MC1090702 – Update to Default blocked file types in Outlook web and new Outlook for Windows

Product:

Microsoft 365 Apps, Microsoft 365 for the web, Outlook

Platform:

Online, Web, World tenant

Status:

Change type:

Feature update, User impact, Admin impact

Links:

Details:

Summary:
Starting in early July 2025, Outlook Web and the New Outlook for Windows will block .library-ms and .search-ms file types. Most organizations will not be affected, but users needing these file types can add them to the AllowedFileTypes property. No action is required if these file types are not used.

Details:
As part of our ongoing efforts to enhance security in Outlook Web and the New Outlook for Windows, we're updating the default list of blocked file types in OwaMailboxPolicy. Starting in early July 2025, the following file types will be added to the BlockedFileTypes list:
.library-ms
.search-ms
[When this will happen:]
Rollout will begin in early July 2025.

Change Category:
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Scope:
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Release Phase:

Created:
2025-06-10

updated:
2025-06-10

Task Type

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Docu to Check

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MS How does it affect me

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MS Preperations

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MS Urgency

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MS workload name

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summary for non-techies**

Starting in July 2025, Outlook Web and the New Outlook for Windows will block .library-ms and .search-ms file types for security reasons, requiring organizations that use these files to update their security settings to allow them.

Direct effects for Operations**

Blocked File Types Impact
Users will be unable to open or download .library-ms and .search-ms file types in Outlook Web and New Outlook for Windows, leading to potential disruption in workflows for those who rely on these file types.
   - roles: End Users, IT Support
   - references: https://learn.microsoft.com/en-us/powershell/module/exchange/set-owamailboxpolicy?view=exchange-ps#-blockedfiletypes, https://support.microsoft.com/en-us/office/blocked-attachments-in-outlook-434752e1-02d3-4e90-9124-8b81e49a8519?preview=true

User Experience Degradation
Users who need to send or receive the newly blocked file types may experience frustration and confusion, impacting their overall experience with Outlook.
   - roles: End Users, Customer Service
   - references: https://learn.microsoft.com/en-us/powershell/module/exchange/set-owamailboxpolicy?view=exchange-ps#-blockedfiletypes, https://support.microsoft.com/en-us/office/blocked-attachments-in-outlook-434752e1-02d3-4e90-9124-8b81e49a8519?preview=true

Increased Support Requests
IT Support may see an increase in support requests related to the inability to access the blocked file types, leading to resource strain.
   - roles: IT Support, Help Desk
   - references: https://learn.microsoft.com/en-us/powershell/module/exchange/set-owamailboxpolicy?view=exchange-ps#-blockedfiletypes, https://support.microsoft.com/en-us/office/blocked-attachments-in-outlook-434752e1-02d3-4e90-9124-8b81e49a8519?preview=true

Policy Management Overhead
Organizations may need to allocate time and resources to update OwaMailboxPolicy to allow the blocked file types if they are in use, creating additional administrative overhead.
   - roles: IT Administrators, Compliance Officers
   - references: https://learn.microsoft.com/en-us/powershell/module/exchange/set-owamailboxpolicy?view=exchange-ps#-allowedfiletypes, https://support.microsoft.com/en-us/office/blocked-attachments-in-outlook-434752e1-02d3-4e90-9124-8b81e49a8519?preview=true

Potential Data Loss
If users are unaware of the change and attempt to send or receive the blocked file types, there may be a risk of data loss or miscommunication regarding important files.
   - roles: End Users, Project Managers
   - references: https://learn.microsoft.com/en-us/powershell/module/exchange/set-owamailboxpolicy?view=exchange-ps#-blockedfiletypes, https://support.microsoft.com/en-us/office/blocked-attachments-in-outlook-434752e1-02d3-4e90-9124-8b81e49a8519?preview=true

Configutation Options**

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Opportunities**

User Training and Communication
As the new file types are being blocked, proactive communication and training for users can mitigate confusion and frustration. Providing clear guidance on the change and how to handle the affected file types can enhance user experience and reduce support tickets.
   - next-steps: Develop training materials and FAQs regarding the blocked file types. Schedule training sessions or webinars for affected users.
   - roles: IT Support, Training Coordinators, HR Managers
   - references: https://support.microsoft.com/en-us/office/blocked-attachments-in-outlook-434752e1-02d3-4e90-9124-8b81e49a8519?preview=true

Policy Review and Customization
Organizations can review their current OwaMailboxPolicy settings to ensure that they align with their needs, especially if they require the use of the blocked file types. This is an opportunity to customize policies to enhance security while maintaining necessary functionality.
   - next-steps: Audit existing mailbox policies and determine if any custom policies are needed. Prepare to update the AllowedFileTypes property for specific users or groups if necessary.
   - roles: IT Administrators, Compliance Officers, Security Officers
   - references: https://learn.microsoft.com/en-us/powershell/module/exchange/set-owamailboxpolicy?view=exchange-ps#-blockedfiletypes

Feedback Mechanism for Users
Implementing a feedback mechanism allows users to report issues related to the blocked file types. This can help the IT department to quickly address concerns and adjust policies or training as needed, improving overall user satisfaction.
   - next-steps: Set up a dedicated channel (like a form or email) for users to submit feedback regarding the blocked file types. Analyze feedback regularly to identify trends and areas for improvement.
   - roles: IT Support, User Experience Teams, Management
   - references: https://learn.microsoft.com/en-us/powershell/module/exchange/set-owamailboxpolicy?view=exchange-ps#-allowedfiletypes

Potentional Risks**

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IT Security**

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explanation for non-techies**

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