K9-Mail is a good app, but unfortunately rarely updated, in the last year. Besides, it has had an annoying cryptography bug and a notification bug.
So we tried FairEmail, that is definitely more accessorized, but not really privacy-friendly as they pretend, and not so open source: impossible to contact the developer, and pro functionalities are blocked until you pay something.
So, we went back to K9-Mail, that is absolutely open-source, even more “spartan”, but in its latest release without cryptography bug and notification bug.