I have a Zimbra behind firewall with separate DNS server on the same LAN. All ports on firewall doing port forwarding. On my Zimbra server, on Network configuration, I have put for DNS local address of my DNS server as they are on the same network.

Now, on Administrator UI, under MTA:
* when Enable DNS lookups are ON I can send mail to other server (yahoo for example), but not between users on my own domain
* when Enable DNS lookups are OFF, and Relay MTA for external delivery: is set to my firewall LAN address, I can not receive mail from yahoo, but can communicate in my own domain

Does anyone can help, what could be the problem?