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.