Mail notifications to external email addresses aren't sent. Error 550 (5.7.1) unable to relay occurs.

Document ID : KB000049645
Last Modified Date : 14/02/2018
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If you are running Exchange Server, then on the Relay properties of the Exchange Server Receive Connector Properties, click on the Authentication tab and select the EXTERNALLY SECURED checkbox. Save the exchange server settings. This will relay email notifications to the external email addresses.