I participated to the funding, and thus, can participate to the development process: https://roundcubenext.com/
http://jmap.io/software.html
https://github.com/cyrusimap/cyrus-imapd/blob/master/docsrc/imap/developer/jmap.rst
https://github.com/cyrusimap/cyrus-imapd/blob/master/docsrc/overview/cyrus_roadmap.rst