Some users were receiving large amounts of spam messages from specific senders in Exchange Online

Title: Some users were receiving large amounts of spam messages from specific senders in Exchange Online

User impact: Users were receiving large amounts of spam messages from specific senders in Exchange Online.

More info: This issue only affected a very specific group of organizations, and your tenancy may not have been affected. Mail may have originated from non-legitimate Microsoft domains.

Final status: We’ve confirmed via an extended period of monitoring that the issue has been resolved after the implementation of our fix.

Scope of impact: Some of your users may have received large amounts of spam messages from potentially fraudulent senders in Exchange Online.

Start Time: Tuesday, September 19, 2023, at 11:26 PM UTC

Preliminary root cause: Our automated Anti-Spam throttling and blocking services were not properly being triggered due to various reasons in logic. A long-term focus remains on this issue as a whole to better understand the cause and methods to address the sources of impact.

End Time: Thursday, September 21, 2023, at 9:53 PM UTC

Next steps:
-We’re reviewing our mitigation steps to reduce resolution time for similar events in the future.
-We’re analyzing our automated services to better improve their efficacy.

A post-incident report will published within five business days.