Clamassassin

clamassassin is a shell script for filtering an email message through ClamAV, using either the command-line scanner clamscan or the client clamdscan. It adds message headers according to the disposition of the mail in a manner similar to Spamassassin, making it convenient to call through Procmail.

Sample procmailrc file
An /etc/procmailrc file passing mail through clamassassin followed by spamc.  