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As your ZCS installation is behind a firewall on a private LAN then you would need to setup a
Wiki :: Split DNS architecture. This has been covered many times on the forums so please try search for "Split DNS"

Though if you do have problems still once you have set it up then you would need to post the following so that we can help you
Code:
cat /etc/hosts
cat /etc/resolv.conf
dig yourdomain mx
dig yourdomain any
host `hostname` <- note backticks and not single quotes