Clam AntiVirus software is often used for scanning by MTAs.
Here is how to list all the signatures in the clam database.
ClamAV is a fine virus scanner which also detects phishing attempts. When complemented with BitDefender v7 (can be used for free on Linux servers) it delivers rock solid virus scanning. BitDefender will detect older viruses, whereas ClamAV will detect the newest.
ClamAV can be updated by using freshclam. It is possible to create a service freshclamd and have that run 24 times a day to fetch updates around the clock.
ClamAV can be called automatically by amavisd-new as an SMTP filter for email delivery.