In previous versions of Outlook you had to do a registry edit to enable this functionality but this is no longer needed in Outlook 2010, except if you are going to recover deleted items from a shared mailbox.
In Office 365 / Exchange Online its more common to create shared mailboxes than ever before since they are essentially free of charge (for up to 5 GB).

These accounts don’t have a username and password so the only way to access them are via the open other mailbox option in OWA or to add it as a secondary mailbox in Outlook.

And in OWA there is no option to mark every item, so let’s say you have 20.000 deleted items that you want to recover, well then this is not an option.
And if you have added it as a secondary mailbox in your outlook 2007/2010 then you will not see recover deleted items for shared mailbox folders, unless you do the old registry edit.

To enable the recover delete item button for shared mailboxes you need to enable a registry setting manually.

1. Start the registry editor
   
2. Locate and then click the following key in the registry:
   HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Exchange\Client\Options
   

   (If there is no Option folder, just create it)
   

3. On the Edit menu, click Add Value, and then add the following DWORD value:
   DWORD name: DumpsterAlwaysOn
   Data type: DWORD
   Value data: 1
   
4. Close the registry editor
   

Now restart Outlook and the option to recover deleted items is now there.

*This post would have had picture in it, but 20.000 items takes like 30 minutes to recover and it kinds of locks Outlook so this is the only picture I could get