Depending on the version of Outlook...
Go to Tools|Options.
Click on the "Other" tab.
Under General, click in the checkbox to the left of "Empty
the Deleted Items folder upon exiting". Every time you
close Outlook, it will empty your Deleted Items
automatically.
It will confirm that you want to empty the folder before
it permanently deletes them. If you don't want that
prompt, click on Advanced Options...under General, and
clear the checkbox next to "Warn before permanently
deleting items."
HTH
Tea
