there are a number of ways to solve this problem.
The easiest way to remove this message if there was limited or no connectivity to your ISP (Data Provider) is to make sure that your iPhone connects to a local hot spot using your Wi-Fi connection, this will send the email and the message will go away.
Another way to remove the message is to turn on your airplane mode then compose and send another email. this will make the outbox visible again then turn the airplane mode off and go to the email account, both emails will be sent and the message will disappear.
You can also delete the unsent message by doing the following:
1. Go to your "Home" screen
2. Click on the "Settings" app icon
3. Switch "Airplane Mode" on
4. Go back to your "Home" screen
5. Click on the "Mail" app icon
6. Compose a message and send it (any message to any recipient will do)
7. Once the message has been sent exit "Mail" by going to your "Home" screen
8. Start "Mail" back up by clicking on the "Mail" app icon
9. The outbox folder should now appear, click on it to open it
10. Delete ALL messages out of your outbox folder by sliding your finger over it and then pressing the "delete" button for all messages.
11. Go to your "Home" screen
12. Click on the "Settings" app icon
13. Switch "Airplane Mode" Off
14. Go to your "Home" screen
15. Open "Mail" back up, the message should now be gone.
1. Go to your "Home" screen
2. Click on the "Settings" app icon
3. Switch "Airplane Mode" on
4. Go back to your "Home" screen
5. Click on the "Mail" app icon
6. Compose a message and send it (any message to any recipient will do)
7. Once the message has been sent exit "Mail" by going to your "Home" screen
8. Start "Mail" back up by clicking on the "Mail" app icon
9. The outbox folder should now appear, click on it to open it
10. Delete ALL messages out of your outbox folder by sliding your finger over it and then pressing the "delete" button for all messages.
11. Go to your "Home" screen
12. Click on the "Settings" app icon
13. Switch "Airplane Mode" Off
14. Go to your "Home" screen
15. Open "Mail" back up, the message should now be gone.
If you need further help you can go to apple's support site by clicking on the following icon: