Email stuck in outbox

This is the second time when the same issue, we wiped the device to fix it. Now happend again, the email does not apprear but the sending message will show 1 of 1. If you send emails, they are being sent but only one is stuck.

We tried everything from turning off the Airplane mode, rebooting, end task for the app, remove other apps from device and no luck.

I can't keep wipe it the deive everytime to fix this issue.

Thx