When sending outbound through Email Security .cloud, an email sent to one or more recipients outside your organization fails to arrive to the intended recipients.
- No Non Delivery Report (NDR)
- Within the Client portal, Email Track and Trace does not show a delivery attempt to the target recipient
The ReturnPath/Envelope Sender of the email for the DL is a Null Sender by default in Office 365.
The "Sender" in your Office 365 report is simply a "Body From" and is not the SMTP sending address.
As a result Symantec .cloud servers cannot verify that the email is legitimately from a customer, so they respond with a "trying to use me as a relay" message.
Microsoft articles related to this issue.