Sentinel Messaging does not currently support the outbound relaying of mail to external domain recipients in a distribution group being sent to. An external recipient consists of an email address that is not part of one of the customer’s listed domains.
You may experience also bounceback messages containing “Relay Access Denied” with the external recipients address listed in the bounceback:
mx1-eu1.ppe-hosted.com #554 5.7.1 firstname.lastname@example.org>: Relay access denied ##”
Such mail sent to a distribution group with external domain’s need to be routed through a different outbound mail route, as Sentinel Messaging smarthost will give the “Relay Access Denied” error. Contact your mail server’s routing guide/expert to set this up correctly.