Most of the IM of today isn't synchronous as well. That's the exact reason OTR was deemed not enough, because it requires synchronization, and the Open Whisper

Most of the IM of today isn't synchronous as well. That's the exact reason OTR was deemed not enough, because it requires synchronization, and the Open Whisper guys came up with a novel encryption scheme that works with asynchronicity in mind. Now of course that doesn't mean you should never be synchronous, and it is true email might not be the best vector here. But from my little experience it's good enough for simple messaging.