I do this by making sure that the 'native' domain name of my SME is non-routing, e.g. clientdomain.local.
Now I can setup local email accounts to my heart's content, or I can create forwarding rules to forward to clientdomain.org without worrying about the SME swallowing the emails - since they are clearly intended for a different domain. Emails to anyone@clientdomain.org will go to the appropriate MX for clientdomain.org - no confustion!