What are some alternatives?
MailScanner - MailScanner is a open source email security system design for Linux-based email gateways.
Mailwasher - Over 8 million people use MailWasher as their spam filter
POPFile - POPFile classifies email into categories you define.
Scrollout F1 - Scrollout is a free and easy to use anti-spam email gateway designed for Linux & Windows administrators without advanced email security experience.
Spamfence.net - Professional e-mail security free for private use: With Spamfence.
SPAMfighter - SPAMfighter and Microsoft have built the strongest and most effective anti spam filter for Outlook, Windows Live Mail, Outlook Express & Thunderbird