I've had 3x clients with long term XTRA email accounts suddenly affected by this situation when sending email from Outlook 2010, and in one case from Apple Mail. These have been emails sent to known contacts, with perfectly legitimate subject and body content. As soon as the messages are sent from their email clients, immediately they receive a message in their inbox with the following Subject: Undeliverable (subject line)

The body is as follows: 

Your message did not reach some or all of the intended recipients.
      Subject:    RE: (subject)
      Sent: 13/04/2021 8:37 p.m.
The following recipient(s) cannot be reached:
      '(Recipient)' on 13/04/2021 8:37 p.m.
            554 5.7.1 Message rejected due to possible spam content

Messages can be successfully sent to other XTRA addresses, and to themselves, but pretty much nowhere else. In one case, one of my clients has 2x computers using the same XTRA address and both using Outlook 2010. One was able to send successfully, and the other couldn't. Careful comparison found that the one that was successful had their sender name set in Outlook as "Dr "First Name" "Second Name". The one that failed had their sender name as "First Name" "Second Name" (without the Dr. in front). This was tested back and forth with consistent results. How crazy is this?  Has XTRA's spam filter been set to ridiculous levels or is it a learning AI system with a bad attitude? I've recommended they get proper domain addresses, but this has fallen on deaf ears. 

Has anyone else been affected by this, and what happened?