Is this a new install or an upgrade? What happened (or was updated) on your server the day this problem started? What errors do you see in the log files when you stop then start Zimbra? Post the output of the following (run on the Zimbra server):
Code:
cat /etc/hosts
cat /etc/resolv.conf
dig yourdomain.com mx
dig yourdomain.com any
host `hostname` <-- use that exact command with backticks not single quotes