It should have sender authentication. E2E encryption that's easy and works by default. Those would be more than enough killer features, SMTP is just too broken and all the workarounds we have in place like DKIM, SPF, Spam ratings etc etc don't make up for it. We still have spam, important mails still end up in our junk boxes, and nobody trusts it enough anymore to put important content in emails. The war has long been lost.
"War"
Few, outside small, likely familiar tech circles, use these terms when discussing email.
Not being SMTP usually means being another messaging island or other, which has only strengthened email.
Lists have huge value today, still!
Now, there is one exception: Marketing
The reason? Everyone else is busy getting work done.
Whatever may transcend email needs that quality, or it will, in fact be, yet another messaging island.
Email must be: * Secure and encrypted; * Have proven identity; * Have easy to fabricate and predictable rendering;
I feel like the ability to have forms and charts is very nice, but adds a lot of complexity, especially from a security point of view. I'd be looking at this kind of "application level" functionally being a layer added optionally on top, not being in the core protocol.
The difference seems to be that ProtonMail don’t allow you to use normal imap and your own client with pgp, but force you to use their client. This is probably a trade-off made to protect users against them selves in one way (disabling encryption for grandpa) and hiding the non tech-savvy from all the technical details.
Again, many assumptions here.
So, to me, mentioning proton here doesn’t make sense as the underlying tech: smtp & pgp have existed and been industry standard for a long time. So there is not issue of adoption.