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: <38D1C1FD-3C35-4568-925C-FC46CAC0DE8A@sendinghost.com>
Content-Type: text/plain;
	charset=US-ASCII;
	format=flowed
To: Other User <otheruser@receivinghost.com>
From: User Name <username@sendinghost.com>
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.