K-9 Mail is an open-source Android email app, and it is still its own app. Thunderbird for Android is the Mozilla-branded sibling built from the same project. Installing one does not install the other. Both talk to your mail server and, if you leave them that way, not to an analytics company.
Neither app contains its own OpenPGP code. Encryption is handed to OpenKeychain. That project is no longer adding features. It still works for many people. It is a dependency you should notice, not a hidden strength. The interface is plain. Search and notifications are the parts worth testing on your own provider before you move a whole family onto it.
Use it when you want IMAP on Android without Gmail’s app. Check that you installed the app you meant, and treat OpenPGP as “only as healthy as OpenKeychain.” thunderbird-android
