SpamAssassin is a mature, widely-deployed open source project that serves as a mail filter to identify Spam. SpamAssassin uses a variety of mechanisms including header and text analysis, Bayesian filtering, DNS blocklists, and collaborative filtering databases. SpamAssassin runs on a server, and filters spam before it reaches your mailbox.

SpamAssassin's practical multi-technique approach, modularity, and extensibility continue to give it an advantage over other anti-spam systems. Due to these advantages, SpamAssassin is widely used in all aspects of email management. You can readily find SpamAssassin in use in both email clients and servers, on many different operating systems, filtering incoming as well as outgoing email, and implementing a very broad range of policy actions. These installations include service providers, businesses, not-for-profit and educational organizations, and end-user systems. SpamAssassin also forms the basis for numerous commercial anti-spam products available on the market today.

Features:

Wide-spectrum
SpamAssassin uses a wide variety of local and network tests to identify spam signatures. This makes it harder for spammers to identify one aspect which they can craft their messages to work around.

Easy to extend
Anti-spam tests and configuration are stored in plain text, making it easy to configure and add new rules.

Flexible
SpamAssassin encapsulates its logic in a well-designed, abstract API so it can be integrated anywhere in the email stream. The Mail::SpamAssassin classes can be used on a wide variety of email systems includingprocmail, sendmail, Postfix, qmail, and many others.

Easy Configuration
SpamAssassin requires very little configuration; you do not need to continually update it with details of your mail accounts, mailing list memberships, etc. Once classified, site and user-specific policies can then be applied against spam. Policies can be applied on both mail servers and later using the user's own mail user-agent application.

Requirements:
OS X 10.2-10.9
Perl
Universal Binary