You've added your post to a thread that's over a year old and has nothing to do with your problem, I'll move your post.
You've given no details of whether this is a new isnatll or an upgrade, whether you have a Split DNS set-up (if you're on a LAN) nor which operating system you're using. I'd suggest you look in the
log files and see what errors you're getting. I'd also hazard a guess at the fact you don't have a DNS server on your LAN, do you? If you don't then follow the instructions here:
Split DNS - Zimbra :: Wiki
You might also want to search the forums as this problem has been covered many times.