A short presentation of MailScanner and ClamAV, as free email scanners alternatives for Linux, from ServerWatch:

I've never been comfortable with Windows-based e-mail scanners because they run on the same porous, malware friendly platform they're supposed to protect. Paying big bucks for software licensing fees year after year isn't a very attractive proposition either.

Fortunately, there are powerful, free alternatives, like MailScanner and ClamAV. Using these two together delivers stout protection again viruses, spam, phishes, and all manner of e-mail-borne malware. Both run on just about any Linux or Unix-type operating system, so you have the benefit of choosing your favorite operating system as well as the benefits of superior security, efficiency and performance.

ClamAV just keeps getting better. It installs with a nice set of default options, and thus requires minimal tweaking. It defaults to checking for new virus signatures several times per day, and it can scan outgoing mail. You should definitely scan outgoing e-mail — if this were a routine practice, 90 percent of e-mail would not be spam or malware.....