I have a m0n0wall box as my router. Inside are two networks. x.x.0.x and x.x.1.x. The 0 range is for outside internet coming in. Everything coming in is controlled with NAT. This NAT will (by choice) forward only to 0 addresses. Each server is configured with two network cards. One on the 0 range, and one on the 1 range. The gigabit cards are set up on the 1 range for obvious reasons.
The m0n0wall box uses DNS Forwarding to resolve internal server IP's. I setup mail.server.com to point back to the mail server.
When I setup the DNS to point mail.server.com to x.x.0.x the internal email works. ie: "echo hi | sendmail recipient" works. Also, when DNS is pointing back to x.x.0.x the WebUI will not work.
On the other hand, when DNS points back to x.x.1.x the internal mail doesn't work, but the WebUI does work.
Any explanations to that? :S