ClamAV

Clam AntiVirus is a free and open source anti-virus toolkit especially designed for e-mail scanning on mail gateways. It provides a number of utilities including a flexible and scalable multi-threaded daemon, a command line scanner and advanced tool for automatic database updates. The core of the package is an anti-virus engine available in a form of shared library. Both ClamAV and its updates are made available free of charge.