As you know Exchange 2007 has moved to the 64-bit space, however Exchange’s implementation of Extended MAPI did not. You can still download Exchange's Extended MAPI here, however it is only a 32-bit version.
If you are moving your Exchange application to 64-bit you have some decisions to make. If all your system requirements can be met by Exchange WebServices then you have nothing to worry about. However, if you are still dependent on Extended MAPI than you can either remain at 32-bit and run in WoW64 or componentize your Extended MAPI calls so that they can be run out-of-proc. A 64-bit process cannot load a 32-bit DLL. Therefore, all calls to Exchange's Extended MAPI need to be made from a 32-bit process.
How do you work around this? That’s where RPC comes in. A 32-bit process can communicate with a 64-bit process via RPC so if you have only a few calls to Extended MAPI then this is your best bet. However if you need to make a lot of calls to Extended MAPI you need to determine whether running out-of-proc is more advantageous than just running in Wow64.
I plan to eventually build a sample of this so check back every so often.
* Pending KB