Check out this:
http://email.about.com/od/outlooktips/qt/How_to_View_the_Complete_Message_Source_in_Outlook.htm
For the sake of not lose the information if the page is down:
Type “regedit”.
Hit Enter.
Go to HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Outlook\Options\Mail.
Select Edit | New | DWord from the menu.
Type “SaveAllMIMENotJustHeaders”.
Hit Enter.
Double-click the newly created SaveAllMIMENotJustHeaders value.
Type “1”.
Click OK.
Close the registry editor.
Restart Outlook if it has been running.
The source of the message will appear under View → Options → internet headers.