Right now, all requests require a callback with a token. user a@a.com sends a request to b@b.com with a generated token. b@b.com then sends a verification request back with that token to a@a.com, and a@a.com sends back true, and b@b.com completes the request.

Which, if I'm not mistaken, is very similar to how XMPP does it.

The protocol also tries and minimizes the amount of actions that a third party node can take. You only trust your own node to take actions on your behalf.