[Mail Headers, MIME, HTML, and ASCII Text][1]

Message-ID: (also Message-Id: or Message-id:) This

field is like a serial number and 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 particular message; subsequent

revisions to the message should each receive new message

identifiers. sendmail constructs the field from the

data, the time, a unique file name, and the originating

machine name.

[1]: http://www.expita.com/header1.html