I discovered a neat trick: if you kill the Mail app while it's sending (by deleting it from the app switcher) Mail will loose the unsent email. Nothing in Sent, Drafts or Trash.
Could be useful for unsending embarrassing messages. You have to be quick as a viper though. You'd probably be better off jailbreaking the phone and creating an activator shortcut for airplane mode. You know, turn off the interwebs before Mail can send.
Kind of annoying when Mail eats your blog posts, though. They're works of art, you know?