Apple - iPhone - Tips and Tricks
To my surprise and delight, I learned something new from Apple’s iPhone tips page:
Get back to your draft.
In Mail, touch and hold the Compose button to switch to your last saved message draft.
Could be useful on occasion, although unfortunately this does not seem to work across quitting Mail.
Update: Just to clarify, I don’t manually quit apps. I mean when Mail is forced to quit by the OS.