Let the email message actually fill up the whole screen in landscape mode.
When I rotate tablet into landscape orientation it's because I can't read the message in vertical orientation (combination of text too small & message being pre-formatted in HTML with expectations of a certain minimum display width). It is counter productive to show message list.
This is #1 reason why I can't give the sparkling review that you're looking for, when you prompt me every 2 weeks to provide a rating.
Rathan Kumar N. commented