Microsoft is reporting some users are unable to view the content of emails in the Outlook client. Outlook on the web and Outlook mobile do not appear to be affected at this time.
For access to messages during this issue, Outlook on the web can be reached at https://outlook.office.com
Update May 11 10:00pm:
Microsoft has identified the cause of the problem and are deploying a fix. This fix should take 4-5 hours to reach all users.
Affected users will need to restart Outlook to apply the fix. In some cases users will need to restart Outlook a second time for the changes to take effect.
Update May 12 9:00am:
This issue is now resolved. Users should restart their Outlook client to apply the fix.