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.
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