qmail-scanner can work on small mail servers but I would not recommend running it on a busy mail server.
Some mail admins will recommend mail front - http://untroubled.org/mailfront/mailfront.html
My preference is to use simscan for calling ClamAV and SpamAssassin - simscan - qmail virus and spam scanner | Download simscan - qmail virus and spam scanner software for free at SourceForge.net