SHutdown Zimbra then kill any remaining Zimbra processes, when you've done that check to see if there is an MTA running on port 25. If there's another MTA then kill it and stop it from starting at boot then restart Zimbra. Check to see if Zimbra starts and if all the services are running and if not post the errors from the log file and see if there's also a /opt/zimbra/log/zmmailboxd.out file and post the output of that.
You should also post the output of the following (just for confirmation) run on the Zimbra server:
dig yourdomain.com mx
dig yourdomain.com any
host `hostname` <- use that exact command with backticks not single quotes