Microsoft 365 Exchange Online Outage Disrupts Email Access on Outlook Mobile App
Microsoft is currently 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. This ongoing issue specifically impacts customers utilising Hybrid Modern Authentication (HMA), a common configuration for organisations 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, resulting in considerable communication delays for those reliant on mobile access. The outage is attributed to 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 that generates an unexpected exception for a subset of users during typical, temporary network failures. This exception improperly triggers a quarantine state for the mail synchronisation job, suspending the process responsible for syncing both inbound and outbound mail for a full 12-hour interval.
Microsoft’s engineering teams have identified the cause and developed a fix, with deployment in progress as of their last update on Friday, August 22, 2025, at 8:50 AM. The company 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 impact is limited to some users on the Outlook mobile app within a hybrid environment, while desktop and web versions of Outlook remain unaffected. This highlights the mobile-centric nature of the bug, targeting 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.
Categories: Service Disruption, Hybrid Modern Authentication, Mobile Application Issues
Tags: Microsoft, Exchange Online, Incident, Outlook, Mobile Application, Hybrid Modern Authentication, Email, Synchronization, Bug, Service Restoration