An excellent article is available over at Howto Forge on “A multi-stage approach to securing your email communication”

When speaking of mail server-related security, one tends to limit the issue to message applied security measures, and even more to Antivirus and Antispam protection. This is however only one stage in the more complex process of securing your server. This article aims at identifying and explaining all security layers, highly important when choosing a certain mail server and consequently when configuring and using it.