SpamAssassin is a mail filter to identify spam.

Using its rule base, it uses a wide range of heuristic tests on mail
headers and body text to identify "spam", also known as unsolicited
commercial email.

The spam-identification tactics used include header analysis, text
analysis, blacklists and Razor.

Once identified, the mail can then be optionally tagged as spam for
later filtering using the user's own mail user-agent application.

SpamAssassin requires very little configuration; you do not need to
continually update it with details of your mail accounts, mailing list
memberships, etc. It accomplishes filtering without this knowledge, as
much as possible.

The distribution provides a command line tool to perform filtering,
along with Mail::SpamAssassin, a set of perl modules which implement a
Mail::Audit plugin, allowing SpamAssassin to be used in a Mail::Audit
filter, or (possibly at some point) in a spam-protection proxy POP/IMAP
server.