Microsoft Addresses Outlook Freezes After Windows Update: Workarounds and Technical Context

Microsoft has acknowledged a significant issue affecting the classic Outlook desktop client following the installation of this month's Windows security updates. The problem manifests as severe freezes and hangs, particularly for users with POP email accounts who have deployed the KB5074109 security update on Windows 11 25H2 and 24H2 systems. The company has shared temporary workarounds while its Windows and Outlook teams investigate a permanent solution.

The Core Problem: How Security Updates Interact with Email and Cloud Storage

The issue stems from a conflict between the security update and Outlook's file handling mechanisms. According to Microsoft's Windows health dashboard, the bug causes any application to "become unresponsive or experience unexpected errors when opening files from or saving files to cloud-backed storage, such as OneDrive or Dropbox."

For Outlook specifically, this creates a cascade of problems:

1. Application Hangs: Outlook becomes unresponsive and fails to reopen unless its process is terminated in Task Manager or the system is restarted.
2. Email Redownloading: Previously downloaded emails may be downloaded again, creating duplicate entries and confusion.
3. Sent Items Missing: Sent emails might not appear in the Sent Items folder, even though they were successfully transmitted.
4. POP Account Specificity: The issue is particularly pronounced for POP email accounts, which download emails to local storage rather than leaving them on the server.

The technical root appears to be related to how the security update modifies file I/O operations, particularly when applications interact with cloud-synced folders. When Outlook attempts to access or save PST files (the local email database) stored in OneDrive, the modified file handling routines create deadlocks or infinite loops, causing the application to freeze.

Affected Platforms and Scope

Microsoft has confirmed the issue affects multiple Windows versions:

• Windows 11: 25H2 and 24H2 editions
• Windows 10: All versions
• Windows Server: 2025, 2022, and 2019

The problem is not limited to Outlook. Any application that frequently reads from and writes to cloud-backed storage may experience similar issues. However, Outlook is particularly vulnerable because:

• It maintains constant access to PST files for email indexing and search
• It performs frequent writes for email synchronization
• Many users store PST files in OneDrive for backup and cross-device access

Microsoft's Temporary Workarounds

Until a permanent fix is available, Microsoft recommends several approaches:

Option 1: Move PST Files Out of Cloud Storage

The most straightforward solution is to move Outlook PST files out of OneDrive or other cloud-backed folders. This prevents the file system conflict entirely.

Steps to implement:

1. Close Outlook completely
2. Navigate to your OneDrive folder and locate the PST file (typically in OneDrive\Documents\Outlook Files\)
3. Copy the PST file to a local folder (e.g., C:\Users\[Username]\Documents\Outlook Files\)
4. In Outlook, go to File > Account Settings > Data Files
5. Remove the existing PST file location and add the new local location
6. Restart Outlook

Option 2: Use Webmail

For users who need immediate access to email without modifying their local setup:

• Access Outlook Web App (OWA) at outlook.office.com
• Use the web interface for sending/receiving emails
• This bypasses the local client entirely

Option 3: Uninstall Problematic Updates (With Security Caveats)

Microsoft provides the option to uninstall the problematic updates, but warns this leaves devices vulnerable:

1. Open Settings
2. Navigate to Windows Update > Update history
3. Click Uninstall updates
4. Find KB5074109 or KB5073724 and click Uninstall

Important Security Warning: Removing security updates exposes your system to vulnerabilities that are actively being exploited in the wild. This should only be considered a last resort for critical business operations, and devices should be reconnected to the internet only after the updates are reinstalled or a fix is applied.

Technical Analysis: Why This Happens

The issue highlights a common challenge in modern computing: the interaction between security hardening and application compatibility. Security updates often modify low-level system APIs to prevent exploitation, but these changes can break applications that rely on specific behaviors.

In this case, the security update likely modifies how Windows handles file operations in cloud-synced folders. When an application like Outlook repeatedly accesses a file (as it does with PST databases), the new security measures may create a deadlock situation:

1. Outlook requests file access
2. The security update adds additional validation checks
3. Cloud sync services (OneDrive) simultaneously attempt to sync the file
4. The combination creates a race condition or resource contention
5. The application hangs waiting for file access that never completes

This type of issue is particularly difficult to diagnose because:

• It only manifests in specific configurations (cloud-synced PST files)
• The symptoms appear in the application layer (Outlook) but originate in the system layer
• Multiple Microsoft teams must coordinate to resolve it

Broader Context: Recent Windows Update Issues

This is not the first time Windows updates have caused Outlook-related problems. Microsoft's update history shows a pattern of issues:

• December 2025: Security updates caused Message Queuing failures
• January 2026: Updates caused some PCs to fail to shut down properly
• Earlier: Updates caused credential prompt failures for remote connections

This pattern underscores the complexity of maintaining Windows' security posture while ensuring application compatibility across millions of configurations. Each update must balance:

• Security: Patching vulnerabilities before they're exploited
• Compatibility: Ensuring existing applications continue to work
• Performance: Maintaining system responsiveness
• Reliability: Preventing crashes and data loss

Recommendations for IT Administrators

For organizations managing multiple Windows systems:

1. Assess Impact: Identify users with POP accounts and cloud-synced PST files
2. Prioritize Workarounds: Implement PST file relocation for affected users
3. Monitor for Fixes: Watch for Microsoft's permanent update release
4. Communicate: Inform users about the issue and available workarounds
5. Consider Alternatives: Evaluate migration to Exchange Online or IMAP accounts where possible

Looking Ahead: Microsoft's Response Timeline

Microsoft has not provided a specific timeline for a permanent fix. The company's Windows and Outlook teams are actively investigating, but the complexity of the issue—requiring coordination between multiple product teams—suggests a solution may take several weeks to develop and test thoroughly.

In the meantime, affected users should:

• Implement one of the temporary workarounds
• Avoid storing PST files in cloud-synced folders
• Monitor the Windows health dashboard for updates
• Consider using webmail for critical email operations

The situation serves as a reminder of the delicate balance between security and usability in modern operating systems. As Microsoft continues to harden Windows against evolving threats, users and administrators must remain vigilant about update impacts and maintain backup plans for critical workflows.

For the latest updates on this issue, users can monitor Microsoft's official channels and the Windows release health dashboard.