5.7.1 smtp; 550 5.7.1 Unable to relay
This is caused when SMG does not have an explicit delivery route for the non-local domain.
By default, SMG always routes email in the inbound queue to a local server. When the recipient is changed to the non-local mailbox, the SMG does not have a hardset routing rule, and attempts to deliver the mail locally, which fails. The local Exchange server does not accept email for the non-local domain, and blocks the email relay.
This will make sure that any email that passes into the Delivery queue for the non-local domain gets routed correctly. This setting will override the default inbound routing for the non-local domain.