computer program released under the Apache License 2.0 used for e-mail spam filtering based on content-matching rules, which also supports DNS-based, checksum-based and statistical filtering, supported by external programs and online databases.
SpamAssassin is generally regarded as one of the most effective spam filters, especially when used in combination with spam databases. While simple text-matching alone may, for most users, be sufficient to correctly classify a majority of incoming mail the complexity involved in the combination of the comparison of words and symbols used in conjunction with the sources of spam may far exceed the average user's capability. For instance, graphic-only spam messages have no text to compare to therefore checking the sender's originating mailserver and included links against various databases of known e-mail abusers enables the prevention of unnecessary or non-personal mail getting through to the end user.
