A serious issue with the Exchange Online service, flagged as EX1137017, is currently under investigation by Microsoft.
Due to this issue, a number of users are unable to send or receive email messages through the Outlook mobile app.
This issue primarily affects those using Hybrid Modern Authentication (HMA), which is often used by organizations that combine internal Exchange servers with Exchange Online.
The outage prevented affected mobile users from accessing new messages or sending outgoing mail from their devices, causing significant communication delays for those who depend on mobile access. The outage is due to a faulty build update recently pushed to production.
Microsoft’s preliminary root cause analysis indicates that the new build introduced a critical error.
This exception improperly triggers a quarantine state for the mail synchronization process. As a result, the process responsible for synchronizing incoming and outgoing mail is suspended for a full 12 hours, effectively interrupting email flow to the user’s mobile app.
Microsoft engineering teams have identified the cause and developed a fix. As of the last update, Friday, August 22, 2025, at 8:50 AM, this fix was being implemented on the affected infrastructure.
The company stated that it is actively monitoring system saturation to ensure the issue is resolved without further complications. However, a definitive timeline for a full resolution has not yet been provided.
The impact is limited to some users of the Outlook mobile app in a hybrid environment. The desktop and web versions of Outlook do not appear to be affected by this specific incident.
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