We are in the beginning steps of integrating into our AD domain a company (we'll call them companyX) that we purchased. One of the first steps in our integration is to set up a two way trust. To do this we plan to use a conditional forwarder however in our testing we became aware of a critical issue. While our public and private domain DNS namespaces are different, companyX has identical public and private namespaces. To top that off, their Exchange 2003 mailserver's public and private dns names are identical.
The problem we are having is that when we put in place the conditional forwarders, our mail servers stop looking at the public address for companyX's mail server and start looking at their internal address. Their Exchange server is configured to only accept email on their public IP address and so email to companyX stops flowing.
We have tried to circumvent this issue by configuring Hosts file entries on our mailservers so that they route to the external IP of companyX's mailservers but this did not work. I am hoping that someone might have other suggestions on how to resolve this. We're looking into configuring other smtp connectors on their Exchange server but no one on had is expert in Exchange 2003 or has the expertise to do this. We're also talking with our Network/firewall admins to see if they can do some fancy NATting of the traffic. Any other suggestions on how to proceed?