Outlook provides a way to recover items after you have permanently deleted them, including after emptying the Deleted Items folder. Your Exchange server administrator specifies the retention time for permanently deleted items on the Exchange server. After this time has elapsed, you cannot recover the deleted items.

By default, to use the Recover Deleted Items command on the Tools menu, you must be viewing the Deleted Items folder. By modifying the registry, this command is available regardless of which Outlook folder that you are viewing. You can view and recover deleted items, including those items that were permanently deleted by using SHIFT+DELETE, for the folder that you are viewing.

To enable this feature follow the steps below.

1. Exit Outlook.
2. Open the Windows registry editor.
3. Browse to My ComputerHKEY_LOCAL_MACHINESoftwareMicrosoftExchangeClientOptions.
4. On the Edit menu, point to New, and then click DWORD Value.
5. Type the name DumpsterAlwaysOn.

Note Do not type any spaces in the name.

6. Set the DWORD value to 1.
7. Restart Outlook.

The Tools menu now has the Recover Deleted Items command for every Outlook folder.

Source: http://office.microsoft.com/en-us/outlook-help/recover-deleted-items-from-any-folder-HA001116528.aspx