Thank you for your response - it turned out to be 'none of the above'. It had been working quite well for some time, however it was setup using a shared mailbox. Apparently, I got away for free for a while, until MS did a sweep. What I missed during setup was the following key line in the MS documentation: Mailbox: You must have a licensed Office 365 mailbox to send email from.
I didn't see that line, and it worked perfectly for quite a while, so I never gave it another thought. So the fix is to make this mailbox a paid licence. I confirmed that it works using a different paid license mailbox.
The Microsoft document is here: https://docs.microsoft.com/en-us/ex...submission