The following is from the Microsoft Office 365 admin center regarding yesterday's issue with Outlook:
Title: Issue affecting viewing email content in Outlook
User Impact: Users may have been unable to view email message content within Outlook.
More info: Impact was specific to the Outlook client and users with access to other protocols, such as Outlook on the web or the Outlook mobile app, could view message content in those platforms as a potential workaround while our fix was applied.
Final status: We've confirmed that the fix successfully resolved the issue, though some users will be required to restart their email client for the change to take effect. In some circumstances, a second restart may be necessary.
Scope of impact: This issue could have affected any user attempting to view an email message in the Outlook client.
Start time: Tuesday, May 11, 2021, 1:24 PM (6:24 PM UTC)
End time: Tuesday, May 11, 2021, 10:00 PM (5/12/2021, 3:00 AM UTC)
Preliminary root cause: A recent change to systems that facilitate text display management for content within the Outlook client caused impact. A full root cause analysis is being performed and additional details will be provided in the PIR.
- We're performing an extensive review of our coding to understand how the issue occurred and why it was missed during the testing and early deployment cycle.
We'll publish a post-incident report within five business days.