Try building an email client that supports presence (online/offline/busy/custom state), subscriptions, group chat with different admin levels (owner, admin, mem

Try building an email client that supports presence (online/offline/busy/custom state), subscriptions, group chat with different admin levels (owner, admin, member ...), message receipt and read notifications, chat state notifications (composing, active, inactive ..), VoIP including video chat (XMPP Jingle), public chat rooms, buddy lists, and many other features that I didn't list. Surely someone can model it on top of email too, but the effort would be enormous and it would be proprietary. XMPP on the other hand is a standardized protocol that supports all of this with great library support on many platforms and that's why most chat clients today use this protocol.