K9/K10 (Kaiten) mail support has been discontinued — K9 no longer gives apps access to its messages, so there's nothing our app can read anymore. This answer is kept for reference only.
If you installed or updated K9 / K10 mail after installing our application, you will need to re-install our application due to how permissions work in Android. After doing this, and assuming that you selected to read the message body, everything will work as expected!
Contact us if you have any other questions or concerns.