For the past day or so we have been getting errors on delivery to Microsoft properties (hotmail*, M365*, etc) with various error messages:
"Not authorized to relay messages through the server that reported this error"
"This message has not yet been delivered. Microsoft Exchange will continue to try delivering the message on your behalf. "
We have removed the IP addresses that Microsoft is no longer processing and we are working with them on determining what the issue is - the IP addresses are not on any RBLs so there is nothing more we are able to do other than to not deliver mail to Microsoft from IP addresses they are rejecting mail from.
As a result of these changes, there should be no issue after 9:30 AM EST today. We will update this ticket once we get more information from Microsoft.