I had the same problem, and I used Troy's solution
Troy29 on Sep 20, 2012 8:34 AM
I know this thread is a bit old, but just in case anyone finds it here is the solution:
1) Put the iPhone in Airplane mode
2) Open the Mail app
3) Go to the Outbox
4) Click Edit, click the circle next to the stuck message to select it
5) Click Delete
6) Take the iPhone out of Airplane mode
7) Return to the Mail app. :-)
with the following modifications:
step -2: turn off Wi-Fi
Step -1: turn off cellular data and put the phone in airplane mode
step 0: turn the phone off, then on again (after waiting a little while)
follow the rest of the steps 2 through 5 exactly as Troy wrote (step 1 is already done), and of course add to step 6
"turn wi-fi / cellular data back on"
Voilà! No more message from **** in the outbox!