Why do I get the error "You CANNOT create an email forwarder to a forbidden domain" when trying to add a forward to aol, yahoo, msn, or hotmail email account in cpanel?

Our terms of service do not allow auto forwards to hotmail, yahoo, aol, or msn email accounts. This is because those providers incorrectly mark any spam that we forward them as coming form our server and will in a very short amount of time block our servers because of the legitimately forwarded emails.