***
*** NOTE: Now %%PREFIX%%/sbin/policyd has renamed to %%PREFIX%%/sbin/postfix-policyd-spf
***

To run this from %%PREFIX%%/etc/postfix/master.cf:

policy  unix  -       n       n       -       -       spawn
  user=nobody argv=%%PREFIX%%/sbin/postfix-policyd-spf

To use this from Postfix SMTPD, use in %%PREFIX%%/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.