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.

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: