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

In addition:

  milter_connect_macros must include the j and _ macros
  milter_rcpt_macros    must include the b, r, v, and Z macros

Most of these are present by default and shouldn't need explicitly setting,
except for the _ macro in milter_connect_macros. You can use:

postconf -d milter_connect_macros

to find the current setting, and add _ to the returned value, e.g.

  milter_connect_macros = j {daemon_name} v _

Note that Postfix users will see the following warning logged at the time of
the first connection to the milter:

  Could not retrieve sendmail macro "i"!.  Please add it to
  confMILTER_MACROS_ENVFROM for better spamassassin results

This is a consequence of different design decisions between Postfix and
Sendmail (which spamass-milter is written for), where Postfix does not assign
a Queue ID to an incoming message until later in an SMTP transaction compared
with Sendmail. The milter wants to use the information before Postfix can
provide it, hence the message. It is harmless and can be ignored.

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