This issue is now resolved.
As of 4am AEST Microsoft de-listing the MPmail IP that was being blocked, and causing issues for outbound email to hotmail being blocked.
Posted 6 months ago. Jul 22, 2017 - 07:35 AEST
We are aware of messages to Hotmail/Live/Outlook bouncing sending through the smarthost. We are in the process of having this de-listed and the IP removed from our sending pool. If you urgently have to send please contact firstname.lastname@example.org with your customers smarthost and we will route you through a different server.