Outlook Mobile Sync Issues Disrupt Users as Microsoft Deploys Fix

Outlook Mobile Sync Issues are disrupting email access for users on Hybrid Modern Authentication. Microsoft confirmed the problem and is deploying a permanent fix.

The Problem

The issue began on August 17, 2025. Outlook mobile users with on-premises mailboxes routed through HMA faced major disruptions. Sync jobs were quarantined after temporary failures, forcing delays of up to 12 hours. This caused email and calendar updates to stall, leaving many accounts unreliable.

Microsoft linked the disruption to a service change designed to improve sync efficiency. Instead of optimizing performance, the update introduced a flaw that placed healthy sync jobs in quarantine. Users saw delayed delivery, missing updates, and mailbox instability.

Microsoft’s Response

Microsoft engineers quickly investigated the root cause. A fix has now been deployed to stop quarantine behavior. Sync jobs will continue running even after minor failures, reducing delays and improving reliability. A previous attempt to shorten delays to one hour was unsuccessful, prompting this broader change.

User Impact

• Affected users: Outlook mobile accounts using Hybrid Modern Authentication.
• Main symptom: 12-hour delays for email and calendar synchronization.
• Microsoft has not disclosed affected regions or user counts.
• Updates are available through the Microsoft 365 Admin Center under incident ID EX1137017.

Related Issues

Microsoft is also addressing separate issues. A Teams desktop bug is causing blank screens during meetings on systems running specific Intel drivers. Users should switch temporarily to the web version of Teams as a workaround.

Other recent disruptions include Outlook client crashes and OTP delivery failures in Exchange Online, highlighting ongoing reliability challenges across Microsoft’s services.

Conclusion

Outlook Mobile Sync Issues highlight how small configuration changes can create large disruptions. Microsoft has deployed a fix and is monitoring its impact. Users depending on Hybrid Modern Authentication should expect improved stability, but continued updates may follow as Microsoft ensures full resolution.