what type of mail exchange?

Hi,
I am starting a project for the completion of my current studies. The situation that I have been given is that There are two differerent locations that use ISDN to form a WAN. They both are not in a domain model and located within one site there is at least one other network. They want to be able to join one site with another to be able to have internal mail without spending a fortune. I have been looking at mercury mail since that is cheap but do not know if it can be used across different locations and networks? or if not what would be a relativeley cheap program that can operate through networks or must it be something like exchange mail. What type domain model would probably be good for this I was thinking a single master? any ideas?