The latest K-9 app does not have next and previous arrows. These were replaced with the swipe left and right gestures. Unfortunately, my Pixel 7 Pro uses left and right swipe to go back to the previous screen, replacing the arrow back buttons. If I flick the message just right with K-9 I can make it go to the next message, but often the Pixel gesture works instead and it takes me back to the message list which was the previous screen.
I’ve brought up the same issue in August 2021.
Here’s @cketti’s reply:
edit/ Correction: A similar issue wrt swipe gestures.
The system gestures only trigger if you start the swipe at the edge of the screen. To be able to move between messages in K-9 Mail, simply start the swipe gesture further away from the edge of the screen.