today android automatically upgrade k9mail to the latest version. As result the app got the new theming including that includes the change of the toolbar from the bottom to the top. Regardless by intent or not, the move of the toolbar breaks the daily one hand / one finger usage handling of k9mail that made k9mail so popular.
Please move the toolbar back to the bottom in the new theme, or provide instructions how to restore to the old theme, or provide intructions how to downgrade to version that supports it. Thanks.
Btw. if you plan such desruptive change, you may well advised to do a survey or introduce it per feature toogle.