nb1 writes:  

Apache SpamAssassin 3.3.0 is a mature, wide-spectrum, extensible email filtering package deployed by hundreds of thousands of organizations worldwide.

Apache SpamAssassin 3.3.0 represents a major shift in how SpamAssassin rules (the actual patterns that help to identify spam) are updated. Rules are now separate from the core product and are instead downloaded using "sa-update," SpamAssassin's automatic update software. This method was optional with the 3.2.x series of releases and has proven to be very popular.

SpamAssassin provides a comprehensive set of features and support for methods and standards such as text based patterns, bayesian scoring, DNS based black and white lists, DKIM and SPF sender authentication, and email signature clearing houses. The software utilizes a principle of identifying multiple reasons for classifying an email as spam to improve accuracy and decrease the chance of legitimate emails being incorrectly identified as spam

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