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RFC 5322


Internet Message Format

Part 2 of 3, p. 10 to 38
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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

   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

   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.

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   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 "\"

   ccontent        =   ctext / quoted-pair / comment

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

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

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   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

   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

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   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
                       "[" / "]" /
                       ":" / ";" /
                       "@" / "\" /
                       "," / "." /

   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)

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

   qcontent        =   qtext / quoted-pair

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

   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

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.

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   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

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   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] /

   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

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   ("<" 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

   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.

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   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 /

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   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

   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.

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   fields          =   *(trace
                         *optional-field /
                         *(resent-date /
                          resent-from /
                          resent-sender /
                          resent-to /
                          resent-cc /
                          resent-bcc /
                       *(orig-date /
                       from /
                       sender /
                       reply-to /
                       to /
                       cc /
                       bcc /
                       message-id /
                       in-reply-to /
                       references /
                       subject /
                       comments /
                       keywords /

   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.

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   | 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

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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

   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

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

      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

   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

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.

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   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.

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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

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      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

   When creating a reply to a message, the "In-Reply-To:" and
   "References:" fields of the resultant message are constructed as

   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.

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      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

   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

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

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   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

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   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

   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.

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   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]

   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.

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   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

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

      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

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   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

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)

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   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

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                       %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

   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.

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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.

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   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 /

   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

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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

Top      Up      ToC       Page 38 
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

(page 38 continued on part 3)

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