MWES is fully compatible with an Exchange cluster (from 2007 onwards) (Front end with Mail stores behind)
There are a couple of caveats however:
• Version 2.80 of MWES or later needs to be installed (it has specifics to handle Exchange clustering)
• SMTP Replay needs to be switched off (registry key 'use_replay' with a value of 0, or create this key if it doesn't exist) -Microsoft use an extended proprietary SMTP command set between the FE and MS servers which is not handled by Replay.
• MWES needs to be installed on the Front end server, and communicate to the front end exchange server - not with the Mail stores.
• All domains accepted by the front end server need to be added into the MWES Domains list (otherwise they will be immediately rejected)