We currently run 2 email system, both work fine. Both systems are behind a firewall.
The problem is when one exchange servers looks up the MX record it gets the external address. The problem is that the firewall blocks the external address on port 25.
You can connect to the other server using the internal address on port 25 without any problems.
What I would like to know is there a way to put in a static MX record for this one email serevr.