Latest update from Microsoft is as follows. We recommend using the workarounds previously provided for the remainder of the day, if you continue to experience issues, please feel free to contact us.
Microsoft: "Current status: We’ve identified the underlying cause of impact and are applying a fix. This fix will reach all affected users incrementally over the course of the next four-to-five hours. Once users receive the fix, they will need to restart their email client to apply the fix. In some circumstances, users may need to restart their client a second time for the changes to take effect. We expect to complete this process and restore service for all affected users by May 12, 2021, at 3:00 AM UTC.
We encourage affected users who are able to do so to leverage the workarounds described above in the "more info" section of this post while we complete the process of fixing this problem."