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.
You can retrieve your permanently deleted emails in Microsoft Outlook by going to "Tools -> Recover Deleted Items". By default it is enabled by in Outlook 2003/2007.
Basically when you click on "Tools -> Recover Deleted Items", it shows the deleted items for the currently selected folder in the Outlook. For example, if you have selected the "Inbox" in the Outlook and then clicked on "Tools -> Recover Deleted Items", then it will show all the deleted items from Inbox. So to retrieve the permanently deleted (shift + delete) mails, you need to select "Deleted Items" folder in Outlook and then click on "Tools -> Recover Deleted Items".
If you cannot see the "Tools -> Recover Deleted Items" in your Outlook, then you just need to use following steps:
1. Click on "Start -> Run"
2. Type "regedit" and press "Enter" button (To open Registry Editor)
3. Go to HKEY_LOCAL_MACHINE\Software\Microsoft\Exchange\Client\Options
4. Add a new double word ‘DumpsterAlwaysON’ of type REG_DWORD with the value 1 in Hex
5. Close the registry editor.
6. Restart the Computer.
7. Open Outlook and go to "Tools -> Recover Deleted Items".