I found the default binding of up/down to only scroll up/down the current message, even when one of the other lists has the focus to be quite obnoxious and frustrating, and just deleted those bindings.  The binding of up and down becomes even less useful when using large monitors where you can see the entirety of the e-mail within the message pane so there's nowhere to scroll anyway.

Why not just let the list views and scroll views handle their own scrolling in the natural way (up/down/page up/page down will scroll or move the cursor within the currently focused view)?  You can certainly keep left/right as prev/next message and other key bindings, but users coming from other multi-view apps expect that they can use those keys to scroll within the currently focused view.  
It seems unnecessary to bind those keys when the views should handle them in the expected way.

Reproducible: Always