2:00pm Pacific — Beginning earlier today, we have seen that Office 365 is deferring some email messages we are delivering from our mail handling servers.  We have inquired with Microsoft as to the cause of the deferral, and are waiting for a response from them.  In the meantime, customers who host their email at Office 365 and are impacted by this issue may wish to configure their Office 365 accounts to bypass IP address-based filters, according to these instructions:

https://community.office365.com/en-us/f/148/t/222563

Note that we generally recommend that our inbound filtering customers disable any secondary IP-based spam filters within their mail hosting infrastructure, whether that is on-premise or hosted externally at a provider like Office 365.  (The IP-based filtering on the customer mail server will not provide any benefit given that all of the inbound mail will be delivered from our systems rather than directly from the original sending IP addresses.)