Outlook error 0x800420c8

0x800420c8 - The message could not be opened from OUTBOX folder.
One cause of this can be that there are two identical accounts in Outlook Express with different names.
To resolve this, you need to remove the duplicate account. To remove it:
1. Open Outlook Express
2. Click on the Tools > Accounts > Mail tab.
3. Choose the duplicate mail account, and then click Remove.
4. Click Close.