I have the identical problem the way MT1414 described it at the top of the thread. I have tried the "Airplane Mode" method (even with battery removal) with no luck because the "deleted" outgoing message never shows up anywhere to delete it. I am motivated to avoid clearing data/cache because I have another e-mail account (without the problem) in this same app. I also want to avoid re-establishing just the account with the problem because of the data loss. Toggling the Sign-In checkbox doesn't work either. Is there any other (new) workaround?