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/mail/main.cf:
smtpd_recipient_restrictions = ...
check_policy_service unix: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/mail/main.cf:
smtpd_recipient_restrictions = ...
check_policy_service inet:127.0.0.1:10023, ...