Jmap was developed by Fastmail, meanwhile all primary FLOSS mail servers do not support it and just new projects like Stalwart are starting to bring it up.
Re:How IMAP works under the hood
by FuriouslyAdrift on Wednesday April 02, 2025 @11:18AM
(#43557660)
Cyrus supports it. https://www.cyrusimap.org/imap/developer/jmap.html
https://www.fastmail.com/dev/