MailScanner is a generic scanner for use by MTAs, intended to drive a spam filter and a virus scanner. It is written in Perl and can be set up with at least SendMail and Exim.
It usually comes packaged for use with the SpamAssassin spam filter. Plug VipulsRazor into SpamAssassin and ClamAV into MailScanner, and you have a complete Free anti virus and spam proxy solution - always a good thing to equip an enterprise mail server with.
If you want to pay someone to do all this for you, there's a product called OpenProtect, which integrates MailScanner, SpamAssassin and ClamAV into an easily installable bundle.
If you're using Exim, you can do most of this directly, and don't really need the overhead of MailScanner. See EximMailFilter.