If that was your external IP in the host file and you are behind a firewall, this is most-likely your problem. If your server is on a LAN, it should have a LAN address and your firewall should be routing the external IP to your internal IP.

I have a similar setup and documented every step in this thread. It may have helpful information in it for you. It is the result of reading many diverse documents in order to get Ubuntu 10.04 and Zimbra 7.1.3 working as a potential replacement for our current email server.