Here is a simple tip to make Outlook empty your Deleted Items folder automatically each time you quit.
WARNING: I am assuming that you know that the Trash Can is not the same as a filing cabinet… and that once you have deleted an email (and sent it to the Deleted Items folder), you no longer want it!
After you make this simple settings change Outlook will display a message similar to the following every time you close Outlook.

In Outlook 2010:
- Click File.
- Click Options
- Select the Advanced tab.
- Tick the box labelled “Empty Deleted Items Folder when exiting Outlook”.

- Click OK
In Outlook 2007 and Outlook 2003:
- Click Tools
- Click Options
- Select the Other tab.
- Tick the box labelled “Empty Deleted Items Folder when exiting Outlook”.
- Click OK
I hope this quick tip helps to keep your Outlook data file cleaner and leaner.