I have a user that cannot forward a calendar appointment to his wife. He is using Outlook 2003 and we recently moved from SBS 2003 to SBS 2008. The error he receives is "You are not allowed to send this message because you are trying to send on behalf of another sender without permission to do so. Please verify that you are sending on behalf of the correct sender, or ask your system administrator to help you get the required permission.".
This has also happened a few times when he tried to set up a meeting and invited a few internal users.