Microsoft Expands Blocked File Types for Outlook: Key Changes Ahead
In a strategic move to bolster security, Microsoft made significant updates to its blocked file types for attachments back in 2019. The company aimed to enhance protection against potentially harmful files by adding new extensions to the BlockedFileTypes property in existing OwaMailboxPolicy objects. This initiative underscores Microsoft’s commitment to safeguarding users from risks posed by malicious actors.
Recent Updates to Blocked File Lists
As part of its ongoing efforts, Microsoft has progressively expanded the list of blocked files for both Outlook on the Web and the New Outlook for Windows application. A recent communication via the Microsoft 365 Admin Center has revealed plans to include two additional file types to this block list. The company reassures users that these newly restricted file types are not commonly used and should not pose a significant impact on the majority of users.
“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
Guidance for Administrators
For system administrators, Microsoft indicates that no immediate action is necessary if their organizations do not utilize the newly blocked file types. However, if these files are still in use, admins are advised to include them in the AllowedFileTypes property of user OwaMailboxPolicy objects prior to the roll-out date. This substantial change is anticipated to commence in early July 2025.
To assist you in this process, additional guidance is available in the official Microsoft documentation, accessible [here](https://learn.microsoft.com/en-us/powershell/module/exchange/set-owamailboxpolicy?view=exchange-ps#-allowedfiletypes).
Overview of Allowed File Types
It’s important to note the following file types are permitted by default in Outlook:
- .avi
- .bmp
- .doc
- .docm
- .docx
- .gif
- .jpg
- .mp3
- .one
- .png
- .ppsm
- .ppsx
- .ppt
- .pptm
- .pptx
- .pub
- .rpmsg
- .rtf
- .tif
- .tiff
- .txt
- .vsd
- .wav
- .wma
- .wmv
- .xls
- .xlsb
- .xlsm
- .xlsx
- .zip
Further Resources and Information
For a comprehensive list of currently blocked file types, refer to the Microsoft support article [here](https://support.microsoft.com/en-us/office/blocked-attachments-in-outlook-434752e1-02d3-4e90-9124-8b81e49a8519?preview=true).Administrators can also find details about this update on the Microsoft 365 Admin Center, specifically under message ID MC1090702.
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