Yes the screen should rotate in the mail app.
If you see a lock icon in the upper right corner of the screen - then the screen orientation is locked.
Try the side switch above the volume rocker first and see if that unlocks the screen. If that doesn't do it, double tap the home button and swipe to the right and look for the lock icon all the way to the left.
If the screen is unlocked but still will not rotate, reset the iPad.
Reset the iPad by holding down on the sleep and home buttons at the same time for about 10-15 seconds until the Apple Logo appears - ignore the red slider - let go of the button