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

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**

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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**

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Potentional Risks**

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IT Security**

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explanation for non-techies**

Starting in July 2025, Outlook Web and the New Outlook for Windows will block two specific file types: .library-ms and .search-ms. Think of this change like a new security measure at an office building. Previously, certain items were allowed through the door without much scrutiny, but now, for safety reasons, those items are being stopped at the entrance unless explicitly permitted.

These file types are not commonly used, so for most organizations, this update will pass unnoticed, much like a new rule that doesn't affect the daily routine of most employees. However, if your organization does use these file types, it’s like having a special tool that needs clearance to enter the building. You’ll need to update your security settings to allow these items through.

If your organization doesn’t use these file types, you don’t need to do anything. The update will automatically apply, much like a building updating its security system without requiring tenants to take any action. But if you do need these file types, you can adjust your settings to allow them, similar to adding someone to an approved guest list.

For those who need to make these adjustments, detailed instructions are available online, akin to a guidebook for navigating the new security protocols. This ensures that everyone who needs access can still get it, while maintaining the overall security of the system.