This field contains a unique identifier (the local-part address unit) which refers to THIS version of THIS message. The uniqueness of the message identifier is guaranteed by the host which generates it. This 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 should each receive new message identifiers.
The contents of this field identify previous correspondence which this message answers. Note that if message identifiers are used in this field, they must use the msg-id specification format.
The contents of this field identify other correspondence which this message references. Note that if message identifiers are used, they must use the msg-id specification format.
This field contains keywords or phrases, separated by commas.