Hello Technet Forms,
I have a requirement to block messages that are being sent to one or more domains on the same message. So as an example someone is trying to email mcdonalds.com but the message was accidently also addressed to a burgerking.com email. So DLP should block the message from sending. So far playing around with the DLP settings within Exchange 2013 it does not seem like this is possible at all. But I figured I would ask in case anyone else has tired this and gotten it to work.