Note: for Zimbra 8.6 and newer

yum -y install epel-release
yum -y install postgrey

vim /etc/rc.d/init.d/postgrey

find

 OPTIONS="--unix=$SOCKET"

replace with

 OPTIONS="--inet=10023"

Centos 7

yum -y install epel-release
 yum -y install postgrey

vim /etc/sysconfig/postgrey

replace the line

POSTGREY_OPTS=”–delay=60″

with

POSTGREY_OPTS=”–delay=300 –inet=10023″

If you are running Zimbra 8.6 or newer

vim /opt/zimbra/conf/zmconfigd/smtpd_recipient_restrictions.cf

insert

 check_policy_service inet:127.0.0.1:10023

before the lines that start with %%
save the file then execute

systemctl start postgrey.service
systemctl enable postgrey.service
su zimbra
zmmtactl restart

Done!

Script to insert line into smtpd_recipient_restrictions.cf

#!/bin/bash
 egrep '\s*check_policy_service\s*inet:127\.0\.0\.1:10023\s*' /opt/zimbra/conf/zmconfigd/smtpd_recipient_restrictions.cf

RETVAL=$?
 if [ $RETVAL -eq 0 ]; then
 exit
 fi

sed -i "1i check_policy_service inet:127.0.0.1:10023" /opt/zimbra/conf/zmconfigd/smtpd_recipient_restrictions.cf

su - zimbra -c "zmmtactl restart"

ref: http://wiki.zimbra.com/index.php?title=Improving_Anti-spam_system#Postgrey