ClamAV is a comprehensive and flexible anti-virus package. It has a database of virus patterns that it uses to check each message. The database is updated frequently, typically within a few hours of a new virus showing up. You can set the system up to install new versions of the database automatically.

ClamAV can be deployed in a number of different ways. I run it as a sendmail milter. Most of the time it "just works", and is very effective.