MDaemon 

MDaemon , Windows-based email server software, contains full mail server functionality and control with a strong emphasis on security to protect your email communication needs. Security features include Greylisting, DKIM, DomainKeys (DK), and SPF.
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VisNetic MailServer 

VisNetic Mail Server is a high performance mail server with an extensive, highly configurable feature set. This secure mail server is also ideal for use as a gateway mail server, features built-in anti spam, Web mail and an instant messaging server.
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0Spam.com Express 

The most powerful, next generation anti spam software, 0Spam.com Express is compatible with all versions of Windows and most E-mail clients, safely removes spam from your existing POP3, Yahoo, Hotmail, GMail, or IMAP accounts, and many cool features.
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