With today's fight between Beeper and iMessage, I would like to remind you of a similar previous fight when the Pidgin chat app (Microsoft) attempted to interoperate with AOL instant messenger.
I believe at one point the server would challenge: "what are bytes X..Y of your .exe file?" It even escalated to the point of the server exploiting a buffer overflow in the client and sending x86 code in the initial handshake!