Edit Postfix's master configuration file, master.cf, and add the following:

    greyfix    unix  -       n       n       -       -       spawn
      user=nobody argv=%%PREFIX%%/sbin/greyfix -/ 24

(The -/ argument specifies how much of the IP address to consider significant,
in this case the first 24 bits.)

Edit Postfix's main configuration file, main.cf and add the following:

    smtpd_recipient_restrictions = permit_mynetworks,
      reject_unauth_destination,
      check_policy_service unix:private/greyfix

If there is already a smtpd_recipient_restrictions configuration line you
should edit it rather than add a new one. The important part for Greyfix is
that you should add check_policy_service unix:private/greyfix to it.

Finally have postfix reload its configuration with:

    %%PREFIX%%/etc/rc.d/postfix reload