Microsoft has 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 service to be restored by 11 p.m.
Posted May 11, 2021 - 19:18 EDT
Microsoft is investigating an issue with email message visibility in Outlook. Outlook on the web and Outlook Mobile appear to be unaffected.