From their Twitter: https://twitter.com/MSFT365Status/status/1392208966231355392
We’re investigating available data to isolate the root cause. Users with access to alternate protocols like Outlook on the web or Outlook mobile can use those as a potential workaround in the interim. More details can be found under EX255650 in the admin center.
We’ve identified the root cause and are applying a fix, which will reach affected users over the next 3 to 4 hours. Users will need to restart the Outlook client to apply the fix after it's received. We expect to restore service to affected users by May 12, 2021, at 3:00 AM UTC.
If you're having issues, there is a work around by running Outlook in Safe mode. More details here: https://www.bleepingcomputer.com/news/microsoft/microsoft-outlook-bug-prevents-viewing-or-creating-email-worldwide/
Tuesday, May 11, 2021