Messages stuck in the outbox, not sent even when trying Send All

Delete the .ost file @:
C:\Users\CurrentUsername\AppData\Local\Microsoft\Outlook

N.B.
1- Non-server side data will be deleted, such as previously unsent items in the outbox.
2- All outlook cached items are going to be automatically downloaded which can be as big as 200 or 400 MBs.

Unable to open an Access .mdb file

This is not the main cause; however, read this strange scenario, you might be another victim:

When I double clicked an .mdb file to open it on my new machine which have MS Office 2010 installed, including Access, it just opened another Office interface rather than Access without any  access features and in the status bar, it was written, "Powered by Microsoft Access".

When I open the Access application from the Start Menu, it initiated the Setup application and configured itself.

Back to the .mdb, when I clicked it, it opened like a charm!!!

Outlook 2010 Folder List

This will display the calendar, contacts, the public folders on the left navigation window trees.

To switch to the Folder list view: