Mail Storage Guard 

Mail Storage Guard is an extension for Microsoft Exchange Server 2000/2003 and IIS SMTP servers, designed to protect your server from spam, prevents mail relaying, mail-bomb and NDR attacks. Works at SMTP and at storage levels.
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PlexMailer 

PlexMailer is an anti-spam software. It supports POP3, IMAP, LDAP, NNTP and MBOX protocols. It comes with dozens of filters such as Bayesian and realtime blacklists. Also, it supports server side options and integration with other products.
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