I know that when I permanently delete something from Outlook (so also from the Deleted Items folder), it will still not be deleted directly and my pst-file won't become smaller unless I run Compact Now. Until then or until it gets overwritten, the data is still stored within the pst-file and thus recoverable as you described here.
I'd prefer the deleted items to be really gone and free up space directly. For this I now manually run Compact Now a couple of times per week.
Can I configure Outlook that it will always automatically compact the pst-file whenever I delete something?
The reason that Outlook’s pst-file doesn’t directly grow and shrink whenever data gets added or deleted is for performance reasons; reserving and returning blocks of space to the file system is a relatively slow process. By maintaining (some of) this deleted space instead of directly returning it, Outlook’s performance could be improved.
Force automatic Compact Now via the Registry
You can force Outlook to return this free space within the pst-file by running Compact Now. You can also force Outlook to return this free space every time that you close Outlook by making the following Registry change;
By setting PSTNullFreeOnClose, Outlook automatically cleans up after you.
Other than the value name would make you assume, this applies to both pst- and ost-files. When the value is set to
1, deleted data will be removed when you close Outlook and when it is set to
0 or the value name is missing, deleted data won’t be removed on close.
This setting does NOT automatically empty the Deleted Items folder when you close Outlook.
Note: This change can cause longer Outlook shutdown times as it will truncate the pst-file or ost-file at shutdown time. Especially the first time that you close Outlook after making this change, it could take an extended period of time for Outlook to close.