HERE IS THE BEST FIX!!
Hello everyone,
I had this exact same issue on my iPhone 6S Plus with an email I sent from my Gmail account that was stuck in the Outbox for many hours. NOTHING I did worked to get to the point where I could "edit" the email to swipe it away and delete the email that was stuck.
In order the get to the "edit" option I first tried putting the phone in airplane mode to break the wifi and cellular connection and that did not work. I then tried powering the phone off and that did not work. So too was a hard reset, which also did not work. Desperate after 2 hours, I actually deleted the Gmail account entirely and that did not work. Finally, I resolved the problem by doing this...
1. Put the phone in Airplane Mode to terminate the wifi and cellular connections. LEAVE THE AIRPLANE MODE ON WHEN MOVING TO STEP 2
2 Then, do a hard reset by pressing and holding the home button at the same time you hold down the power button. Wait 5 seconds or so until the phone turns off, resets entirely and powers back on.
3. Once the phone is back on, LEAVING AIRPLANE MODE ON, go back to the outbox and "edit" and delete the email entirely.
4. Turn Airplane mode OFF once the previously stuck email has been deleted and you are good to go.