Microsoft 365 Alert – Service Degradation – Exchange Online – Users may have been unable to open the new Outlook desktop client app on Windows devices – RESOLVED

Announcements Sourav Nandi

20/08/2025 08:35:00 AM

NHSmail Reference: INC46608521

Microsoft Reference: EX1137908

Issue Status: RESOLVED

Issue Description: Users may have been unable to open the new Outlook desktop client app on Windows devices.

More info: As a workaround, affected users could’ve used the Outlook classic desktop client.

Final Update: 20/08/2025 08:35:00 AM – Microsoft have successfully reverted the service to the last known healthy build to remediate the impact and confirmed from telemetry that this issue is no longer occurring.

Scope of impact: Approximately five percent of users attempting to open the new Outlook desktop client on Windows devices may have been impacted.

Root cause: A recent deployment intended to optimize services dependent in the affected scenario caused crashes to occur on these services, leading to the impact.

Next update by: Microsoft are reviewing their validation and updating procedures to better detect and prevent issues like this prior to deployment in the future.