Everyone knows – or should know – that you can click on the recycle bin in outlook and then recover deleted emails. Unfortunately if you have a shared mailbox, Outlook does not allow (by default) the recovery of deleted messages. Welcome registry edit!

http://go.microsoft.com/?linkid=9643538
That’ll fix it for you.

Or you can add it manually:
Start Registry Editor.
Locate and then click the following key in the registry:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Exchange\Client\Options
On the Edit menu, click Add Value, and then add the following registry value:
Value name: DumpsterAlwaysOn
Data type: DWORD
Value data: 1

Looks like it’s in the HKLM\SOFTWARE\Wow6432Node\Microsoft\Exchange\Client\Options area now.
If options does not exist, just create a new key, then create the dword below
“DumpsterAlwaysOn”=dword:00000001