When you delete an item from your mailbox, it's moved to the Deleted Items folder. You can delete the entire contents of a folder, including all sub-folders, by clicking Manage Folders and then using the Delete Folder option.
Click the Deleted Items folder in Mail.
To permanently delete the entire contents of your Deleted Items folder, click Empty Deleted Items Folder on the toolbar.
To permanently delete only some of the items in your Deleted Items folder, select the check box next to each item you want to delete and then click Delete.
What else do I need to know?
There may be a policy on your Deleted Items folder that will cause items in it to be permanently deleted after they have been in the folder for a set period of time.
What if I want to know more?