Clam AntiVirus (also known as ClamAV) is a multi-platform and open source anti-virus software that runs on top of the UNIX/Linux command-line interface and can be easily integrated into any mail server for virus scanning of attachments.
Key features include a command-line scanner, fast and multi-threaded daemon with support for on-access scanning, Milter interface for sendmail, built-in support for a plethora of mail file formats, as well as advanced database updater with support for digital signatures and scripted updates.
A virus scanner C library, on-access scanning (Linux and FreeBSD), built-in support for almost all mail file formats, built-in support for a wide range of archive formats, including zip, tar, rar, gzip, bzip2, cabinet, ole2, chm, sis, binhex, etc., are also supported by Clam AntiVirus.
In addition, the program comes with built-in support for Portable and ELF executable files that are compressed with UPX, Petite, FSG, NsPack, MEW, wwpack32, and Upack, as well as obfuscated with Y0da Cryptor, SUE and other compression algorithms.
Another interesting feature is the ability to update its virus database multiple times per day. The total number of virus signatures can be found on the project's homepage (see the link above). Furthermore, it features built-in support for well known document formats, including HTML, PDF, RTF, MS Office, and other.
A plethora of websites, non-profit organization and respected businesses are using this free anti-virus software to protect their mail gateways for malware and other threats that surround us each day, among which we can mention Register.it, Netlynx Technologies, FuturaHost, DynDNS, XS4ALL, etc.
Officially supported operating systems include all GNU/Linux distributions and BSD flavors, several Solaris OSes, Microsoft Windows, AIX, HP-UX, OSF, OpenVMS, and others. It is available for download as a universal source archive, but it can be easily installed in your Linux distro from its official software channels.