While I have posted a ticket, I though I might post the question here too.
I asked the Zimbra support for advise on moving from an old box to a new one and they gave me this wiki post to follow.
Network Edition: Moving from 32-bit to 64-bit Server - Zimbra :: Wiki
Something went amiss and Zimbra won't start. I know why, just not the how.
The old hardware was running RHEL 4.5 and Zimbra 5.0.13. The new hardware is RHEL 5.3. I first upgraded the old box to 5.0.16 and then went through the procedure using 5.0.16 on the new box. It went well until the last step. zmcontrol start showed everything starting, but a backup would not run. I started and stopped Zimbra watching the log.
The log showed something at 0.0.0.0:25 preventing postfix from running. I made sure that neither postfix or sendmail was installed before starting the 5.0.16 install on the new box.
The only possibility is something called fetchmail, which seems to be a dependency of sendmail (which I uninstalled from the default RHEL install.) On next go around, I'll check it.
Where else should I look for port 25 conflicts? Other thoughts?