Let's check that everything is working correctly. I don't know if Ubuntu installs another MTA but do you have any other mailserver running on your system? Run the following commands on your server:
Code:
su - zimbra
zmcontrol status
That should show all the services running, if the MTA isn't running then do this:
Code:
telnet localhost 25
If you get a valid response from that then you have another MTA running and it needs to be stopped and disabled. If you need to stop it then restart Zimbra again and see if you can send email. If there's still the same problem then run these commands again and post the output:
Code:
cat /etc/hosts
cat /etc/resolv.conf
dig yourdomain mx
dig yourdomain any
host `hostname` <- note backticks and not single quotes
Ubuntu also has Apparmor but I don't know if it's enabled by default, can you check and disable it if it's running and you should also make sure there's no firewall running on that server.