The issue appears to have commenced around 2pm today AEDT. It appears to be an intermittent issue between MPmail and Office 365. This is not impacting MPexchange or other MPmail tenants. Messages that are deferred will retry and deliver once the issue has been resolved. These are visible in the MPmail portal.
Messages are also accessible through individual users "archive" tab in MPmail or through message continuity at: https://webmail.mpmail.com.au/
We have escalated this to both MS & MPmail ops centres.