Exchange is hybrid o365/on-premises (2016)
From some applications we're getting issues sending (anonymous) emails, specifically 550 5.7.4 "Unable to relay recipient in non-accepted domain".
So, following an article by Paul Cunningham
we created a new Receive Connector called it 'Outbound Anonymous SMTP Relay', listing my PC's IP address as a permitted server, with 'Anonymous Users' ticked under 'Permission Groups'.
This has not changed the behaviour, but looking at the logs (following a telnet test) the test email attempt is not being processed by the new Receive Connector, instead it's being picked up by the 'Default Frontend' connector.