4.1. SYNTAX

Note: Due to an artifact of the notational conventions, the syntax indicates that, when present, some fields, must be in a particular order. Header fields are NOT required to occur in any particular order, except that the message body must occur AFTER the headers. It is recommended that, if present, headers be sent in the order "Return-Path", "Received", "Date", "From", "Subject", "Sender", "To", "cc", etc.

This specification permits multiple occurrences of most fields. Except as noted, their interpretation is not specified here, and their use is discouraged.

The following syntax for the bodies of various fields should be thought of as describing each field body as a single long string (or line). The "Lexical Analysis of Message" section on "Long Header Fields", above, indicates how such long strings can be represented on more than one line in the actual transmitted message.

     message     =  fields *( CRLF *text )       ; Everything after
                                                 ;  first null line
                                                 ;  is message body

     fields      =    dates                      ; Creation time,
                      source                     ;  author id & one
                    1*destination                ;  address required
                     *optional-field             ;  others optional

     source      = [  trace ]                    ; net traversals
                      originator                 ; original mail
                   [  resent ]                   ; forwarded

     trace       =    return                     ; path to sender
                    1*received                   ; receipt tags

     return      =  "Return-path" ":" route-addr ; return address

     received    =  "Received"    ":"            ; one per relay
                       ["from" domain]           ; sending host
                       ["by"   domain]           ; receiving host
                       ["via"  atom]             ; physical path
                      *("with" atom)             ; link/mail protocol
                       ["id"   msg-id]           ; receiver msg id
                       ["for"  addr-spec]        ; initial form
                        ";"    date-time         ; time received

     originator  =   authentic                   ; authenticated addr
                   [ "Reply-To"   ":" 1#address] )

     authentic   =   "From"       ":"   mailbox  ; Single author
                 / ( "Sender"     ":"   mailbox  ; Actual submittor
                     "From"       ":" 1#mailbox) ; Multiple authors
                                                 ;  or not sender

     resent      =   resent-authentic
                   [ "Resent-Reply-To"  ":" 1#address] )

     resent-authentic =
                 =   "Resent-From"      ":"   mailbox
                 / ( "Resent-Sender"    ":"   mailbox
                     "Resent-From"      ":" 1#mailbox  )

     dates       =   orig-date                   ; Original
                   [ resent-date ]               ; Forwarded

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

     resent-date =  "Resent-Date" ":"   date-time

     destination =  "To"          ":" 1#address  ; Primary
                 /  "Resent-To"   ":" 1#address
                 /  "cc"          ":" 1#address  ; Secondary
                 /  "Resent-cc"   ":" 1#address
                 /  "bcc"         ":"  #address  ; Blind carbon
                 /  "Resent-bcc"  ":"  #address

     optional-field =
                 /  "Message-ID"        ":"   msg-id
                 /  "Resent-Message-ID" ":"   msg-id
                 /  "In-Reply-To"       ":"  *(phrase / msg-id)
                 /  "References"        ":"  *(phrase / msg-id)
                 /  "Keywords"          ":"  #phrase
                 /  "Subject"           ":"  *text
                 /  "Comments"          ":"  *text
                 /  "Encrypted"         ":" 1#2word
                 /  extension-field              ; To be defined
                 /  user-defined-field           ; May be pre-empted

     msg-id      =  "<" addr-spec ">"            ; Unique message id

     extension-field =
                   <Any field which is defined in a document
                    published as a formal extension to this
                    specification; none will have names beginning
                    with the string "X-">

     user-defined-field =
                   <Any field which has not been defined
                    in this specification or published as an
                    extension to this specification; names for
                    such fields must be unique and may be
                    pre-empted by published extensions>