A couple of notes regarding the use of Postgrey on Centos 4 (RHEL4):
You can get the postgrey RPM from the DAG
repository. Setup the DAG repository and use yum to install the postgrey package.
Update Postfix config as described in the parent post, then create /etc/sysconfig/postgrey and add the following to it:
Then restart postfix and start postgrey:
service postfix restart
service postgrey start