Microsoft Exchange Online experiencing intermittent mail flow issues

This incident has been resolved.

We have begun reprocessing and redelivering emails that bounced during the affected time period. Here is what to expect:

Most emails should be delivered, but not all of them. Some bounces during the incident may have been legitimate, and those messages will not be recoverable through reprocessing.

Duplicates are possible. If a sender resent their email after the incident and the original message is also redelivered through our reprocessing, recipients may receive both. This is expected behavior and not a cause for concern.

Reprocessing takes time. We expect this to be completed by midnight ET tonight, though it may finish sooner.

Earlier today, Microsoft identified some of Shield's services as suspicious or compromised. This caused emails to bounce or be delayed. As of approximately 2:30 PM ET, the status began to lift, and we observed a return to expected mail flow.

A support request has been opened with Microsoft to help us determine how and why this incident happened. We have not received a response yet. We are not closing this issue as resolved. Our intention is to get to the root of the incident.

Our first priority was to find a way to resume normal mail flow, and it appears we have done so. We are continuing to monitor the situation, including the Microsoft notice regarding mail flow issues in Exchange Online, which appear to have been resolved.

Microsoft Defender is restricting connectors due to false-positive reports. Please go to Microsoft Defender > Restricted Entities and unblock the listed connectors. (https://security.microsoft.com/restrictedentities)

You will need to monitor and check on this restriction for the time being. We have observed that Microsoft restricts the connector after a short time, and the unblock request must be made again.

We are continuing to monitor and investigate possible mitigation options.

Microsoft is experiencing intermittent issues with mail flow. Please see their notice for reference.
https://admin.cloud.microsoft/#/servicehealth/:/alerts/EX1331830

Mailprotector is monitoring the situation. However, mail flow with CloudFilter and Shield appears to be normal. Please check logs to identify bounces or delays from Microsoft hosts to confirm potential effects from their systems.