If you need to hook it up to other stuff (where there is a solution using postfix), it’s probably easier to stick with postfix. As an all-in-one mail server I prefer stalwart over docker-mailserver, mailcow, etc. because it’s one unified software with sensible configuration instead of a clusterfuck of services put together using string and duckt tape.

Check out stalwart mail! Not sure it has all the features you need, but it is really flexible through scripting and has got a nice admin web-interface!

I think if every contributor ever agreed, you could switch to a more permissive license that permits a superset of the original license.