Is there any known WhatsApp-Like chat for mobile-phones using PGP/Mail technique? If not, why? Are there limitations for a (near)real-time-chat? If yes, why are they not successful?

What if you can chat from within your app with all characteristics you already know from WhatsApp and your chat-partner just uses his thunderbird with enigmail and you do not have to take care of encryption, but it happens from end-to-end?