BES comes in slightly different versions - Domino, Exchange or GroupWise. When you buy a BES server, you have to specify which version you want.
There is no technical way to make a Domino BES talk to an Exchange system and vice-versa.
The differences in the architecture of mail systems means that there is not one boxed product of BES, they have to be mail-platform specific to work.
: Infrastructure Architect, Denver CO