The Message: header is used to mark an early end to a mail message's headers. When sendmail finds this header, it immediately stops gathering the message's header lines and treats the rest of the header as the start of the message body. This header is useful for including non-Internet headers in the header portion of a mail message. For example:

To: george@wash.dc.gov (George Washington)
Subject: Re: More text
Date: Sun, 6 May 2001 17:32:45 EDT
Message-Id: <200105061723.f46NIY7f028392@wash.dc.gov>
Received: by wash.dc.gov (4.1/1.12 $)
        id AA01513; Sun, 6 May 2001 17:32:45 EDT
From: Ben Franklin <ben@philly.dc.gov>
Message:
ROUTED BY BITNET/CO=US/ROUTE=INTERNET/
FORMAT OF MESSAGE /LANG=USENGLISH/FORM=PLAINTEXT/

Here, the last two header lines are non-Internet headers that might confuse some programs. But the Message: header that precedes them tells sendmail to treat them as message body, and problems are avoided.

Note that Message: is not defined by any RFC but is a convention that is shared by all versions of sendmail and a few other MTAs. It is included in sendmail for backward compatibility with a few old messaging systems, so should be considered deprecated. The Message: header should never be declared in the configuration file, and should probably never be used.