Hello and welcome to Apple Support Communities lewrail,
I understand that you have an email stuck in the Outbox on your iPhone. I'd be happy to help.
You've already done some good troubleshooting steps on your own to try to resolve this issue. Next, let's have you force close the Mail app on your iPhone, and then also do a force restart on your iPhone. If needed, those steps are listed in the articles below.
How to force an app to close on your iPhone, iPad, or iPod touch
Force restart iPhone
If the issue remains after trying these steps, delete the email account from your iPhone and then add it back and test to see if the issue persists. You can find those steps in the article below under the section titled "Remove your email account and set it up again".
If you can’t send email on your iPhone, iPad, or iPod touch
Let me know how this goes.
Have a great day!