Outlook Sending From Shared Mailbox Sent Items Not Going To Shared Sent Items

If using a shared mailbox and you want the sent items to go into the shared sent items the easiest way is to add a registry item

Go to the following:

HKEY_CURRENT_USER\Software\Microsoft\Office\x.0\Outlook\Preferences

Add a DWORD  DelegateSentItemsStyle and give it a value of 1

Got this from here.