Step 2: Configure Postfix to use a content filter

  The next step is to configure Postfix to use DSPAM as a content filter.
  This is relatively simple and requires only a minor change to your
  master.cf file:

  Change:

smtp      inet  n       -       n       -       -        smtpd

  To:

smtp      inet  n       -       n       -       -        smtpd
            -o content_filter=lmtp:unix:/tmp/dspam.sock

  This tells Postfix to send all mail to DSPAM for content filtering.

Step 3: Configure a Reinjection Port

  You'll also need to configure Postfix to listen on a local port for
  reinjection. This is where DSPAM sends back the "good" mail (or alternatively,
  tagged mail also). Add this to your master.cf:

localhost:10026 inet  n -       n       -       -        smtpd
  -o content_filter=
  -o receive_override_options=no_unknown_recipient_checks,no_header_body_checks
  -o smtpd_helo_restrictions=
  -o smtpd_client_restrictions=
  -o smtpd_sender_restrictions=
  -o smtpd_recipient_restrictions=permit_mynetworks,reject
  -o mynetworks=127.0.0.0/8
  -o smtpd_authorized_xforward_hosts=127.0.0.0/8

  Any mail sent to localhost:10026 will be delivered in whatever way you
  have configured Postfix, without being passed through DSPAM again. This is
  also where DSPAM will deliver false positives to when they are retrained by
  the user.