This is straight from the link I posted in the comments. It assumes the use of Exchange.
Recover deleted items This feature requires you to use a Microsoft Exchange Server 2000, Exchange Server 2003, or Exchange Server 2007 account. Most home and personal accounts do not use Microsoft Exchange.
Microsoft Office Outlook 2007 provides a way to recover items after you delete them permanently, even after you empty the Deleted Items folder.
NOTE Your Exchange administrator specifies the retention time for items that are deleted permanently on the server running Exchange. After this time elapses, you cannot recover the deleted items.
You can view and recover deleted items, including the items that were deleted permanently, when you selected items and pressed SHIFT+DELETE or SHIFT+ Button image.
In the folder from where you deleted the item, or in the Deleted Items folder, click Recover Deleted Items on the Tools menu.
Recover items that you have previously deleted
The Recover Deleted Items command is missing
You must use an Exchange account for this command to appear. If you use an Exchange account and do not see the command, contact your Exchange administrator.
Click an item and then click Recover Selected Items Button image.
TIP To select multiple items, press CTRL as you click each item.
Each recovered item is restored to the folder from which it was deleted.
NOTE You cannot recover an item if it does not appear in the Recover Deleted Items dialog box.