How can I setup an Exchange server to run on the same domain as a Merak Mail server?

I have a server in my office which hosts a nunber of customer domains and includes a Merak Mail server. I now wish to add an Exchange mail server so we can make better use of our Windows Mobile Devices and also internal outlook email.

I would prefer to leave the Merak mail server in place since it has superior anti spam and virus control, but if I install Exchange 2003 in the same server it will take over ALL the domains, and I only want it to handle one domain, cowboy.ca.

For now I removed Exchange 03 from the domain server ( webdns ) on local address 192.168.10.22 and installed it on a second internal server called ( exchserv ) local address 192.168.10.23.

I understand I can setup Merak as a backup mail server poiting towards exchserv.cowboy.local, but this does not seem to work. Further more, it seems in order to use the exchange server for Mobile Devices I will need it on the edge of the wan so the mobiles can find the incoming ports. I can easily setup exchange so it can be accessed from the internet using a web browser by changing port 80 to port 8080 as an example, but this does not resolve the issue of using exchange with our Mobile Devices.

So I'm hoping rather than "experiment" and mess up an entire domain, I would check to see if anyone has experience in this area.

Thanks again for any input in this matter. :-)

Russ