The emerging consensus among experts is that it's not worth the trouble, or, worse, incapable of doing much more than generating a false sense of security. That's for a bunch of reasons:
* An enormous installed base of clients that won't do encryption, meaning that at best you're attempting to tunnel encrypted messaging over an unencrypted transport.
* A protocol that leaks metadata, including some message content, at the envelope layer.
* Hundreds of millions of users that primarily access messages through browser clients that can't meaningfully implement crypto.
* An archive-always UX that ensures that huge amounts of plaintext are scattered around the Internet by both senders and receivers.
* An unencrypted installed base that ensures encryption will be opt-in for the foreseeable future, meaning that users will routinely reveal plaintext accidentally by, for instance, quoting messages and forgetting to encrypt.
* End user demands for things like search that can only be delivered efficiently at scale by databases of plaintext (most likely at centralized servers).
All these problems are probably surmountable (with enormous, concerted effort). But: why bother? Email is just one of dozens of messaging systems available to Internet users. Better to move sensitive conversations to things like Signal, WhatsApp, or Wire --- the double ratchet construction is designed specifically to make IM-like protocols secure even when conversations are sporadic and last months.