Is that error being seen in an ndr from "mailer demon at Messagelabs.com"? If so then it is not a email security cloud error but one generated by the server that the service was attempting to deliver to. https://knowledge.broadcom.com/external/article/173431/550-571-message-rejected-as-spam-by-cont.html
if this is the case then the service is reporting what it is being told by the next hop and has no control over the text of the error
In general it is normal for DSN/ndr errors to be relatively high level. Customers can find full information as to why an email has been convicted or rejected in the track and trace logs.
Most MTAs keep their DSN messages at a high level, the DSN isn't the place to give forensic details on the disposition of an email for a number of reasons including giving attackers clues as to how to change their messages to beat threat scanning.
Original Message:
Sent: Jun 06, 2022 01:56 PM
From: Admin Strator
Subject: Why does Messagelabs not return a clear error message
Reason: 550 5.7.1 Message rejected as spam by Content Filtering.
This error message is meaningless and customers don't know where to search.