2.6.2 Version Negotiation Using MAPI Extensions for HTTP
When a client attempts to connect with an Exchange server via the MAPI extensions for HTTP, as described in [MS-OXCMAPIHTTP], the client sends its version in the X-ClientApplication header of the POST request. The server returns its own version back to the client in the X-ServerApplication header of the POST response.
Upon exchange of this information, the client can determine the level of functionality offered by that Exchange server and the most appropriate functionality to provide to the end user.
The server cannot perform any client protocol version negotiation.