BES gets the user mailbox information from the global address list and then connects automatically to the right server through MAPI.
The only time you need to put in the exchange server is to set up the MAPI profile for BESAdmin account, so put in the exchange mailbox server for the mailbox that is set up for the BESAdmin account.
As long as the users are in the same forest and organisation, as well as the global address list you don't need to specify anything else.