Hacker News Reader

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?