rfc2822 April 2001

3.6.4. Identification fields

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

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

   The message identifier (msg-id) is similar in syntax to an angle-addr
   construct without the internal CFWS.

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 / no-fold-quote / obs-id-left

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

no-fold-quote   =       DQUOTE *(qtext / quoted-pair) DQUOTE
no-fold-literal =       "[" *(dtext / quoted-pair) "]"

   The "Message-ID:" field provides a unique message identifier that
   refers to a particular version of a particular message.  The
   uniqueness of the message identifier is guaranteed by the host that
   generates it (see below).  This message identifier is intended to be
   machine readable and not necessarily meaningful to humans.  A message
   identifier pertains to exactly one instantiation of a particular
   message; subsequent revisions to the message each receive new message
   identifiers.

   Note: There are many instances when messages are "changed", but those
   changes do not constitute a new instantiation of that message, and
   therefore the message would not get a new message identifier.  For
   example, when messages are introduced into the transport system, they
   are often prepended with additional header fields such as trace
   fields (described in section 3.6.7) and resent fields (described in
   section 3.6.6).  The addition of such header fields does not change
   the identity of the message and therefore the original "Message-ID:"
   field is retained.  In all cases, it is the meaning that the sender
   of the message wishes to convey (i.e., whether this is the same
   message or a different message) that determines whether or not the
   "Message-ID:" field changes, not any particular syntactic difference
   that appears (or does not appear) in the message.

   The "In-Reply-To:" and "References:" fields are used when creating a
   reply to a message.  They hold the message identifier of the original
   message and the message identifiers of other messages (for example,
   in the case of a reply to a message which was itself a reply).  The
   "In-Reply-To:" field may be used to identify the message (or
   messages) to which the new message is a reply, while the
   "References:" field may be used to identify a "thread" of
   conversation.

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

   The "In-Reply-To:" field will contain the contents of the "Message-
   ID:" field of the message to which this one is a reply (the "parent
   message").  If there is more than one parent message, then the "In-
   Reply-To:" field will contain the contents of all of the parents'
   "Message-ID:" fields.  If there is no "Message-ID:" field in any of
   the parent messages, then the new message will have no "In-Reply-To:"
   field.
   The "References:" field will contain the contents of the parent's
   "References:" field (if any) followed by the contents of the parent's
   "Message-ID:" field (if any).  If the parent message does not contain
   a "References:" field but does have an "In-Reply-To:" field
   containing a single message identifier, then the "References:" field
   will contain the contents of the parent's "In-Reply-To:" field
   followed by the contents of the parent's "Message-ID:" field (if
   any).  If the parent has none of the "References:", "In-Reply-To:",
   or "Message-ID:" fields, then the new message will have no
   "References:" field.

   Note: Some implementations parse the "References:" field to display
   the "thread of the discussion".  These implementations assume that
   each new message is a reply to a single parent and hence that they
   can walk backwards through the "References:" field to find the parent
   of each message listed there.  Therefore, trying to form a
   "References:" field for a reply that has multiple parents is
   discouraged and how to do so is not defined in this document.

   The message identifier (msg-id) itself MUST be a globally unique
   identifier for a message.  The generator of the message identifier
   MUST guarantee that the msg-id is unique.  There are several
   algorithms that can be used to accomplish this.  Since the msg-id has
   a similar syntax to angle-addr (identical except that comments and
   folding white space are not allowed), a good method is to put the
   domain name (or a domain literal IP address) of the host on which the
   message identifier was created on the right hand side of the "@", and
   put a combination of the current absolute date and time along with
   some other currently unique (perhaps sequential) identifier available
   on the system (for example, a process id number) on the left hand
   side.  Using a date on the left hand side and a domain name or domain
   literal on the right hand side makes it possible to guarantee
   uniqueness since no two hosts use the same domain name or IP address
   at the same time.  Though other algorithms will work, it is
   RECOMMENDED that the right hand side contain some domain identifier
   (either of the host itself or otherwise) such that the generator of
   the message identifier can guarantee the uniqueness of the left hand
   side within the scope of that domain.

   Semantically, the angle bracket characters are not part of the
   msg-id; the msg-id is what is contained between the two angle bracket
   characters.