Mail Headers, MIME, HTML, and ASCII Text

Mail Headers, MIME, HTML, and ASCII Text
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.