Outlook 2007: the messaging interface has returned an unknown error

If you receive "the messaging interface has returned an unknown error" in Outlook, this usually indicates a corrupt OST file. The easiest thing to do is delete the user's OST file and let Outlook rebuild it.

On XP:

C:\Documents and Settings\USERNAME\Local Settings\Application Data\Microsoft\Outlook\outlook.ost

On Vista/7

C:\Users\USERNAME\AppData\Local\Microsoft\Outlook\outlook.ost