From their Twitter: https://twitter.com/MSFT365Status/status/1392208966231355392
We’re investigating an issue with email message visibility in Outlook. Outlook on the web appears to be unaffected. More details can be found under EX255650 in the admin center.
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.
The deployment of our fix is progressing as expected and we continue to anticipate that it'll complete on Wednesday, May 12, 2021 by 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