This is a post on the Signal blog. They are obsessed with E2E encryption, and will never be talked down from it.
Also, I presented several problems with email, not just poor support for encryption.
> Email can be e2e encrypted easily if you want it.
E2E encryption should have forward secrecy. The way that’s usually implemented requires “handshake” messages to be exchanged before the payload is sent anywhere, which cannot be easily retrofitted onto PGP (adding it would be less “encrypted email” and more “an entirely different communication protocol, tunneled over SMTP”). The Axolotl ratchet that Signal uses requires the key for a communication channel to change over time, which is also pretty far away from what PGP and S/MIME currently do.
> The fact is that you generally don't, because you want to access your mail from any device and you want a server-side search
Signal and WhatsApp both exist and people use them, so they obviously aren’t deal-breakers.
> Also, proper e2ee requires identity verification of your contacts, where you check their key fingerprints using an uncompromised channel
It requires you to verify the identity of one, initial contact to bootstrap your social network. From then on, the “uncompromised channel” can be the previous E2E encrypted channel you already created.