Installing the spamass-milter-postfix package changes the default behaviour
of spamass-milter to be more Postfix-friendly, i.e.:

   The Unix-domain socket used for MTA communication is changed to
   /var/run/spamass-milter/postfix/sock, and that socket is
   writable by the postfix group.

To configure Postfix to use the milter, add to /etc/postfix/main.cf:

  smtpd_milters = unix:/var/run/spamass-milter/postfix/sock

For further information, see:
http://www.postfix.org/MILTER_README.html