The submitByMailPlugin is ready for wider testing. A candidate for release is available at my GitHub repository https://github.com/arnoldle/phplist-plugin-submitByMailPlugin. It has been thoroughly debugged, but not yet tested in actual operation. I am planning to do such testing myself, starting next week, but I would appreciate feedback from others.
Please consult the README and, of course, do not try to use the plugin in critical settings.