Outlook [Express] "deletes" mail by making a copy in the TRASH folder, and marking the original as deleted, making the problem worse when you're trying to get back some space.

To permanantly delete messages, from Outlook, without making copies in the TRASH folder, select the messages you want to get rid of, then holding down the <Shift> key on your keyboard, press the <delete> key. You will be asked to confirm that you really want to permanantly delete the messages.

Don't forget you can copy messages from your IMAP folders to folders on your local disk before you delete them from your IMAP folder