Hello Tom,
The Symantec Cloud service provides client with tools to track any messages that is scanned and treated by the service. While I can understand the challenges faced when communicating email delivery issues to the recipient, Symantec Cloud clients are fully equipped to review any issues pertaining to receiving emails and the Symantec.Cloud service is largely client managed.
Additionally, the Symantec Cloud service, if it does block emails would generate an NDR back to the sender. If your logs shows the cluster accepting and responding with a 250 OK, then the email was passed on for delivery. What happens to the email then becomes fully visible to the client and this information is private to that account and to the client.
Contacting the recipient, you can also reach out to support.cloud@symantec.com and explain the issue you are facing and providing all relevant logs, information, emails details etc that is pertinent to the issue you are facing. In some circumstances the support team may be able to provide feedback on what is ocurring, but they are also under obligation to respect their client's privacy.