I have Trend Micro Maximum Security (Version 17.8, although this problem has occurred for a while) on Windows 10, running Outlook as part of Microsoft Office Professional Plus 2021; that is, desktop, not Office 365.

Trend detects some spam itself - with some false positives too - and sends emails to the Spam Folder.

When I manually Flag spam from the Inbox, I get a message back the next time Outlook polls. Something like this:
From    System Administrator     No email information
To      Me                       No email information
Subject Undeliverable: FW: Tax Lien List
Your message did not reach some or all of the intended recipients.

      Subject:     FW: Tax Lien List
      Sent:  12/19/2024 9:08 AM

The following recipient(s) cannot be reached:

      spam@cs.trendmicro.com on 12/19/2024 9:08 AM
            550 extension.        Apparently this is the error message - blocking this message at space@cs.trendmicro.com  
 

Following a similar process for a false positive generates almost the same result - but for false@cs.trendmicro.com
There are no Email headers visible. This isn't just an Outlook quirk - it's the messages coming in this way.

I wondered if the problem was changed receiving email addresses at cs.trendmicro.com, in which case what are the correct addresses, and how could I change them. But that doesn't seem to be it. Composing and sending Manual emails to the two addresses hasn't generated the Undeliverable responses.

What's going on? Is there a Hot Fix?  Coming?
Thanks, 
Mike