ZCS can become confused if no MX is present, so I would recommend running a
Split DNS - Zimbra :: Wiki
If you believe the configuration is okay as is then please post the following :-
Code:
cat /etc/hosts
cat /etc/resolv.conf
cat /etc/nsswitch.conf
dig zim.sub.domain.com mx
dig zim.sub.domain.com ns
host `hostname` <- note backticks