Installing
----------

 1. Add the following to /etc/postfix/master.cf:

        policy  unix  -       n       n       -       -       spawn
            user=nobody argv=/usr/bin/perl /usr/sbin/postfix-policyd-spf-perl

 2. Configure the Postfix policy service in /etc/postfix/main.cf:

        smtpd_recipient_restrictions =
            ...
            reject_unauth_destination
            check_policy_service unix:private/policy
            ...

    NOTE:  Specify check_policy_service AFTER reject_unauth_destination or
    else your system can become an open relay.

 3. Restart Postfix.