DMZ/internal DNS problem

We have an Exchange 5.5 server on our “inside” network and a Kerio 6.0 mail server in our DMZ. I am having trouble with email not resolving so that messages from the Exchange server are showing “host not found” when I send mail to a domain on the Kerio DMZ mail server.

I have set up the DNS zones and have set up the host records so that mail clients “inside” can resolve to the server, but actual mail isn’t resolving.

I’ve tried loading MX records that doesn’t seem to solve the problem.