User impact: Users may have experienced a delay in sending and receiving email messages from external domains in Exchange Online.

More info: Users may have experienced intermittent “Server busy” errors, affecting multiple mailboxes.

Depending on the client, the service should attempt to periodically retry sending the email previously impacted by the issue. If the initial sender received an NDR after 24-72 hours (different clients or email providers can vary), they will need to send a new email.

Final status: We’ve confirmed that the issue is resolved after an extended period of monitoring, and receiving additional confirmations from previously impacted organizations.

Scope of impact: This issue impacted users who may have been attempting to send and receive email messages from external domains in Exchange Online.

Start time: Friday, September 4, 2026, at 9:00 AM UTC

End time: Friday, September 4, 2026, at 3:15 PM UTC

Preliminary root cause: The enforcement of a specific anti-spam model contributed to impact for a subset of email traffic. We’ll provide more details within the Post-Incident Report.

Next Steps:
– We’re reviewing model classifications, rule configurations and upstream data pipelines to determine why the anti-spam model started to impact a subset of email traffic.

We’ll provide a preliminary Post-Incident Report within two business days and a final Post-Incident Report within five business days.