Some weeks ago, I’ve come across Delta Chat, whose main thing is “(near) instant messaging using your email”
That left me thinking, has this been attempted before? If not, why? Also, why (besides servers’ limitations as means to fight spam) isn’t this solution used more often, given that e-mail has been a decentralized solution for well over 40 years now?
Jabber/XMPP is similar, Google EEE’d though…
Then you have Matrix which is what the French security services based their internal chat system on.