Sending to distribution groups with external domain recipients
Spambrella 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:
Such mail sent to a distribution group with external domains needs to be routed through a different outbound mail route, as Spambrella smarthost will give the “Relay Access Denied” error. Contact your mail server’s routing guide/expert to set this up correctly.
If you are using Office 365, this Microsoft article will assist in creating outbound connectors to change mail routing