Stop Zimbra with 'zmcontrol stop' then check if there are any zimbra processes running and kill them if there is. When you've done that start zimbra again with 'zmcontrol start' and check the
log files for error messagse, post details here of what you find.
BTW, you should also make sure that there isn't another mail server running on port 25 and that any firewall and SElinux is disabled on the zimbra server.