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There's TMTP, "a sane network protocol for email, to end attacks and promote productivity."

Serious money may be required.

Support for MNM by way of Patreon is requested on the home page.

There's also JMAP, with credible standards people involved:

Serious industry involvement however appears limited to FastMail.

JMAP is an extension of the email protocol stack; it only replaces IMAP.

TMTP is an alternative to the entire email protocol stack. It will be standardized after it's been proven in a range of real-world scenarios.

The mnm client & server, which implement TMTP, work well today, and have nearly all the features most users need. See docs menu in the online demo.

Re "serious money", could you elaborate?