Some users' email messages from on-premises Exchange servers to Exchange Online may intermittently fail
ID: EX1491694
Issue type: Incident
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Impacted services
Exchange Online
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Details
Title: Some users' email messages from on-premises Exchange servers to Exchange Online may intermittently fail
User impact: Users' email messages from on-premises Exchange servers to Exchange Online may have intermittently failed.
More info: Impacted users received the following error "Mail sent to the wrong Office 365 region. ATTR35."
Final status: We have stopped the service-side testing and reversed the changes it had made. We've confirmed with both service-side telemetry and multiple users' feedback that this issue is now resolved. We've stopped the service-side testing, reversed the changes it had made, and confirmed with service-side telemetry and user feedback that this issue is now resolved.
Scope of impact: Some users' email messages from on-premises Exchange servers to Exchange Online may have intermittently failed.
Start time: Thursday, October 8, 2026, at 10:06 AM UTC
End time: Thursday, October 8, 2026, at 4:05 PM UTC
Root cause: A routing service-side testing process created a Domain Name System (DNS) issue resulting in this impacting event.
Next steps:
-We're reviewing our service side testing processes and checks to ensure that impact is avoided or to notify admins ahead of time to potential downtime periods so that preventative actions can be taken.
This is the final update for this event.
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