If you have ever been composing a new message in iOS Mail when you need to refer back to an earlier email then what you can do is instead of saving the new message as a draft you can just swipe it out of the way.
If you’re composing a new message and need to take a look through your older messages, just drag the title bar of the new message down toward the bottom of the screen to move it out of the way.
You can now run through your mail folders, read old message, and whatever else you need to do. To return to the new message, just tap its title bar at the bottom of the screen.
It’s a pretty simple trick–and somewhat obvious in hindsight–but Mail doesn’t make it really obvious that you can slide your new message down to get it out of the way.