Microsoft releases emergency OOB update to fix Outlook freezes

Microsoft has released emergency out-of-band (OOB) updates for Windows 10, Windows 11, and Windows Server systems to address a critical issue that prevents Microsoft Outlook classic from opening when using PST files stored in cloud storage. The problem, which emerged after the January 2026 Patch Tuesday updates, causes Outlook to freeze and become unresponsive when accessing PST files stored on services like OneDrive or Dropbox.

The Core Problem

A Microsoft Outlook PST file is a data file used by the application to store a user's email and other data locally on their computer instead of on the mail server. In enterprise environments, these files are commonly used to access mail when offline and back up important messages. The issue specifically affects classic Outlook, which is typically tied to enterprise licensing and not included with most home installations of Windows.

Microsoft explains the technical details: "After installing Windows updates released on or after January 13, 2026 (the Originating KBs listed above), some applications might become unresponsive or experience unexpected errors when opening files from or saving files to cloud-backed storage, such as OneDrive or Dropbox."

The symptoms are particularly disruptive for daily workflow:

• Outlook becomes unresponsive when opening the application
• The application fails to reopen unless its process is terminated in Task Manager or the system is restarted
• Sent emails might not appear in the Sent Items folder
• Previously downloaded emails might be downloaded again

Enterprise Impact and Scope

This issue primarily affects enterprise environments where classic Outlook is the standard. The problem occurs in specific configurations where PST files are stored on cloud storage services. While home users typically don't encounter this issue, IT administrators managing large deployments have reported significant productivity disruptions.

The timing is particularly problematic as it affects systems immediately after applying the standard January 2026 security updates, creating a conflict between security compliance and application functionality.

Emergency Out-of-Band Updates Released

To address this critical issue, Microsoft released the following out-of-band updates on Saturday, January 24, 2026:

• Windows 11 25H2 and 24H2: KB5078127
• Windows 11 23H2: KB5078132
• Windows 10 23H2 and 22H2: KB5078129
• Windows Server 2022: KB5078136
• Windows Server 2025: KB5078135
• Windows Server 2019: KB5078131

Installation and Deployment Guidance

Affected users and administrators have several options for deploying these fixes:

Immediate Installation: For those experiencing the Outlook freeze issue, the updates can be installed immediately through:

1. Windows Update (if the updates have been pushed to your organization's update channels)
2. The Microsoft Update Catalog using the direct links above

Strategic Consideration: Microsoft notes that if these issues do not affect your devices, there is no need to install the out-of-band updates immediately. Instead, administrators can wait for the upcoming preview update or next month's Patch Tuesday, when the fixes will be rolled out as part of the regular update schedule. This approach helps minimize unnecessary changes in stable environments.

Related OOB Updates

Last Saturday, Microsoft also released OOB updates for two additional bugs:

1. Issues preventing access to Microsoft 365 Cloud PC sessions
2. Problems preventing some PCs with Secure Launch enabled from shutting down or entering hibernation

Microsoft states that these previous fixes are now included in today's updates, consolidating the emergency patches into a single installation package for affected systems.

Technical Context and Broader Pattern

This incident highlights a recurring challenge in enterprise IT management: the balance between security updates and application compatibility. PST files stored in cloud storage represent a specific configuration that became problematic after the January 2026 updates. The issue likely stems from changes in file system handling, cloud storage integration, or security hardening that inadvertently affected how Outlook interacts with these files.

For IT administrators, this situation reinforces the importance of:

• Testing updates in non-production environments before broad deployment
• Maintaining awareness of specific application configurations in their environment
• Having rollback procedures ready for emergency situations

Practical Recommendations

For affected users:

1. Install the appropriate OOB update for your Windows version
2. Verify Outlook functionality after installation
3. Monitor for any additional issues with PST file access

For IT administrators:

1. Assess whether your environment is affected by this specific configuration
2. Consider the deployment strategy: immediate installation vs. waiting for regular updates
3. Communicate the issue and solution to affected users
4. Review PST file storage policies to consider alternative storage locations if cloud storage continues to cause issues

For future prevention:

1. Maintain test environments that mirror production configurations
2. Document application dependencies on specific file storage configurations
3. Establish clear communication channels with Microsoft for enterprise support

Looking Ahead

Microsoft will likely include these fixes in the next regular Patch Tuesday cycle, reducing the need for emergency updates in the future. However, this incident serves as a reminder that even well-tested updates can have unforeseen impacts on specific configurations, particularly in complex enterprise environments with legacy application dependencies.

The company's decision to release an out-of-band update demonstrates the severity of the issue for affected users, prioritizing productivity over the standard update cadence. For organizations that haven't yet deployed the January 2026 updates, this presents an opportunity to apply both the security updates and the fix simultaneously, avoiding the freeze issue entirely.

Additional Resources

For more information about Outlook configuration and feature comparisons:

• Feature comparison between new Outlook and classic Outlook
• Microsoft Update Catalog for manual download of KB updates
• Windows 10 update history
• Windows 11 update history

This emergency update cycle underscores the importance of maintaining flexible update strategies in enterprise environments, where the impact of security updates must be weighed against application functionality and user productivity.