***
*** NOTE: Now /usr/local/sbin/policyd has renamed to /usr/local/sbin/postfix-policyd-spf
***

To run this from /usr/local/etc/postfix/master.cf:

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

To use this from Postfix SMTPD, use in /usr/local/etc/postfix/main.cf:

smtpd_recipient_restrictions = ...
                               reject_unknown_sender_domain
                               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.