You are implementing DLP Cloud Service for Email with forwarding mode Gmail > Cloud Service for Email > Email Security.cloud, but there is no traffic and you see a response from the remote server error "550 5.7.1 Domain was not authorized" when testing email.
Component: DLP Cloud Service for Email, Email Security.cloud
Any sending domains not registered are rejected by the downstream MTA (Email Security.cloud for most customers). These domains need to be added to their account via the ClientNet Self Service Portal.
Add domain from which email is being sent is listed as an "Active Domain", under My Domains section in the Email Security.cloud portal.
Related Article ID: 170072: Emails rejected by DLP Cloud Service when sending messages from new domains (broadcom.com)