The message is not delivered with bounce back error:
554 5. 7. 1. Message rejected due to local policy. Please visit https://support.apple.com/en-us/HT204137
Email security cloud
The issue is caused by iCloud settings
The issue isn't generated by Email security cloud but by the recipient side. The local policy of the recipient set on Apple cloud rejects the message delivery. ESS doesn't block the email but released for delivery after checking.
It is highly recommend to open Apple ticket support and check with them the reason of the issue as it is 3rd party problem.