Does Microsoft publish the remote procedure call (RPC) interface that Exchange uses, or is it hidden by the Messaging API (MAPI) interface?

By design, the RPC interfaces that Microsoft uses for Windows OSs and Windows applications aren't public. Although this policy is an annoyance to developers, it gives Microsoft the flexibility to change RPC code at any time without fear of breaking third-party applications. If you really, really try, you can do what the open-source software provider Samba project team did and reverse-engineer the interfaces, but reverse-engineering is a lot of work for marginal benefit. MAPI will be going away one of these days, so you'd be better off to target your development at WWW Distributed Authoring and Versioning (WebDAV) and HTTP.