@Adam Most people in the last 2 years onboarded on one of the many relays which enforce end-to-end encryption already. Others use their own mail server and that works well, as they can relax rate limits as needed.

Nothing is taken away from interoperability between mail servers.

Delta remains a messaging solution that allows IETF standard compliant server implementations to fully participate. We ceased to advertise delta as plaintext email client starting 2 years ago.