The messaging interface has returned an unknown error

This issue commonly occurs in Microsoft Outlook when Outlook has reached its file size limit. To fix this you need to allow Outlook to use a larger sized table.

1. Launch Microsoft Outlook
2. Right click the folder list (on the left hand side) and left click Properties.
3. Click Advanced
4. Check “Allow upgrade to large tables”.
5. Click Ok
6. Click Ok
7. Restart Outlook

Note: You can also force emails to be deleted thereby clearing the Outlook table by selecting emails and select SHIFT-DEL at the same time.