Ask HN: Chat-App based on Mail and PGP?

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 could 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?

What if you could host the mail and app-backend on your own infrastructure? But it is still possible to communicate with others using their own server or using public server or with someone who does not want to use the app, but receives your messages via email - encrypted?

What if you could use multiple devices?

Wouldn't you want such an chat-app?