SpamAssassin is a mail filter to identify spam. It will examine
each message presented to it, and assign a score indicating the
likelihood that the mail is spam.

It applies a wide range of heuristic tests on mail headers and body text
to identify "spam", also known as unsolicited commercial email. The mail
can then be optionally tagged as spam for later filtering.

The spam-identification tactics used include header analysis, text analysis,
a Bayesian-style form of probability-analysis classification and DNS
blacklists. It also includes plugins to support reporting spam messages to
collaborative filtering databases such as Pyzor, DCC, and Vipul's Razor.

The distribution provides a command line tool to perform filtering, along
with a set of perl modules which allows SpamAssassin to be used in a
variety of different spam-blocking scenarios.

In addition, "spamd", a daemonized version of SpamAssassin which runs
persistently, is available. Using its counterpart, "spamc", a lightweight
client written in C, an MTA can process large volumes of mail through
SpamAssassin without having to fork/exec a perl interpreter for each message.

SpamAssassin does not deliver mail to the users mailbox. You need a
different program (procmail is recommended) for local mail delivery.