MailScanner is a virus & spam email scanner, that (in its' most common configuration) sits on your MTA and checks all mail that runs through it.
MailScanner's website can be found here. Look at MailScannerNotes for other info.
Debian Woody Backport of Mailscanner: http://www.wlug.org.nz/archive/debian/. (That will eventually become an Apt? repository...)
7 pages link to MailScanner: