SpamAssassin is a mail filter to identify spam.

Using its rule base and optional network sources (DNS-based lists and
collaborative spam-tracking databases), it carries out a wide range of
heuristic tests on mail headers and body text to identify "spam", also
known as unsolicited commercial email.

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.

Read ${PREFIX}/share/doc/SpamAssassin/README
for further installation instructions.

Or if you are in a hurry you can read:
${PREFIX}/share/doc/SpamAssassin/OpenBSD-SpamAssassin-mini-howto.html

The SpamAssassin Wiki is available at <http://wiki.apache.org/spamassassin>
and contains up-to-date information on configuring SpamAssassin, and using
some of the more advanced features of it.