Microsoft is investigating a significant service incident within Exchange Online, identified as EX1137017, which is preventing some users from sending or receiving emails through the Outlook mobile application.

The issue, which remains ongoing, specifically impacts customers utilizing Hybrid Modern Authentication (HMA), a common configuration for organizations that integrate on-premises Exchange servers with Exchange Online.

The disruption has left affected mobile users unable to access new messages or send outbound mail from their devices, causing considerable communication delays for those reliant on mobile access. The outage stems from a flawed build update recently pushed to the production environment.

According to Microsoft’s preliminary root cause analysis, the new build introduced a critical bug. When the system encounters a typical, temporary network failure an event usually handled gracefully it now generates an unexpected exception for a subset of users.

This exception improperly triggers a quarantine state for the mail synchronization job. As a result, the process responsible for syncing both inbound and outbound mail is suspended for a full 12-hour interval, effectively halting email flow to the user’s mobile app.

Microsoft’s engineering teams have identified the cause and developed a fix. As of their last update on Friday, August 22, 2025, at 8:50 AM, the deployment of this fix was in progress across the affected infrastructure.

The company stated it is actively monitoring the deployment’s saturation to ensure it resolves the issue without introducing further complications. However, a definitive timeline for full resolution has not yet been provided.

The scope of the impact is limited to some users on the Outlook mobile app within a hybrid environment. Desktop and web versions of Outlook do not appear to be affected by this specific incident.

This distinction highlights the mobile-centric nature of the bug, which targets the mail sync mechanism essential for on-the-go device connectivity.

Administrators and affected users are advised to monitor the Microsoft 365 Service Health Dashboard for updates regarding EX1137017.

Microsoft has committed to providing its next major update on the situation by Monday, August 25, 2025, at 9:00 PM UTC, by which time they hope to have more clarity on the deployment’s progress and a timeline for complete service restoration.

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