Add to Recipient Policy and you will still not be able to relay
Add New SMTP Connector
Add the Default Server
Address Space Tab Add Type: SMTP, Address: yourexternaldomain.com
Check the Box that says Allow message to be relayed to these domains
Test for Domain relay through command prompt for yourexternaldomain.com and another to make sure you are not a wide open relay.
A Case Study: The Relationship Between a Managed Service Provider and Their Customer
Longtime AngelCom customer, Palace Law, provides the inside scoop on what the customer/MSP relationship looks like from a business perspective.