OK, MAPI (Messaging Application Programming Interface) isn't really dead, but it has, at least from a marketing perspective, undergone a bit of an overhaul. MAPI is an API — Application Programming Interface — that allows client programs to become messaging-aware. The Outlook/Exchange client/server combination relies heavily on MAPI, which uses RPC as its transport mechanism. After all, an API, which includes MAPI, is just a set of interfaces. In order for communication to occur, you still need some kind of transport, and that's the role served by RPC.
Because of pressure from regulatory bodies to open up proprietary standards, and the need to have good sounding names for everything, something had to be done about "MAPI over RPC."
Enter the Outlook-Exchange Transport Protocol.
The Outlook-Exchange Transport Protocol is still MAPI riding on RPC, but it has a new name. Even better (depending on your perspective), the Outlook-Exchange Transport Protocol can be licensed from Microsoft for inclusion in other vendor's products. According to Microsoft, it will "license its trade secrets, copyrighted information, and necessary patent claims for implementation of this protocol specification."