From Fedora Project Wiki
SQLgrey
SQLgrey is a Postfix policy service implementing a greylisting policy.
The Fedora sqlgrey package comes pre-configured to use a SQLite database. To turn it on, just start the sqlgrey service (service sqlgrey start and don't forget to chkconfig --level 2345 sqlgrey on) and add the following to smtpd_recipient_restrictions in /etc/postfix/main.cf:
check_policy_service inet:127.0.0.1:2501
If you don't have an uncommented smtpd_recipient_restrictions, you probably want to add a block like this:
smtpd_recipient_restrictions = permit_mynetworks,
reject_unauth_destination,
reject_unknown_sender_domain,
reject_non_fqdn_sender,
reject_non_fqdn_recipient,
reject_invalid_hostname,
check_policy_service inet:127.0.0.1:2501