Sometimes when sending a large attachment (ie Photo or video full size) the email app on an iPhone or iPad will freeze and crash.
The Edit option is greyed out and you have no way to stop or delete the sending email.
Go into settings and select Airplane mode which will disable all internet access
Then go back into the email -> Outbox -> delete the email or open and remove the attachment
Remeber to switch Airplane mode back off once the email has been removed or updated.
If you cannot remove the email by setting Airplane mode to On try the following;
In order the get to the "edit" option first put the phone in airplane mode to disable the wifi and mobile data connection. If you cannot remove or delete the email carry out a Hard Reset by following the steps below;.
Put the phone in Airplane Mode to terminate the wifi and cellular
Then, do a hard reset by pressing and holding the home button and the Powewr button at the same time until you see the Apple symbol and a white screen.
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 email has been deleted.