MailStripper Pro is a mail scanner that aims to remove spam and viruses from incoming mail. It recognises and decodes many obfuscation techniques used by spammers and uses several different mechanisms to recognise spam which are immune to Bayes poisoning, and has achieved real-world success of over 99.8%. It is MTA-independent and does not implant itself into your mail server. Instead, it functions as a proxy, allowing the MTA to reside on a different server.
spfmilter is a milter (sendmail mail filter) which implements the Sender Policy Framework (SPF). SPF is a strategy for preventing junk mail. The SMTP standard for email allows anyone to forge anyone else's email address. SPF verifies that the Sender address of an email message matches (according to some policy) the client IP address that submitted it.
spmfilter is a mail filtering framework. It attempts to be a general filtering framework for any purpose. The API enables spmfilter plugins to access messages as they are being processed by the MTA. This allows them to examine and modify message content and meta-information during the SMTP transaction. Plugins are loaded at runtime and can be processed in any sequence. The processing chain can also be altered by a single plugin, for example if the plugin has to stop further processing.