Microsoft provided RCA regarding the issue under EX255429 in Microsoft 365 admin center.
Here’s the summary:
Title: Some users may have been unable to send and receive external email messages within the Exchange Online service User
Impact: Users may have been unable to send and receive external email messages within the Exchange Online service.
Final status: We've confirmed that cache has finished updating and the affected infrastructure has returned to service. After further monitoring, we've confirmed that this has resolved the issue.
Scope of impact: Impact was specific to users who were served through the affected infrastructure located in Europe.
Start time: Monday, May 10, 2021, 8:40 AM (6:40 AM UTC)
End time: Monday, May 10, 2021, 11:40 AM (9:40 AM UTC)
Root cause: A section of infrastructure, responsible for connecting email message attributes to end users, was not performing as expected due to the associated cache not fully updating.
Next steps: - We're reviewing our caching infrastructure to find ways to prevent this problem from happening again. This is the final update for the event.