I would have to say X-wall. I have tried the rest but for the price of $349 you can't beat it. It does all the filtering and is constantly being updated. Blacklists, Whitelist, Bayes, Heuristic, Subject, Body, HTML, headers, Attachments, Invalid emails like iamspam@yourdomain.com, individual whitelisting, use of your antivirus software or F-prot for scanning incoming emails, blah blah blah.......
Dev