Having secured authentication, encrypted message transfer, and protected your server from being used by spammers, you have just one other major security task to complete before turning on your mail server: doing your users a favor by filtering out as much junk mail as possible, including viruses that may have been sent by email. Of course, your users should also use spam filtering software that runs on their own computers, but with the proper settings, you can eliminate some of the most egregious junk mail before it's ever delivered —or at least mark it to help users identify it as likely spam.
For more on client-side spam filtering, see Chapter 9.
Mac OS X employs three main mechanisms for filtering junk mail. First, there's a Bayesian junk mail filter — one that, like the filter built into Mail, can learn about the characteristics of good and bad messages as you use it, becoming more accurate over time. You can adjust the sensitivity of this filter and configure several other attributes. Second, there's a separate virus filter, which matches the contents of attachments against the signatures of known viruses. Both of these filters are powered by open-source software included with Mac OS X Server (SpamAssassin for spam filtering and ClamAV for virus filtering).
The third filtering method is a bit different: blocking incoming SMTP traffic based on information supplied by Real-time Blacklist (RBL) servers, also known as black-hole ...