UPDATE: Deferrals from Office365

10:45AM Pacific — Microsoft’s Product Engineering team has confirmed that this issue should now be resolved in their systems.  This is consistent with our own testing, which shows immediate, undeferred acceptance of new messages from our previously impacted mail handlers to Office365 servers.  Mail which is currently queued in our system as a result of this problem should be transmitted shortly as our servers retry delivery.

Microsoft has not been able to provide any information regarding a definitive cause for this issue, but we are continuing to press them for details, as well as assurances that this will not recur.