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

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

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.
In addition, there is "spamd", a persistently running daemonized
version of the command line tool "spamassassin". Together with the small
client "spamc" this allows processing of large volumes of mail without the
cost of starting a perl interpreter for each one.

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