Ok, gottcha.
I quickly experimented with my bluetooth keyboard, just to make sure this isn't normal but annoying behavior. Mail rotates in all orientations for me, keyboard on, and editor open. So this is not normal behavior.
Unfortunately all I can suggest are the usual fixes. If they don't work say so, and maybe someone with a better idea of what is going on will chime in.
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When you have problems with an app it's a good idea to clear it from RAM.
Close/Clear the app: With the problem app closed, double tap on the Home button to get the multitask bar. Tap and hold on any icon in the bar until they jiggle, then tap the minus sign on the problem app. This will remove it from RAM and insure a clean start the next time you launch the app.
If that does not work try a restart,
Restart: Hold down the power button until the power off slider appears. Slide it off. When the iPad finishes shutting down, hold down the power button again until the Apple logo appears.
and then a reset if that does not work.
Reset: Hold down the power and home buttons until the Apple logo appears. The iPad will reboot. This usually takes a bit longer than a restart.
