I agree 100% with Nico57. For all benefits, BES only brings to device the first 2K header of a message. Replicating mail files "local to BES" will be an effort gone wasted because BES honours the home server settings in the user's Person doc in the Domino Directory.
Lake_Puma is an interesting one but you should leave the server to service their respective functions in their own box.
i.e
BES Server - let it be BES in its own box
Mail Servers - let them do what they are supposed to do in their own boxes
This is call distribution of load. You are going to kill BES by loading it with the mail function.
Hope I help