Vote for JunkJam
Designed to work in front of your email server, JunkJam(tm) allows both users and administrators the ability to create “allowed” and “blocked” lists for a specific email address and entire domains. JunkJam does not force all users into a single blocking list nor does it require the maintenance of text matching software. JunkJam blocks spam before it reaches your email system as an SMTP mail relay running inside your firewall.