Mail in Mac OS X 10.3.7, when generating a Message-ID header, generates a GUUID that includes information that identifies the Ethernet hardware being used, which allows remote attackers to link mail messages to a particular machine.
https://exchange.xforce.ibmcloud.com/vulnerabilities/19085
http://www.kb.cert.org/vuls/id/464662
http://securitytracker.com/id?1013001
http://secunia.com/advisories/14005
http://lists.apple.com/archives/security-announce/2005/Jan/msg00001.html