The problem is that the moment you try and add even the most basic value-add features, other email clients can't understand you anymore. And this applies to fundamental things like identity verification, encryption and group chats.

Technically I don't see an issue with doing these things in a portable, standard way. Email signature and encryption (s/mime) has existed for a long time and most mainstream clients know how to handle it. The nut that has still not been cracked very well AFAIK is how to create and share public keys in a seamless way.

For S/MIME you don’t need to share keys. Your private key gets signed by a CA that you both trust.

You can even get free S/‘MIME certificates that validate on all clients.

People don’t do it because encrypted mail can’t be searched or archived, and it becomes cumbersome to have a wider array of clients.

Like DeltaChat.[1]

[1] https://delta.chat/en/

That's the basic idea behind "Chat over IMAP", which for various reasons hasn't seen as much effort put into it as many of us would like (not that I can talk, I haven't done anything).

HN discussion from when it was launched is here: https://news.ycombinator.com/item?id=19216077

I agree. Email is fast enough to have a real-time conversation, I do it often. Maybe not as instant as WhatsApp, etc, but with other advantages.