This problem is now definitively resolved and has been since 1PM AEDT yesterday.
No further blockages have appeared, and all mail has been cleared from the queue since around 3:45PM yesterday.
If you encounter any issues with users claiming not to have received mail, or unexpected bounces from known valid email addresses, please contact Support.
Posted Nov 09, 2021 - 10:43 AEDT
Monitoring
The problem is now largely resolved.
We have confirmed that this does not affect Office365 domains, only Microsoft's free public email services.
We have also lodged a request with Microsoft to clear the block definitively, but since that communication, emails to the services listed are now delivering.
You are clear to re-deliver any emails to these domains and they should arrive without issue.
We will monitor this for the next hour before marking it resolved.
Posted Nov 08, 2021 - 12:54 AEDT
Identified
Despite the blacklist entry resulting from client spam activity last week being removed, Microsoft have chosen to start blocking mail from our outbound servers.