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

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.

[How this will affect your organization:]

The newly blocked file types are rarely used, so most organizations will not be affected by the change. However, if your users are sending and receiving affected attachments, they will report that they are no longer able to open or download them in Outlook Web or the New Outlook for Windows.

We will be adding the two file types to the Blocked File Type list on the OWA Mailbox Policy – Default as well as any custom policies created in your tenant.

Learn more about the BlockedFileType Policy here: -BlockedFileTypes 

[What you need to do to prepare:]

No action is required if your organization does not rely on these file types. The update will automatically apply to all OWA Mailbox policies in your organization.

If your organization needs to allow these file types, you can add them to the AllowedFileTypes property of your users’ OwaMailboxPolicy objects before the rollout.

Learn more:

• -AllowedFileTypes  
• Blocked attachments in Outlook 

Source: Microsoft

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