Automatically Empty the Trash When Quitting in Microsoft Outlook
If you constantly find yourself trying to earn back precious space in your email program, setting the trash to automatically empty when you quit Outlook is a great place to start. Go to the File tab and click Options.
Look under the Advanced tab for “Outlook start and exit,” and check the box next to “Empty Deleted Items folder when exiting Outlook.”
If you want Outlook to double-check with you before it permanently deletes anything, look farther down in the Advanced area for the section called Other, and select the Prompt for confirmation before permanently deleting items.
Source: PC Magazine – http://www.pcmag.com/article2/0,2817,2418518,00.aspl