Try implementing -(UIInterfaceOrientation)preferredInterfaceOrientationForPresentation
in your parent ViewController and set it to UIInterfaceOrientationPortrait.
You can also view what your MFMailComposeViewController is doing, but I think you'd have to subclass it and implement -(void)didRotateFromInterfaceOrientation:
and then send info back via notifications or a delegate...but you shouldn't need to with preferredInterfaceOrientationForPresentation.
Hope this helps!