Havent played with exchange RPC over HTTPS yet, however if i read the article below right, you need a the Exchange RPC Extention installed behind the FW, the port forward the connections to that. I would assume for security the RPCProxy would be a separate machine, not your exchange box, although i guess you could. I think it would go something like this:
Outlook client -> EFW(443) port forward -> RPCProxy -> Exchange Server on ports (6001, 6002, 6004)
See Microsoft support articles:
http://support.microsoft.com/kb/832017http://technet.microsoft.com/en-us/library/aa996644(EXCHG.65).aspx