Only compressing the folders will delete that email permanently from the computer system. That’s great if you deleted an email by accident and want to recover it but bad if an unauthorized third party tries to read your emails.

Microsoft Outlook provides the means to compress a folder immediately but no option to automate the task. It will however display a popup every 100th time that Outlook got closed asking if the user wants to compress the folders.

A far better solution to force Outlook to delete deleted mails completely at every shutdown is to change a setting in the Windows Registry.

Open the Windows Registry by pressing [Windows R], typing [regedit] and hitting [enter]. Navigate to the following Registry key:

HKEY_CURRENT_USER\Software\Microsoft\Office

The next step depends on the version of Microsoft Outlook or Microsoft Office that is installed on the computer. When in doubt perform the operation for all of the following subkeys.

Find the 9.0, 10.0, 11.0 or 12.0 keys in that folder. Click in the Outlook subkey and locate the key PST. If PST is not existing create it. A full Registry path to the right location could look like this:

HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Outlook\PST\

Only the 12.0 can change in the path. If you are in the PST key right-click in the right pane and select New > DWord.

Name the Dword PSTNullFreeOnClose. Double-click it afterwards and give it the value 1 to enable it. This ensures that Microsoft Outlook will remove deleted mails completely on every exit from the computer system.