The Android app's good, and available from F-Droid. And the features are solid. There's definitely room for improvement, but no glaring deficiencies in my experience.

To be specific though, their roadmap [1] includes "Import External Mailboxes into TutaNota", and I don't use their desktop client, so I'm not sure what export options are available there.

[1]: https://www.tutanota.com/secure-email/