Purgy 

Anti-spam software for Windows. It accepts emails from senders you trust while asking unknown senders to verify themselves by replying to an email. After positive verification it adds them to your approved sender list and delivers all their messages.
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BusinessMail Email Server + Anti Spam System 

Fully multi threaded email server and anti-spam system with POP/SMTP plus web interface, offering powerful fully asynchronous operations, fully user configurable anti-spam system, user configurable filtering, aliasing, list handling plus lots more.
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FTGate 

FTGate4 is a powerful Windows mail server with comprehensive Groupware functionality. Features include calendaring, contacts, IMAP, anti-spam , anti-virus support, multiple domains, Web Administration, WebMail, instant messaging, SPF,SSL + more.
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NUCEM Spam Terminator 

NUCEM Spam Terminator compared to the best competitors anti-spam solutions is 3 times more accurate at differentiating between valid e-mail and SPAM. The best competitors anti-spam solution barely achieves a TCR of 12 while NUCEM has a TCR of 37.
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Extensible Messaging Platform Anti-Spam Filter 

Server based Anti-Spam Filter protects organizational mail servers (Exchange Server, Novell GroupWise, Lotus Domino, Sendmail, etc) from spam, email-borne viruses and objectionable content using contextual pattern signatures (not simple keywords).
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Spam Alarm for POP3 

Spam Alarm 2.0 is an anti-spam and junk e-mail filtering tool that is designed to prevent unwanted mail from your POP 3 accounts. Spam Alarm 2.0 works directly with POP3 servers.
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