My recommendation here is Signal: https://whispersystems.org/
My recommendation here is Signal: https://whispersystems.org/
And it also makes sense. A lot of these services are from companies that need to make money. And there isn't much money in the journalists and dissidents who don't have a bespoke solution.
I think what we're going to need is a new, non-SMTP protocol, which preserves all of the good things about email, while providing e2e encryption and (pseudonymous) identity assurance. I don't know enough to be involved in designing that protocol, though, other than saying what I want to see as an end-user.
and Signal/WhatsApp comes to replace (and kill) xmpp, not email. Another issue is the generational shift away from email, that is only for Spam and Work, more and more everytime...
### Protocol
* Easily federated
* Identifiers can be memorable/meaningful (unlike phone numbers) while still being globally unique (thanks to federation)
* Device independent (not tied to a phone number, can generally use the same account on different devices)
* Can contact people you don't know/haven't met (this is possible with Signal, but they'd have to publicly share their personal cell phone number, which is a no-go).
### Client
* Optimized for longer-form, less immediate messaging (folders, drafts, rich text)
* MIME attachments (Signal supports only a limited number of predefined types of attachments)
I feel like you could probably layer an email-equivalent on top of Matrix, but I'm not 100% sure about that.