . . . . . . . . . . . . 10
       3.2.2.  Folding White Space and Comments . . . . . . . . . . . 11
       3.2.3.  Atom . . . . . . . . . . . . . . . . . . . . . . . . . 12
       3.2.4.  Quoted Strings . . . . . . . . . . . . . . . . . . . . 13
       3.2.5.  Miscellaneous Tokens . . . . . . . . . . . . . . . . . 14
     3.3.  Date and Time Specification  . . . . . . . . . . . . . . . 14
     3.4.  Address Specification  . . . . . . . . . . . . . . . . . . 16
       3.4.1.  Addr-Spec Specification  . . . . . . . . . . . . . . . 17
     3.5.  Overall Message Syntax . . . . . . . . . . . . . . . . . . 18
     3.6.  Field Definitions  . . . . . . . . . . . . . . . . . . . . 19
       3.6.1.  The Origination Date Field . . . . . . . . . . . . . . 22
       3.6.2.  Originator Fields  . . . . . . . . . . . . . . . . . . 22
       3.6.3.  Destination Address Fields . . . . . . . . . . . . . . 23
       3.6.4.  Identification Fields  . . . . . . . . . . . . . . . . 25
       3.6.5.  Informational Fields . . . . . . . . . . . . . . . . . 27
       3.6.6.  Resent Fields  . . . . . . . . . . . . . . . . . . . . 28
       3.6.7.  Trace Fields . . . . . . . . . . . . . . . . . . . . . 30
       3.6.8.  Optional Fields  . . . . . . . . . . . . . . . . . . . 30
   4.  Obsolete Syntax  . . . . . . . . . . . . . . . . . . . . . . . 31
     4.1.  Miscellaneous Obsolete Tokens  . . . . . . . . . . . . . . 32
     4.2.  Obsolete Folding White Space . . . . . . . . . . . . . . . 33
     4.3.  Obsolete Date and Time . . . . . . . . . . . . . . . . . . 33
     4.4.  Obsolete Addressing  . . . . . . . . . . . . . . . . . . . 35
     4.5.  Obsolete Header Fields . . . . . . . . . . . . . . . . . . 35
       4.5.1.  Obsolete Origination Date Field  . . . . . . . . . . . 36
       4.5.2.  Obsolete Originator Fields . . . . . . . . . . . . . . 36
       4.5.3.  Obsolete Destination Address Fields  . . . . . . . . . 37
       4.5.4.  Obsolete Identification Fields . . . . . . . . . . . . 37
       4.5.5.  Obsolete Informational Fields  . . . . . . . . . . . . 37

       4.5.6.  Obsolete Resent Fields . . . . . . . . . . . . . . . . 38
       4.5.7.  Obsolete Trace Fields  . . . . . . . . . . . . . . . . 38
       4.5.8.  Obsolete optional fields . . . . . . . . . . . . . . . 38
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 38
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 39
   Appendix A.     Example Messages . . . . . . . . . . . . . . . . . 43
   Appendix A.1.   Addressing Examples  . . . . . . . . . . . . . . . 44
   Appendix A.1.1. A Message from One Person to Another with
                   Simple Addressing  . . . . . . . . . . . . . . . . 44
   Appendix A.1.2. Different Types of Mailboxes . . . . . . . . . . . 45
   Appendix A.1.3. Group Addresses  . . . . . . . . . . . . . . . . . 45
   Appendix A.2.   Reply Messages . . . . . . . . . . . . . . . . . . 46
   Appendix A.3.   Resent Messages  . . . . . . . . . . . . . . . . . 47
   Appendix A.4.   Messages with Trace Fields . . . . . . . . . . . . 48
   Appendix A.5.   White Space, Comments, and Other Oddities  . . . . 49
   Appendix A.6.   Obsoleted Forms  . . . . . . . . . . . . . . . . . 50
   Appendix A.6.1. Obsolete Addressing  . . . . . . . . . . . . . . . 50
   Appendix A.6.2. Obsolete Dates . . . . . . . . . . . . . . . . . . 50
   Appendix A.6.3. Obsolete White Space and Comments  . . . . . . . . 51
   Appendix B.     Differences from Earlier Specifications  . . . . . 52
   Appendix C.     Acknowledgements . . . . . . . . . . . . . . . . . 53
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 55
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 55
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 55

1. Introduction

1.1. Scope

   This document specifies the Internet Message Format (IMF), a syntax
   for text messages that are sent between computer users, within the
   framework of "electronic mail" messages.  This specification is an
   update to [RFC2822], which itself superseded [RFC0822], updating it
   to reflect current practice and incorporating incremental changes
   that were specified in other RFCs such as [RFC1123].

   This document 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 specification.

   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 [RFC5321] for a
   discussion of the envelope.)  The contents comprise the object to be
   delivered to the recipient.  This specification 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 specification
   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 specification, local storage is
   outside of the scope of this specification.

      Note: This specification 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 document 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.

1.2. Notational Conventions

1.2.1. Requirements Notation

   This document occasionally uses terms that appear in capital letters.
   When the terms "MUST", "SHOULD", "RECOMMENDED", "MUST NOT", "SHOULD
   NOT", and "MAY" appear capitalized, they are being used to indicate
   particular requirements of this specification.  A discussion of the
   meanings of these terms appears in [RFC2119].

1.2.2. Syntactic Notation

   This specification uses the Augmented Backus-Naur Form (ABNF)
   [RFC5234] notation 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).

1.2.3. Structure of This Document

   This document is divided into several sections.

   This section, 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 lays out the actual rules for the structure
   of each part of a message (the syntax) as well as a description of
   the parts and instructions for their interpretation (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 specification.

   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 versions of this specification 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
   specification.  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 specification.

   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 specification and
   earlier specifications for Internet messages.

   Appendix C contains acknowledgements.

2. Lexical Analysis of Messages

2.1. General Description

   At the most basic level, a message is a series of characters.  A
   message that is conformant with this specification is composed of
   characters with values in the range of 1 through 127 and interpreted
   as US-ASCII [ANSI.X3-4.1986] characters.  For brevity, this document
   sometimes refers to this range of characters as simply "US-ASCII
   characters".

      Note: This document 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], [RFC2049], [RFC4288], [RFC4289]), that
      extend this specification to allow for values outside of that
      range.  Discussion of those mechanisms is not within the scope of
      this specification.

   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
   section of the message") followed, optionally, by a body.  The header
   section is a sequence of lines of characters with special syntax as
   defined in this specification.  The body is simply a sequence of
   characters that follows the header section and is separated from the
   header section by an empty line (i.e., a line with nothing preceding
   the CRLF).

      Note: Common parlance and earlier versions of this specification
      use the term "header" to either refer to the entire header section
      or to refer to an individual header field.  To avoid ambiguity,
      this document does not use the terms "header" or "headers" in
      isolation, but instead always uses "header field" to refer to the
      individual field and "header section" to refer to the entire
      collection.

2.1.1. Line Length Limits

   There are two limits that this specification places on the number of
   characters in a line.  Each line of characters MUST be no more than
   998 characters, and SHOULD be no more than 78 characters, excluding
   the CRLF.

   The 998 character limit is due to limitations in many implementations
   that send, receive, or store IMF messages which simply cannot handle
   more than 998 characters on a line.  Receiving implementations would
   do well to handle an arbitrarily large number of characters in a line
   for robustness sake.  However, there are so many implementations that
   (in compliance with the transport requirements of [RFC5321]) do not
   accept messages containing more than 1000 characters 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 [RFC5321] if they actually cause
   information to be lost).  Again, even though this limitation is put
   on messages, it is incumbent upon implementations that 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.

2.2. Header Fields

   Header fields are lines beginning with a field name, followed by a
   colon (":"), followed by a field body, and terminated by CRLF.  A
   field name MUST be composed of printable US-ASCII characters (i.e.,
   characters that have values between 33 and 126, inclusive), except
   colon.  A field body may be composed of printable US-ASCII characters
   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).  A field body MUST NOT include CR and LF except
   when used in "folding" and "unfolding", as described in section
   2.2.3.  All field bodies MUST conform to the syntax described in
   sections 3 and 4 of this specification.

2.2.1. Unstructured Header Field Bodies

   Some field bodies in this specification are defined simply as
   "unstructured" (which is specified in section 3.2.5 as any printable
   US-ASCII characters plus white space characters) with no further
   restrictions.  These are referred to as unstructured field bodies.
   Semantically, unstructured field bodies are simply to be treated as a
   single line of characters with no further processing (except for
   "folding" and "unfolding" as described in section 2.2.3).

2.2.2. Structured Header Field Bodies

   Some field bodies in this specification have a syntax that is more
   restrictive than the unstructured field bodies described above.
   These are referred to as "structured" field bodies.  Structured field
   bodies are sequences of specific lexical tokens as described in
   sections 3 and 4 of this specification.  Many of these tokens are
   allowed (according to their syntax) to be introduced or end with
   comments (as described in section 3.2.2) as well as the white space
   characters, and those white space characters are subject to "folding"
   and "unfolding" as described in section 2.2.3.  Semantic analysis of
   structured field bodies is given along with their syntax.

2.2.3. Long Header Fields

   Each header field is logically a single line of characters comprising
   the field name, the colon, and the field body.  For convenience
   however, and to deal with the 998/78 character limitations per line,
   the field body portion of a header field can be split into a
   multiple-line representation; this is called "folding".  The general
   rule is that wherever this specification 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.  An unfolded header field has no length restriction and
   therefore may be indeterminately long.

2.3. Body

   The body of a message is simply lines of US-ASCII characters.  The
   only two limitations on the body are as follows:

   o  CR and LF MUST only occur together as CRLF; they MUST NOT appear
      independently in the body.
   o  Lines of characters in the body MUST be limited to 998 characters,
      and SHOULD be limited to 78 characters, excluding the CRLF.

      Note: As was stated earlier, there are other documents,
      specifically the MIME documents ([RFC2045], [RFC2046], [RFC2049],
      [RFC4288], [RFC4289]), that extend (and limit) this specification
      to allow for different sorts of message bodies.  Again, these
      mechanisms are beyond the scope of this document.

3. Syntax

3.1. Introduction

   The syntax as given in this section defines the legal syntax of
   Internet messages.  Messages that are conformant to this
   specification MUST conform to the syntax in this section.  If there
   are options in this section where one option SHOULD be generated,
   that is indicated either in the prose or in a comment next to the
   syntax.

   For the defined expressions, a short description of the syntax and
   use is given, followed by the syntax in ABNF, followed by a semantic
   analysis.  The following primitive tokens that are used but otherwise
   unspecified are taken from the "Core Rules" of [RFC5234], Appendix
   B.1: CR, LF, CRLF, HTAB, SP, WSP, DQUOTE, DIGIT, ALPHA, and VCHAR.

   In some of the definitions, there will be non-terminals 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 the
   generation of messages, with "obs-" elements that are to be ignored,
   while section 4 adds grammar for the interpretation of messages.

3.2. Lexical Tokens

   The following rules are used to define an underlying lexical
   analyzer, which feeds tokens to the higher-level parsers.  This
   section defines the tokens used in structured header field bodies.

      Note: Readers of this specification need to pay special attention
      to how these lexical tokens are used in both the lower-level and
      higher-level syntax later in the document.  Particularly, the
      white space tokens and the comment tokens defined in section 3.2.2
      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, white space and comments may be allowed
      in the higher-level tokens even though they may not explicitly
      appear in a particular definition.

3.2.1. Quoted characters

   Some characters are reserved for special interpretation, such as
   delimiting lexical tokens.  To permit use of these characters as
   uninterpreted data, a quoting mechanism is provided.

   quoted-pair     =   ("\" (VCHAR / WSP)) / obs-qp

   Where any quoted-pair appears, it is to be interpreted as the
   character alone.  That is to say, the "\" character that appears as
   part of a quoted-pair is semantically "invisible".

      Note: The "\" character may appear in a message where it is not
      part of a quoted-pair.  A "\" character that does not appear in a
      quoted-pair is not semantically invisible.  The only places in
      this specification where quoted-pair currently appears are
      ccontent, qcontent, and in obs-dtext in section 4.

3.2.2. Folding White Space and Comments

   White space characters, including white space used in folding
   (described in 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 3.2.4.  Comments may nest.

   There are several places in this specification 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 specification, 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) /  obs-FWS
                                          ; Folding white space

   ctext           =   %d33-39 /          ; Printable US-ASCII
                       %d42-91 /          ;  characters not including
                       %d93-126 /         ;  "(", ")", or "\"
                       obs-ctext

   ccontent        =   ctext / quoted-pair / comment

   comment         =   "(" *([FWS] ccontent) [FWS] ")"

   CFWS            =   (1*([FWS] comment) [FWS]) / FWS

   Throughout this specification, where FWS (the folding white space
   token) appears, it indicates a place where folding, as discussed in
   section 2.2.3, may take place.  Wherever folding appears in a message
   (that is, a header field body containing a CRLF followed by any WSP),
   unfolding (removal of the CRLF) is performed before any further
   semantic analysis is performed on that header field according to this
   specification.  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 header field are semantically interpreted as a single
   space character.

3.2.3. Atom

   Several productions in structured header field bodies are simply
   strings of certain basic characters.  Such productions are called
   atoms.

   Some of the structured header field bodies also allow the period
   character (".", ASCII value 46) within runs of atext.  An additional
   "dot-atom" token is defined for those purposes.

      Note: The "specials" token does not appear anywhere else in this
      specification.  It is simply the visible (i.e., non-control, non-
      white space) characters that do not appear in atext.  It is
      provided only because 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.

   atext           =   ALPHA / DIGIT /    ; Printable US-ASCII
                       "!" / "#" /        ;  characters not including
                       "$" / "%" /        ;  specials.  Used for atoms.
                       "&" / "'" /
                       "*" / "+" /
                       "-" / "/" /
                       "=" / "?" /
                       "^" / "_" /
                       "`" / "{" /
                       "|" / "}" /
                       "~"

   atom            =   [CFWS] 1*atext [CFWS]

   dot-atom-text   =   1*atext *("." 1*atext)

   dot-atom        =   [CFWS] dot-atom-text [CFWS]

   specials        =   "(" / ")" /        ; Special characters that do
                       "<" / ">" /        ;  not appear in atext
                       "[" / "]" /
                       ":" / ";" /
                       "@" / "\" /
                       "," / "." /
                       DQUOTE

   Both atom and dot-atom are interpreted as a single unit, comprising
   the string of characters that make it up.  Semantically, the optional
   comments and FWS surrounding the rest of the characters are not part
   of the atom; the atom is only the run of atext characters in an atom,
   or the atext and "." characters in a dot-atom.

3.2.4. Quoted Strings

   Strings of characters that include characters other than those
   allowed in atoms can be represented in a quoted string format, where
   the characters are surrounded by quote (DQUOTE, ASCII value 34)
   characters.

   qtext           =   %d33 /             ; Printable US-ASCII
                       %d35-91 /          ;  characters not including
                       %d93-126 /         ;  "\" or the quote character
                       obs-qtext

   qcontent        =   qtext / quoted-pair

   quoted-string   =   [CFWS]
                       DQUOTE *([FWS] qcontent) [FWS] DQUOTE
                       [CFWS]

   A quoted-string is treated as a unit.  That is, quoted-string is
   identical to atom, semantically.  Since a quoted-string is allowed to
   contain FWS, folding is permitted.  Also note that since quoted-pair
   is allowed in a quoted-string, the quote and backslash characters may
   appear in a quoted-string so long as they appear as a quoted-pair.

   Semantically, neither the optional CFWS outside of the quote
   characters nor the quote characters themselves are part of the
   quoted-string; the quoted-string is what is contained between the two
   quote characters.  As stated earlier, the "\" in any quoted-pair and
   the CRLF in any FWS/CFWS that appears within the quoted-string are
   semantically "invisible" and therefore not part of the quoted-string
   either.

3.2.5. Miscellaneous Tokens

   Three additional tokens are defined: word and phrase for combinations
   of atoms and/or quoted-strings, and unstructured for use in
   unstructured header fields and in some places within structured
   header fields.

   word            =   atom / quoted-string

   phrase          =   1*word / obs-phrase

   unstructured    =   (*([FWS] VCHAR) *WSP) / obs-unstruct

3.3. Date and Time Specification

   Date and time values occur in several header fields.  This section
   specifies the syntax for a full date and time specification.  Though
   folding white space is permitted throughout the date-time
   specification, it is RECOMMENDED that a single space be used in each
   place that FWS appears (whether it is required or optional); some
   older implementations will not interpret longer sequences of folding
   white space correctly.

   date-time       =   [ day-of-week "," ] date time [CFWS]

   day-of-week     =   ([FWS] day-name) / obs-day-of-week

   day-name        =   "Mon" / "Tue" / "Wed" / "Thu" /
                       "Fri" / "Sat" / "Sun"

   date            =   day month year

   day             =   ([FWS] 1*2DIGIT FWS) / obs-day

   month           =   "Jan" / "Feb" / "Mar" / "Apr" /
                       "May" / "Jun" / "Jul" / "Aug" /
                       "Sep" / "Oct" / "Nov" / "Dec"

   year            =   (FWS 4*DIGIT FWS) / obs-year

   time            =   time-of-day zone

   time-of-day     =   hour ":" minute [ ":" second ]

   hour            =   2DIGIT / obs-hour

   minute          =   2DIGIT / obs-minute

   second          =   2DIGIT / obs-second

   zone            =   (FWS ( "+" / "-" ) 4DIGIT) / obs-zone

   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
   additional 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 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-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 [RFC1305]), and the last two digits
   of the zone MUST be within the range 00 through 59.

3.4. Address Specification

   Addresses occur in several message header fields to indicate senders
   and recipients of messages.  An address may either be an individual
   mailbox, or a group of mailboxes.

   address         =   mailbox / group

   mailbox         =   name-addr / addr-spec

   name-addr       =   [display-name] angle-addr

   angle-addr      =   [CFWS] "<" addr-spec ">" [CFWS] /
                       obs-angle-addr

   group           =   display-name ":" [group-list] ";" [CFWS]

   display-name    =   phrase

   mailbox-list    =   (mailbox *("," mailbox)) / obs-mbox-list

   address-list    =   (address *("," address)) / obs-addr-list

   group-list      =   mailbox-list / CFWS / obs-group-list

   A mailbox receives mail.  It is a conceptual entity that 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 composed of two parts: (1) an optional display
   name that indicates the name of the recipient (which can 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 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 any of those recipients.

3.4.1. Addr-Spec Specification

   An addr-spec is a specific Internet identifier that contains a
   locally interpreted string followed by the at-sign character ("@",
   ASCII value 64) followed by an Internet domain.  The locally
   interpreted string is either a quoted-string or a dot-atom.  If the
   string can be represented as a dot-atom (that is, it contains no
   characters other than atext characters or "." surrounded by atext
   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.

      Note: A liberal syntax for the domain portion of addr-spec is
      given here.  However, the domain portion contains addressing
      information specified by and used in other protocols (e.g.,
      [RFC1034], [RFC1035], [RFC1123], [RFC5321]).  It is therefore
      incumbent upon implementations to conform to the syntax of
      addresses for the context in which they are used.

   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] dtext) [FWS] "]" [CFWS]

   dtext           =   %d33-90 /          ; Printable US-ASCII
                       %d94-126 /         ;  characters not including
                       obs-dtext          ;  "[", "]", 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 [RFC1034], [RFC1035], and [RFC1123].  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 separate documents, such as [RFC5321].  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.

3.5. Overall Message Syntax

   A message consists of header fields, optionally followed by a message
   body.  Lines in a message MUST be a maximum of 998 characters
   excluding the CRLF, but it is RECOMMENDED that lines be limited to 78
   characters excluding the CRLF.  (See 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) / obs-body

   text            =   %d1-9 /            ; Characters excluding CR
                       %d11 /             ;  and LF
                       %d12 /
                       %d14-127

   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
   that are uninterpreted for the purposes of this specification.

3.6. Field Definitions

   The header fields of a message are defined here.  All header fields
   have the same general syntactic structure: a field name, followed by
   a colon, followed by the field body.  The specific syntax for each
   header field is defined in the subsequent sections.

      Note: In the ABNF syntax for each field in subsequent sections,
      each field name is followed by the required colon.  However, for
      brevity, sometimes the colon is not referred to in the textual
      description of the syntax.  It is, nonetheless, required.

   It is important to note that the header fields are not guaranteed to
   be in a particular order.  They may appear in any order, and they
   have been known to be reordered occasionally when transported over
   the Internet.  However, for the purposes of this specification,
   header fields SHOULD NOT be reordered when a message is transported
   or transformed.  More importantly, the trace header fields and resent
   header fields MUST NOT be reordered, and SHOULD be kept in blocks
   prepended to the message.  See 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
                         *optional-field /
                         *(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 the header section of a message 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    | Max number | Notes                      |
   |                | number |            |                            |
   +----------------+--------+------------+----------------------------+
   | trace          | 0      | unlimited  | Block prepended - see      |
   |                |        |            | 3.6.7                      |
   | resent-date    | 0*     | unlimited* | One per block, required if |
   |                |        |            | other resent fields are    |
   |                |        |            | 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.

3.6.1. The Origination Date Field

   The origination date field consists of the field name "Date" followed
   by a date-time specification.

   orig-date       =   "Date:" date-time CRLF

   The origination date specifies the date and time at which the creator
   of the message indicated that the message was complete and ready to
   enter the mail delivery system.  For instance, this might be the time
   that a user pushes the "send" or "submit" button in an application
   program.  In any case, it is specifically not intended to convey the
   time that the message is actually transported, but rather the time at
   which the human or other creator of the message has put the message
   into its final form, ready for transport.  (For example, a portable
   computer user who is not connected to a network might queue a message
   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.)

3.6.2. Originator Fields

   The originator fields of a message consist of the from field, the
   sender field (when applicable), and optionally the reply-to field.
   The from field consists of the field name "From" and a comma-
   separated list of one or more mailbox specifications.  If the from
   field contains more than one mailbox specification in the mailbox-
   list, then the sender field, containing the field name "Sender" and a
   single mailbox specification, MUST appear in the message.  In either
   case, an optional reply-to field MAY also be included, which contains
   the field name "Reply-To" and a comma-separated list of one or more
   addresses.

   from            =   "From:" mailbox-list CRLF

   sender          =   "Sender:" mailbox CRLF

   reply-to        =   "Reply-To:" address-list CRLF

   The originator fields indicate the mailbox(es) of the source of the
   message.  The "From:" field specifies the author(s) of the message,
   that is, the mailbox(es) of the person(s) or system(s) responsible
   for the writing of the message.  The "Sender:" field specifies the
   mailbox of the agent responsible for the actual transmission of the
   message.  For example, if a secretary were to send a message for
   another person, the mailbox of the secretary would appear in the
   "Sender:" field and the mailbox of the actual author would appear in
   the "From:" field.  If the originator of the message can be indicated

   by a single mailbox and the author and transmitter are identical, the
   "Sender:" field SHOULD NOT be used.  Otherwise, both fields SHOULD
   appear.

      Note: The transmitter information is always present.  The absence
      of the "Sender:" field is sometimes mistakenly taken to mean that
      the agent responsible for transmission of the message has not been
      specified.  This absence merely means that the transmitter is
      identical to the author and is therefore not redundantly placed
      into the "Sender:" field.

   The originator fields also provide the information required when
   replying to a message.  When the "Reply-To:" field is present, it
   indicates the address(es) to which the author of the message suggests
   that replies be sent.  In the absence of the "Reply-To:" field,
   replies SHOULD by default be sent to the mailbox(es) specified in the
   "From:" field unless otherwise specified by the person composing the
   reply.

   In all cases, the "From:" field SHOULD NOT contain any mailbox that
   does not belong to the author(s) of the message.  See also section
   3.6.3 for more information on forming the destination addresses for a
   reply.

3.6.3. Destination Address Fields

   The destination fields of a message consist of three possible fields,
   each of the same form: the field name, which is either "To", "Cc", or
   "Bcc", followed by a comma-separated list of one or more addresses
   (either mailbox or group syntax).

   to              =   "To:" address-list CRLF

   cc              =   "Cc:" address-list CRLF

   bcc             =   "Bcc:" [address-list / CFWS] CRLF

   The destination fields specify the recipients of the message.  Each
   destination field may have one or more addresses, and the addresses
   indicate the intended recipients of the message.  The only difference
   between the three fields is how each is used.

   The "To:" field contains the address(es) of the primary recipient(s)
   of the message.

   The "Cc:" field (where the "Cc" means "Carbon Copy" in the sense of
   making a copy on a typewriter using carbon paper) contains the
   addresses of others who are to receive the message, though the
   content of the message may not be directed at them.

   The "Bcc:" field (where the "Bcc" means "Blind Carbon Copy") contains
   addresses of recipients of the message whose addresses are not to be
   revealed to other recipients of the message.  There are three ways in
   which the "Bcc:" field is used.  In the first case, when a message
   containing a "Bcc:" field is prepared to be sent, the "Bcc:" line is
   removed even though all of the recipients (including those specified
   in the "Bcc:" field) are sent a copy of the message.  In the second
   case, recipients specified in the "To:" and "Cc:" lines each are sent
   a copy of the message with the "Bcc:" line removed as above, but the
   recipients on the "Bcc:" line get a separate copy of the message
   containing a "Bcc:" line.  (When there are multiple recipient
   addresses in the "Bcc:" field, some implementations actually send a
   separate copy of the message to each recipient with a "Bcc:"
   containing only the address of that particular recipient.)  Finally,
   since a "Bcc:" field may contain no addresses, a "Bcc:" field can be
   sent without any addresses indicating to the recipients that blind
   copies were sent to someone.  Which method to use with "Bcc:" fields
   is implementation dependent, but refer to the "Security
   Considerations" section of this document for a discussion of each.

   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 they 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.

3.6.4. Identification Fields

   Though listed as optional in the table in section 3.6, every message
   SHOULD have a "Message-ID:" field.  Furthermore, reply messages
   SHOULD have "In-Reply-To:" and "References:" fields as appropriate
   and as described below.

   The "Message-ID:" field contains a single unique message identifier.
   The "References:" and "In-Reply-To:" fields each contain one or more
   unique message identifiers, optionally separated by CFWS.

   The message identifier (msg-id) syntax is a limited version of the
   addr-spec construct enclosed in the angle bracket characters, "<" and
   ">".  Unlike addr-spec, this syntax only permits the dot-atom-text
   form on the left-hand side of the "@" and does not have internal CFWS
   anywhere in the message identifier.

      Note: As with addr-spec, a liberal syntax is given for the right-
      hand side of the "@" in a msg-id.  However, later in this section,
      the use of a domain for the right-hand side of the "@" is
      RECOMMENDED.  Again, the syntax of domain constructs is specified
      by and used in other protocols (e.g., [RFC1034], [RFC1035],
      [RFC1123], [RFC5321]).  It is therefore incumbent upon
      implementations to conform to the syntax of addresses for the
      context in which they are used.

   message-id      =   "Message-ID:" msg-id CRLF

   in-reply-to     =   "In-Reply-To:" 1*msg-id CRLF

   references      =   "References:" 1*msg-id CRLF

   msg-id          =   [CFWS] "<" id-left "@" id-right ">" [CFWS]

   id-left         =   dot-atom-text / obs-id-left

   id-right        =   dot-atom-text / no-fold-literal / obs-id-right

   no-fold-literal =   "[" *dtext "]"

   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 version 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 that 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; 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 addr-spec (identical except that quoted strings,
   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 "@" (since domain names and IP addresses are normally unique),
   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.  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.

3.6.5. Informational Fields

   The informational fields are all optional.  The "Subject:" and
   "Comments:" fields are unstructured fields as defined in section
   2.2.1, and therefore may contain text or folding white space.  The
   "Keywords:" field contains a comma-separated list of one or more
   words or quoted-strings.

   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: " (an abbreviation of the Latin "in re", meaning "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.

3.6.6. Resent Fields

   Resent fields SHOULD be added to any message that is reintroduced by
   a user into the transport system.  A separate set of resent fields
   SHOULD be added each time this is done.  All of the resent fields
   corresponding to a particular resending of the message SHOULD be
   grouped together.  Each new set of resent fields is prepended to the
   message; that is, the most recent set of resent fields appears
   earlier in the message.  No other fields in the message are changed
   when resent fields are added.

   Each of the resent fields corresponds to a particular field elsewhere
   in the syntax.  For instance, the "Resent-Date:" field corresponds to
   the "Date:" field and the "Resent-To:" field corresponds to the "To:"
   field.  In each case, the syntax for the field body is identical to
   the syntax given previously for the corresponding field.

   When resent fields are used, the "Resent-From:" and "Resent-Date:"
   fields MUST be sent.  The "Resent-Message-ID:" field SHOULD be sent.
   "Resent-Sender:" SHOULD NOT be used if "Resent-Sender:" would be
   identical to "Resent-From:".

   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.  Forwarding may
      also mean that 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.

3.6.7. Trace Fields

   The trace fields are a group of header fields consisting of an
   optional "Return-Path:" field, and one or more "Received:" fields.
   The "Return-Path:" header field contains a pair of angle brackets
   that enclose an optional addr-spec.  The "Received:" field contains a
   (possibly empty) list of tokens followed by a semicolon and a date-
   time specification.  Each token must be a word, angle-addr, addr-
   spec, or a domain.  Further restrictions are applied to the syntax of
   the trace fields by specifications that provide for their use, such
   as [RFC5321].

   trace           =   [return]
                       1*received

   return          =   "Return-Path:" path CRLF

   path            =   angle-addr / ([CFWS] "<" [CFWS] ">" [CFWS])

   received        =   "Received:" *received-token ";" date-time CRLF

   received-token  =   word / angle-addr / addr-spec / domain

   A full discussion of the Internet mail use of trace fields is
   contained in [RFC5321].  For the purposes of this specification, the
   trace fields are strictly informational, and any formal
   interpretation of them is outside of the scope of this document.

3.6.8. Optional Fields

   Fields may appear in messages that are otherwise unspecified in this
   document.  They MUST conform to the syntax of an optional-field.
   This is a field name, made up of the printable US-ASCII characters
   except SP and colon, followed by a colon, followed by any text that
   conforms to the unstructured syntax.

   The field names of any optional field MUST NOT be identical to any
   field name specified elsewhere in this document.

   optional-field  =   field-name ":" unstructured CRLF

   field-name      =   1*ftext

   ftext           =   %d33-57 /          ; Printable US-ASCII
                       %d59-126           ;  characters not including
                                          ;  ":".

   For the purposes of this specification, any optional field is
   uninterpreted.

4. Obsolete Syntax

   Earlier versions of this specification allowed for different (usually
   more liberal) syntax than is allowed in this version.  Also, there
   have been syntactic elements used in messages on the Internet whose
   interpretations have never been documented.  Though these syntactic
   forms MUST NOT be generated according to the grammar in 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 the interpretation of
   messages.

      Note: This section identifies syntactic forms that any
      implementation MUST reasonably interpret.  However, there are
      certainly Internet messages that 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.  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 could be freely inserted between any syntactic tokens.  This
   allowed 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 3.2.2 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.  Similarly, US-
   ASCII control characters other than CR, LF, SP, and HTAB (ASCII
   values 1 through 8, 11, 12, 14 through 31, and 127) were allowed to
   appear in header field bodies.  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.

4.1. Miscellaneous Obsolete Tokens

   These syntactic elements are used elsewhere in the obsolete syntax or
   in the main syntax.  Bare CR, bare LF, and NUL are added to obs-qp,
   obs-body, and obs-unstruct.  US-ASCII control characters are added to
   obs-qp, obs-unstruct, obs-ctext, and obs-qtext.  The period character
   is added to obs-phrase.  The obs-phrase-list provides for a
   (potentially empty) comma-separated list of phrases that may include
   "null" elements.  That is, there could be two or more commas in such
   a list with nothing in between them, or commas at the beginning or
   end of the list.

      Note: The "period" (or "full stop") character (".") in obs-phrase
      is not a form that was allowed in earlier versions of this or any
      other specification.  Period (nor any other character from
      specials) was not allowed in phrase because it introduced a
      parsing difficulty distinguishing between phrases and portions of
      an addr-spec (see 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.

   obs-NO-WS-CTL   =   %d1-8 /            ; US-ASCII control
                       %d11 /             ;  characters that do not
                       %d12 /             ;  include the carriage
                       %d14-31 /          ;  return, line feed, and
                       %d127              ;  white space characters

   obs-ctext       =   obs-NO-WS-CTL

   obs-qtext       =   obs-NO-WS-CTL

   obs-utext       =   %d0 / obs-NO-WS-CTL / VCHAR

   obs-qp          =   "\" (%d0 / obs-NO-WS-CTL / LF / CR)

   obs-body        =   *((*LF *CR *((%d0 / text) *LF *CR)) / CRLF)

   obs-unstruct    =   *((*LF *CR *(obs-utext *LF *CR)) / FWS)

   obs-phrase      =   word *(word / "." / CFWS)

   obs-phrase-list =   [phrase / CFWS] *("," [phrase / CFWS])

   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 US-ASCII control characters with their traditional
   ASCII meanings.

4.2. Obsolete Folding White Space

   In the obsolete syntax, any amount of folding white space MAY be
   inserted where the obs-FWS rule is allowed.  This creates the
   possibility of having two consecutive "folds" in a line, and
   therefore the possibility that a line which makes up a folded header
   field could be composed entirely of white space.

   obs-FWS         =   1*WSP *(CRLF 1*WSP)

4.3. Obsolete Date and Time

   The syntax for the obsolete date format allows a 2 digit year in the
   date field and allows for a list of alphabetic time zone specifiers
   that were used in earlier versions of this specification.  It also
   permits comments and folding white space between many of the tokens.

   obs-day-of-week =   [CFWS] day-name [CFWS]

   obs-day         =   [CFWS] 1*2DIGIT [CFWS]

   obs-year        =   [CFWS] 2*DIGIT [CFWS]

   obs-hour        =   [CFWS] 2DIGIT [CFWS]

   obs-minute      =   [CFWS] 2DIGIT [CFWS]

   obs-second      =   [CFWS] 2DIGIT [CFWS]

   obs-zone        =   "UT" / "GMT" /     ; Universal Time
                                          ; 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 Savings" (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 [RFC0822] 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 [RFC0822], 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.

4.4. Obsolete Addressing

   There are four primary differences in addressing.  First, mailbox
   addresses were allowed to have a route portion before the addr-spec
   when enclosed in "<" and ">".  The route is simply a comma-separated
   list of domain names, each preceded by "@", and the list terminated
   by a colon.  Second, CFWS were allowed between the period-separated
   elements of local-part and domain (i.e., dot-atom was not used).  In
   addition, local-part is allowed to contain quoted-string in addition
   to just atom.  Third, mailbox-list and address-list were allowed to
   have "null" members.  That is, there could be two or more commas in
   such a list with nothing in between them, or commas at the beginning
   or end of the list.  Finally, US-ASCII control characters and quoted-
   pairs were allowed in domain literals and are added here.

   obs-angle-addr  =   [CFWS] "<" obs-route addr-spec ">" [CFWS]

   obs-route       =   obs-domain-list ":"

   obs-domain-list =   *(CFWS / ",") "@" domain
                       *("," [CFWS] ["@" domain])

   obs-mbox-list   =   *([CFWS] ",") mailbox *("," [mailbox / CFWS])

   obs-addr-list   =   *([CFWS] ",") address *("," [address / CFWS])

   obs-group-list  =   1*([CFWS] ",") [CFWS]

   obs-local-part  =   word *("." word)

   obs-domain      =   atom *("." atom)

   obs-dtext       =   obs-NO-WS-CTL / quoted-pair

   When interpreting addresses, the route portion SHOULD be ignored.

4.5. Obsolete Header Fields

   Syntactically, the primary difference in the obsolete field syntax is
   that it allows multiple occurrences of any of the fields and they may
   occur in any order.  Also, any amount of white space is allowed
   before the ":" at the end of the field name.

   obs-fields      =   *(obs-return /
                       obs-received /
                       obs-orig-date /
                       obs-from /
                       obs-sender /
                       obs-reply-to /
                       obs-to /
                       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 that 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.

4.5.1. Obsolete Origination Date Field

obs-orig-date = "Date" *WSP ":" date-time CRLF

4.5.2. Obsolete Originator Fields

   obs-from        =   "From" *WSP ":" mailbox-list CRLF

   obs-sender      =   "Sender" *WSP ":" mailbox CRLF

   obs-reply-to    =   "Reply-To" *WSP ":" address-list CRLF

4.5.3. Obsolete Destination Address Fields

   obs-to          =   "To" *WSP ":" address-list CRLF

   obs-cc          =   "Cc" *WSP ":" address-list CRLF

   obs-bcc         =   "Bcc" *WSP ":"
                       (address-list / (*([CFWS] ",") [CFWS])) CRLF

   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.

4.5.4. Obsolete Identification Fields

   The obsolete "In-Reply-To:" and "References:" fields differ from the
   current syntax in that they allow phrase (words or quoted strings) to
   appear.  The obsolete forms of the left and right sides of msg-id
   allow interspersed CFWS, making them syntactically identical to
   local-part and domain, respectively.

   obs-message-id  =   "Message-ID" *WSP ":" msg-id CRLF

   obs-in-reply-to =   "In-Reply-To" *WSP ":" *(phrase / msg-id) CRLF

   obs-references  =   "References" *WSP ":" *(phrase / msg-id) CRLF

   obs-id-left     =   local-part

   obs-id-right    =   domain

   For purposes of interpretation, the phrases in the "In-Reply-To:" and
   "References:" fields are ignored.

   Semantically, none of the optional CFWS in the local-part and the
   domain is part of the obs-id-left and obs-id-right, respectively.

4.5.5. Obsolete Informational Fields

   obs-subject     =   "Subject" *WSP ":" unstructured CRLF

   obs-comments    =   "Comments" *WSP ":" unstructured CRLF

   obs-keywords    =   "Keywords" *WSP ":" obs-phrase-list CRLF

4.5.6. Obsolete Resent Fields

   The obsolete syntax adds a "Resent-Reply-To:" field, which consists
   of the field name, the optional comments and folding white space, the
   colon, and a comma separated list of addresses.

   obs-resent-from =   "Resent-From" *WSP ":" mailbox-list CRLF

   obs-resent-send =   "Resent-Sender" *WSP ":" mailbox CRLF

   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] ",") [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.

4.5.7. Obsolete Trace Fields

   The obs-return and obs-received are again given here as template
   definitions, just as return and received are in section 3.  Their
   full syntax is given in [RFC5321].

   obs-return      =   "Return-Path" *WSP ":" path CRLF

   obs-received    =   "Received" *WSP ":" *received-token CRLF

4.5.8. Obsolete optional fields

obs-optional = field-name *WSP ":" unstructured CRLF

5. Security Considerations

   Care needs to be taken when displaying messages on a terminal or
   terminal emulator.  Powerful terminals may act on escape sequences
   and other combinations of US-ASCII control characters with a variety
   of consequences.  They can remap the keyboard or permit other
   modifications to the terminal that could lead to denial of service or
   even damaged data.  They can trigger (sometimes programmable)

   answerback messages that can allow a message to cause commands to be
   issued on the recipient's behalf.  They can also affect the operation
   of terminal attached devices such as printers.  Message viewers may
   wish to strip potentially dangerous terminal escape sequences from
   the message prior to display.  However, other escape sequences appear
   in messages for useful purposes (cf. [ISO.2022.1994], [RFC2045],
   [RFC2046], [RFC2047], [RFC2049], [RFC4288], [RFC4289]) 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], [RFC2049], [RFC4288], and [RFC4289].

   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:"
   may disclose confidential information that 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 header section of a message.  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.

6. IANA Considerations

   This document updates the registrations that appeared in [RFC4021]
   that referred to the definitions in [RFC2822].  IANA has updated the
   Permanent Message Header Field Repository with the following header
   fields, in accordance with the procedures set out in [RFC3864].

   Header field name:  Date
   Applicable protocol:  Mail
   Status:  standard
   Author/Change controller:  IETF
   Specification document(s):  This document (section 3.6.1)

   Header field name:  From
   Applicable protocol:  Mail
   Status:  standard
   Author/Change controller:  IETF
   Specification document(s):  This document (section 3.6.2)

   Header field name:  Sender
   Applicable protocol:  Mail
   Status:  standard
   Author/Change controller:  IETF
   Specification document(s):  This document (section 3.6.2)

   Header field name:  Reply-To
   Applicable protocol:  Mail
   Status:  standard
   Author/Change controller:  IETF
   Specification document(s):  This document (section 3.6.2)

   Header field name:  To
   Applicable protocol:  Mail
   Status:  standard
   Author/Change controller:  IETF
   Specification document(s):  This document (section 3.6.3)

   Header field name:  Cc
   Applicable protocol:  Mail
   Status:  standard
   Author/Change controller:  IETF
   Specification document(s):  This document (section 3.6.3)

   Header field name:  Bcc
   Applicable protocol:  Mail
   Status:  standard
   Author/Change controller:  IETF
   Specification document(s):  This document (section 3.6.3)

   Header field name:  Message-ID
   Applicable protocol:  Mail
   Status:  standard
   Author/Change controller:  IETF
   Specification document(s):  This document (section 3.6.4)

   Header field name:  In-Reply-To
   Applicable protocol:  Mail
   Status:  standard
   Author/Change controller:  IETF
   Specification document(s):  This document (section 3.6.4)

   Header field name:  References
   Applicable protocol:  Mail
   Status:  standard
   Author/Change controller:  IETF
   Specification document(s):  This document (section 3.6.4)

   Header field name:  Subject
   Applicable protocol:  Mail
   Status:  standard
   Author/Change controller:  IETF
   Specification document(s):  This document (section 3.6.5)

   Header field name:  Comments
   Applicable protocol:  Mail
   Status:  standard
   Author/Change controller:  IETF
   Specification document(s):  This document (section 3.6.5)

   Header field name:  Keywords
   Applicable protocol:  Mail
   Status:  standard
   Author/Change controller:  IETF
   Specification document(s):  This document (section 3.6.5)

   Header field name:  Resent-Date
   Applicable protocol:  Mail
   Status:  standard
   Author/Change controller:  IETF
   Specification document(s):  This document (section 3.6.6)

   Header field name:  Resent-From
   Applicable protocol:  Mail
   Status:  standard
   Author/Change controller:  IETF
   Specification document(s):  This document (section 3.6.6)

   Header field name:  Resent-Sender
   Applicable protocol:  Mail
   Status:  standard
   Author/Change controller:  IETF
   Specification document(s):  This document (section 3.6.6)

   Header field name:  Resent-To
   Applicable protocol:  Mail
   Status:  standard
   Author/Change controller:  IETF
   Specification document(s):  This document (section 3.6.6)

   Header field name:  Resent-Cc
   Applicable protocol:  Mail
   Status:  standard
   Author/Change controller:  IETF
   Specification document(s):  This document (section 3.6.6)

   Header field name:  Resent-Bcc
   Applicable protocol:  Mail
   Status:  standard
   Author/Change controller:  IETF
   Specification document(s):  This document (section 3.6.6)

   Header field name:  Resent-Reply-To
   Applicable protocol:  Mail
   Status:  obsolete
   Author/Change controller:  IETF
   Specification document(s):  This document (section 4.5.6)

   Header field name:  Resent-Message-ID
   Applicable protocol:  Mail
   Status:  standard
   Author/Change controller:  IETF
   Specification document(s):  This document (section 3.6.6)

   Header field name:  Return-Path
   Applicable protocol:  Mail
   Status:  standard
   Author/Change controller:  IETF
   Specification document(s):  This document (section 3.6.7)

   Header field name:  Received
   Applicable protocol:  Mail
   Status:  standard
   Author/Change controller:  IETF
   Specification document(s):  This document (section 3.6.7)
   Related information:  [RFC5321]

Appendix A. Example Messages

   This section presents a selection of messages.  These are intended to
   assist in the implementation of this specification, but should not be
   taken as normative; that is to say, although the examples in this
   section were carefully reviewed, if there happens to be a conflict
   between these examples and the syntax described in sections 3 and 4
   of this document, the syntax in those sections is to be taken as
   correct.

   In the text version of this document, messages in this section are
   delimited between lines of "----".  The "----" lines are not part of
   the message itself.

Appendix A.1. Addressing Examples

The following are examples of messages that might be sent between two
individuals.

Appendix A.1.1. A Message from One Person to Another with Simple
Addressing

   This could be called a canonical message.  It has a single author,
   John Doe, a single recipient, Mary Smith, a subject, the date, a
   message identifier, and a textual message in the body.

   ----
   From: John Doe <jdoe@machine.example>
   To: Mary Smith <mary@example.net>
   Subject: Saying Hello
   Date: Fri, 21 Nov 1997 09:55:06 -0600
   Message-ID: <1234@local.machine.example>

   This is a message just to say hello.
   So, "Hello".
   ----

   If John's secretary Michael actually sent the message, even though
   John was the author and replies to this message should go back to
   him, the sender field would be used:

   ----
   From: John Doe <jdoe@machine.example>
   Sender: Michael Jones <mjones@machine.example>
   To: Mary Smith <mary@example.net>
   Subject: Saying Hello
   Date: Fri, 21 Nov 1997 09:55:06 -0600
   Message-ID: <1234@local.machine.example>

   This is a message just to say hello.
   So, "Hello".
   ----

Appendix A.1.2. Different Types of Mailboxes

   This message includes multiple addresses in the destination fields
   and also uses several different forms of addresses.

   ----
   From: "Joe Q. Public" <john.q.public@example.com>
   To: Mary Smith <mary@x.test>, jdoe@example.org, Who? <one@y.test>
   Cc: <boss@nil.test>, "Giant; \"Big\" Box" <sysservices@example.net>
   Date: Tue, 1 Jul 2003 10:52:37 +0200
   Message-ID: <5678.21-Nov-1997@example.com>

   Hi everyone.
   ----

   Note that the display names for Joe Q. Public and Giant; "Big" Box
   needed to be enclosed in double-quotes because the former contains
   the period and the latter contains both semicolon and double-quote
   characters (the double-quote characters appearing as quoted-pair
   constructs).  Conversely, the display name for Who? could appear
   without them because the question mark is legal in an atom.  Notice
   also that jdoe@example.org and boss@nil.test have no display names
   associated with them at all, and jdoe@example.org uses the simpler
   address form without the angle brackets.

Appendix A.1.3. Group Addresses

   ----
   From: Pete <pete@silly.example>
   To: A Group:Ed Jones <c@a.test>,joe@where.test,John <jdoe@one.test>;
   Cc: Undisclosed recipients:;
   Date: Thu, 13 Feb 1969 23:32:54 -0330
   Message-ID: <testabcd.1234@silly.example>

   Testing.
   ----

   In this message, the "To:" field has a single group recipient named
   "A Group", which contains 3 addresses, and a "Cc:" field with an
   empty group recipient named Undisclosed recipients.

Appendix A.2. Reply Messages

   The following is a series of three messages that make up a
   conversation thread between John and Mary.  John first sends a
   message to Mary, Mary then replies to John's message, and then John
   replies to Mary's reply message.

   Note especially the "Message-ID:", "References:", and "In-Reply-To:"
   fields in each message.

   ----
   From: John Doe <jdoe@machine.example>
   To: Mary Smith <mary@example.net>
   Subject: Saying Hello
   Date: Fri, 21 Nov 1997 09:55:06 -0600
   Message-ID: <1234@local.machine.example>

   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 <mary@example.net>
   To: John Doe <jdoe@machine.example>
   Reply-To: "Mary Smith: Personal Account" <smith@home.example>
   Subject: Re: Saying Hello
   Date: Fri, 21 Nov 1997 10:01:10 -0600
   Message-ID: <3456@example.net>
   In-Reply-To: <1234@local.machine.example>
   References: <1234@local.machine.example>

   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" <smith@home.example>
   From: John Doe <jdoe@machine.example>
   Subject: Re: Saying Hello
   Date: Fri, 21 Nov 1997 11:00:00 -0600
   Message-ID: <abcd.1234@local.machine.test>
   In-Reply-To: <3456@example.net>
   References: <1234@local.machine.example> <3456@example.net>

   This is a reply to your reply.
   ----

Appendix A.3. Resent Messages

   Start with the message that has been used as an example several
   times:

   ----
   From: John Doe <jdoe@machine.example>
   To: Mary Smith <mary@example.net>
   Subject: Saying Hello
   Date: Fri, 21 Nov 1997 09:55:06 -0600
   Message-ID: <1234@local.machine.example>

   This is a message just to say hello.
   So, "Hello".
   ----

   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 <mary@example.net>
   Resent-To: Jane Brown <j-brown@other.example>
   Resent-Date: Mon, 24 Nov 1997 14:22:01 -0800
   Resent-Message-ID: <78910@example.net>
   From: John Doe <jdoe@machine.example>
   To: Mary Smith <mary@example.net>
   Subject: Saying Hello
   Date: Fri, 21 Nov 1997 09:55:06 -0600
   Message-ID: <1234@local.machine.example>

   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.  (Note that for brevity, trace fields are not shown.)

Appendix A.4. Messages with Trace Fields

   As messages are sent through the transport system as described in
   [RFC5321], 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 <mary@example.net>;  21 Nov 1997 10:05:43 -0600
   Received: from node.example by x.y.test; 21 Nov 1997 10:01:22 -0600
   From: John Doe <jdoe@node.example>
   To: Mary Smith <mary@example.net>
   Subject: Saying Hello
   Date: Fri, 21 Nov 1997 09:55:06 -0600
   Message-ID: <1234@local.node.example>

   This is a message just to say hello.
   So, "Hello".
   ----

Appendix A.5. White Space, Comments, and Other Oddities

   White space, including folding white space, and comments can be
   inserted between many of the tokens of fields.  Taking the example
   from A.1.3, white space and comments can be inserted into all of the
   fields.

   ----
   From: Pete(A nice \) chap) <pete(his account)@silly.test(his host)>
   To:A Group(Some people)
        :Chris Jones <c@(Chris's host.)public.example>,
            joe@example.org,
     John <jdoe@one.test> (my dear friend); (the end of the group)
   Cc:(Empty list)(start)Hidden recipients  :(nobody(that I know))  ;
   Date: Thu,
         13
           Feb
             1969
         23:32
                  -0330 (Newfoundland Time)
   Message-ID:              <testabcd.1234@silly.test>

   Testing.
   ----

   The above example is aesthetically displeasing, but perfectly legal.
   Note particularly (1) the comments in the "From:" field (including
   one that has a ")" character appearing as part of a quoted-pair); (2)
   the white space absent after the ":" in the "To:" field as well as
   the comment and folding white space after the group name, the special
   character (".") in the comment in Chris Jones's address, and the
   folding white space before and after "joe@example.org,"; (3) the
   multiple and nested comments in the "Cc:" field as well as the
   comment immediately following the ":" after "Cc"; (4) the folding
   white space (but no comments except at the end) and the missing
   seconds in the time of the date field; and (5) the white space before
   (but not within) the identifier in the "Message-ID:" field.

Appendix A.6. Obsoleted Forms

   The following are examples of obsolete (that is, the "MUST NOT
   generate") syntactic elements described in section 4 of this
   document.

Appendix A.6.1. Obsolete Addressing

   Note in the example below the lack of quotes around Joe Q. Public,
   the route that appears in the address for Mary Smith, the two commas
   that appear in the "To:" field, and the spaces that appear around the
   "." in the jdoe address.

   ----
   From: Joe Q. Public <john.q.public@example.com>
   To: Mary Smith <@node.test:mary@example.net>, , jdoe@test  . example
   Date: Tue, 1 Jul 2003 10:52:37 +0200
   Message-ID: <5678.21-Nov-1997@example.com>

   Hi everyone.
   ----

Appendix A.6.2. Obsolete Dates

   The following message uses an obsolete date format, including a non-
   numeric time zone and a two digit year.  Note that although the day-
   of-week is missing, that is not specific to the obsolete syntax; it
   is optional in the current syntax as well.

   ----
   From: John Doe <jdoe@machine.example>
   To: Mary Smith <mary@example.net>
   Subject: Saying Hello
   Date: 21 Nov 97 09:55:06 GMT
   Message-ID: <1234@local.machine.example>

   This is a message just to say hello.
   So, "Hello".
   ----

Appendix A.6.3. Obsolete White Space and Comments

   White space and comments can appear between many more elements than
   in the current syntax.  Also, folding lines that are made up entirely
   of white space are legal.

   ----
   From  : John Doe <jdoe@machine(comment).  example>
   To    : Mary Smith
   __
             <mary@example.net>
   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.

Appendix B. Differences from Earlier Specifications

   This appendix contains a list of changes that have been made in the
   Internet Message Format from earlier specifications, specifically
   [RFC0822], [RFC1123], and [RFC2822].  Items marked with an asterisk
   (*) below are items which appear in section 4 of this document and
   therefore can no longer be generated.

   The following are the changes made from [RFC0822] and [RFC1123] to
   [RFC2822] that remain in this document:

   1.   Period allowed in obsolete form of phrase.
   2.   ABNF moved out of document, now in [RFC5234].
   3.   Four or more digits allowed for year.
   4.   Header field ordering (and lack thereof) made explicit.
   5.   Encrypted header field removed.
   6.   Specifically allow and give meaning to "-0000" time zone.
   7.   Folding white space is not allowed between every token.
   8.   Requirement for destinations removed.
   9.   Forwarding and resending redefined.
   10.  Extension header fields no longer specifically called out.
   11.  ASCII 0 (null) removed.*
   12.  Folding continuation lines cannot contain only white space.*
   13.  Free insertion of comments not allowed in date.*
   14.  Non-numeric time zones not allowed.*
   15.  Two digit years not allowed.*
   16.  Three digit years interpreted, but not allowed for generation.*
   17.  Routes in addresses not allowed.*
   18.  CFWS within local-parts and domains not allowed.*
   19.  Empty members of address lists not allowed.*
   20.  Folding white space between field name and colon not allowed.*
   21.  Comments between field name and colon not allowed.
   22.  Tightened syntax of in-reply-to and references.*
   23.  CFWS within msg-id not allowed.*
   24.  Tightened semantics of resent fields as informational only.
   25.  Resent-Reply-To not allowed.*
   26.  No multiple occurrences of fields (except resent and received).*
   27.  Free CR and LF not allowed.*
   28.  Line length limits specified.
   29.  Bcc more clearly specified.

   The following are changes from [RFC2822].
   1.   Assorted typographical/grammatical errors fixed and
        clarifications made.
   2.   Changed "standard" to "document" or "specification" throughout.
   3.   Made distinction between "header field" and "header section".
   4.   Removed NO-WS-CTL from ctext, qtext, dtext, and unstructured.*
   5.   Moved discussion of specials to the "Atom" section.  Moved text
        to "Overall message syntax" section.
   6.   Simplified CFWS syntax.
   7.   Fixed unstructured syntax.
   8.   Changed date and time syntax to deal with white space in
        obsolete date syntax.
   9.   Removed quoted-pair from domain literals and message
        identifiers.*
   10.  Clarified that other specifications limit domain syntax.
   11.  Simplified "Bcc:" and "Resent-Bcc:" syntax.
   12.  Allowed optional-field to appear within trace information.
   13.  Removed no-fold-quote from msg-id.  Clarified syntax
        limitations.
   14.  Generalized "Received:" syntax to fix bugs and move definition
        out of this document.
   15.  Simplified obs-qp.  Fixed and simplified obs-utext (which now
        only appears in the obsolete syntax).  Removed obs-text and obs-
        char, adding obs-body.
   16.  Fixed obsolete date syntax to allow for more (or less) comments
        and white space.
   17.  Fixed all obsolete list syntax (obs-domain-list, obs-mbox-list,
        obs-addr-list, obs-phrase-list, and the newly added obs-group-
        list).
   18.  Fixed obs-reply-to syntax.
   19.  Fixed obs-bcc and obs-resent-bcc to allow empty lists.
   20.  Removed obs-path.

Appendix C. Acknowledgements

   Many people contributed to this document.  They included folks who
   participated in the Detailed Revision and Update of Messaging
   Standards (DRUMS) Working Group of the Internet Engineering Task
   Force (IETF), the chair of DRUMS, the Area Directors of the IETF, and
   people who simply sent their comments in via email.  The editor is
   deeply indebted to them all and thanks them sincerely.  The below
   list includes everyone who sent email concerning both this document
   and [RFC2822].  Hopefully, everyone who contributed is named here:

   +--------------------+----------------------+---------------------+
   | Matti Aarnio       | Tanaka Akira         | Russ Allbery        |
   | Eric Allman        | Harald Alvestrand    | Ran Atkinson        |
   | Jos Backus         | Bruce Balden         | Dave Barr           |

7. References

7.1. Normative References

   [ANSI.X3-4.1986]  American National Standards Institute, "Coded
                     Character Set - 7-bit American Standard Code for
                     Information Interchange", ANSI X3.4, 1986.

   [RFC1034]         Mockapetris, P., "Domain names - concepts and
                     facilities", STD 13, RFC 1034, November 1987.

   [RFC1035]         Mockapetris, P., "Domain names - implementation and
                     specification", STD 13, RFC 1035, November 1987.

   [RFC1123]         Braden, R., "Requirements for Internet Hosts -
                     Application and Support", STD 3, RFC 1123,
                     October 1989.

   [RFC2119]         Bradner, S., "Key words for use in RFCs to Indicate
                     Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5234]         Crocker, D. and P. Overell, "Augmented BNF for
                     Syntax Specifications: ABNF", STD 68, RFC 5234,
                     January 2008.

7.2. Informative References

   [RFC0822]         Crocker, D., "Standard for the format of ARPA
                     Internet text messages", STD 11, RFC 822,
                     August 1982.

   [RFC1305]         Mills, D., "Network Time Protocol (Version 3)
                     Specification, Implementation", RFC 1305,
                     March 1992.

   [ISO.2022.1994]   International Organization for Standardization,
                     "Information technology - Character code structure
                     and extension techniques", ISO Standard 2022, 1994.

   [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., "MIME (Multipurpose Internet Mail
                     Extensions) Part Three: Message Header Extensions
                     for Non-ASCII Text", RFC 2047, November 1996.

   [RFC2049]         Freed, N. and N. Borenstein, "Multipurpose Internet
                     Mail Extensions (MIME) Part Five: Conformance
                     Criteria and Examples", RFC 2049, November 1996.

   [RFC2822]         Resnick, P., "Internet Message Format", RFC 2822,
                     April 2001.

   [RFC3864]         Klyne, G., Nottingham, M., and J. Mogul,
                     "Registration Procedures for Message Header
                     Fields", BCP 90, RFC 3864, September 2004.

   [RFC4021]         Klyne, G. and J. Palme, "Registration of Mail and
                     MIME Header Fields", RFC 4021, March 2005.

   [RFC4288]         Freed, N. and J. Klensin, "Media Type
                     Specifications and Registration Procedures",
                     BCP 13, RFC 4288, December 2005.

   [RFC4289]         Freed, N. and J. Klensin, "Multipurpose Internet
                     Mail Extensions (MIME) Part Four: Registration
                     Procedures", BCP 13, RFC 4289, December 2005.

   [RFC5321]         Klensin, J., "Simple Mail Transfer Protocol",
                     RFC 5321, October 2008.

Author's Address

   Peter W. Resnick (editor)
   Qualcomm Incorporated
   5775 Morehouse Drive
   San Diego, CA  92121-1714
   US

   Phone: +1 858 651 4478
   EMail: presnick@qualcomm.com
   URI:   http://www.qualcomm.com/~presnick/

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