It says I have "1 unsent message" but there is no message

Go to the Outbox and delete the message. The Outbox mailbox is only available if there are unsent messages.

Switch/power off the phone completely by holding down the Home and top buttons until it completely shuts down.

Lave it like that and go onto webmail and visit every account you have on the phone and use webmail to make sure the outbox is empty. Doa 'Retrieve messages' from each account on webmail.

When you have done all of that for each account, turn on the phone 30 minutes later - any garbage data stuck in the cache should be cleared out when the phone switches on a again.

Worked for me ....