Apple mail header format

Apple Mail version 2.1 generates email headers in the following format:

Mime-Version: 1.0 (Apple Message framework v752.2)
Content-Transfer-Encoding: 7bit
Message-Id: <>
Content-Type: text/plain;
To: Other User <>
From: User Name <>
Subject: Subject Line
Date: Mon, 26 Feb 2007 20:21:57 -0500

The message-id appears to be a random, or version 4, Universally Unique Identifier. By default Mac OS X includes a command line program uuidgen that can be used to generate such values.