Description:

Postgrey is a Postfix policy server implementing greylisting.
When a request for delivery of a mail is received by Postfix
via SMTP, the triplet CLIENT_IP / SENDER / RECIPIENT is built.
If it is the first time that this triplet is seen, or if the
triplet was first seen less than 5 minutes, then the mail gets
rejected with a temporary error. Hopefully spammers or viruses
will not try again later, as it is however required per RFC.
Edit your configuration files:
/etc/postfix/main.cf:
smtpd_recipient_restrictions = ...
check_policy_service unix:extern/postgrey/socket, ...
or if you like to use inet sockets (modify the IP if needed):
/etc/sysconfig/postgrey:
OPTIONS="--inet=127.0.0.1:10023"
/etc/postfix/main.cf:
smtpd_recipient_restrictions = ...
check_policy_service inet:127.0.0.1:10023, ...