I had this problem myself, and tried everything in the forums about resetting network accounts and deleting then re-adding email accounts and it's completely unnecessary. It's a glitch, one that no amount of factory resetting has solved, so I suggest just working around it:
The easiest way to recover your email in limbo is to go to settings, flip your phone into aeroplane mode, then go back to the mail account you're dealing with. Write a new message to anyone and click send. Of course it won't be able to send it with all the connections off, but if you go back to your mailbox, voila! The outbox is has suddenly appeared, with 2 messages in it: the mystery "unsent" email you originally composed as well as the one you just wrote in aeroplane mode.
From the outbox I usually delete the dummy email and MOVE the stuck email into the drafts folder (if you don't move it it will just get stuck again). The switch out of aeroplane mode, go back to your mailbox, and try sending again. It usually works, but if it doesn't at least you can look at the email and retype it or forward it, etc.
That's the best advice I've got. Hope it helps!