This isn't a Postfix issue, just an FYI for those running updated versions of Postfix on CentOS.
I recently updated one of my CentOS 5.5 systems (which was running Postfix 2.8.2 compiled from source) to CentOS 5.6. The Postfix package appeared nowhere on the upgrade list, and my /etc/yum.conf has "exclude=postfix*" However, after the upgrade and a reboot, Postfix wouldn't start. The maillog complained about the "smtpd pass" settings in master.cf that I had uncommented to enable Postscreen. A postconf -d | grep version revealed that somehow, my Postfix version had reverted to 2.3.3 (the default for new CentOS installs). I thought this was strange, but since I had previously downloaded and compiled Postfix 2.8.2 on that system, I did a "cd /usr/local/src/postfix-2.8.2" and a "make upgrade" and was able to start Postfix 2.8.2 successfully within a few seconds. I wasn't sure if this was a one time thing, but confirmed the issue last night when the same thing happened after upgrading another system from CentOS 5.5 -> 5.6. I haven't been able to find the exact cause yet, but I at least wanted to post a workaround for the archives in case anyone else goes searching for this issue. Thanks, SteveJ
