Emails sent from the upstream cloud email provider are not accepted for delivery to the DLP Cloud Email Service.
DLP Cloud Service for Email can work with either Microsoft® Office 365® Exchange Online and Google Gmail™ for Work (although it can only be configured for one or the other - not both at once). Presumably, this issue can also happen for either setup.
"Google tried to deliver your message, but it was rejected by the server for the recipient domain"
In this case, the mail route to the DLP Cloud Email Service had been misconfigured in the Google Gmail™ for Work setup.
It is necessary that the port specified for the DLP Cloud Service to receive messages should be 25.
Note, there is a related issue, for DLP Cloud Service and Prevent customers who forward their messages to Email Security.cloud - for a technical article on that issue, see link to TECH236455 in related articles.
The IP address for routing to DLP Cloud Email should be provided in the "Welcome Email" sent to customers who have successfully applied for server provisioning.
These details should be part of the "mail route": Settings for Gmail > Advanced settings:
<Customer-Cloud-Detector-ID> : 25
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