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>