Tutanota (https://tutanota.com/ https://tutanota.com/) does that, and even has compatibility with the existing SMTP network. It seems to be web-only at the mome

Tutanota (https://tutanota.com/ https://tutanota.com/) does that, and even has compatibility with the existing SMTP network. It seems to be web-only at the moment though. I'm pretty sure other systems exist, unfortunately as long as we stay with SMTP nothing will change.