This incident has been resolved.
As of 16:15 UTC, we have applied a fix and new notifications are no longer getting blocked.
We are closely monitoring the email quarantine service and will provide update as soon as more information becomes available.
The issue has been identified and a fix is being implemented.
We are currently investigating an issue with EMEA instance of Symantec Email Quarantine service. Quarantine notification emails from Symantec Email Quarantine are incorrectly getting a Spam verdict. This issue is only impacting notification delivery and the core Quarantine view/release functionality remains fully operational.
We are handling this issue as a priority and we will provide next update or as soon as new information becomes available.