This will delete all your messages, not just the one in the outbox.
Set the phone's messaging system to use the internal memory (C: drive) for messaging. If you're asked if you want to copy the existing messages thnen say no (or you'll copy the problem over). Your messages will revert to whatever state they were in before you switched over to the E: mass memory.
Connect the phone in "mass storage" mode and go to the mass memory using your file explorer on the computer. Don't forget to enable the display of hidden and system files.
Navigate to E:\Private\1000484b
In there you'll see a folder called Mail2.
Delete it.
Disconnect the phone from the computer, Set it to use the E: mass storage for messaging and say "no" when asked if you want to copy your existing messages over.
Done.
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