Microsoft to block more file types in Web, New Outlook

Microsoft is quietly tightening security around email attachments in Outlook for the Web and the new Outlook app on Windows. Starting July 2025, Microsoft is blocking two more file types for Outlook attachments—”.library-ms” and “.search-ms.”

This update builds on a policy first rolled out in 2019. Back then, Microsoft expanded its list of blocked file types to prevent users from accidentally opening risky attachments. With additional attachment file type blocking, Microsoft wants to protect Outlook users from sneaky malware and threat actors who hide behind obscure file formats.

Also read: Microsoft Outlook: Best Ways to Organize Emails

In a recent post on the Microsoft 365 Admin Center (MC1090702), the company said this latest update affects both Outlook for the Web and the new Windows app. While most people probably haven’t even seen .library-ms or .search-ms files, they’re being blocked by default now.

But don’t worry—if your organization still uses those formats, admins can override the change by adding them to the “AllowedFileTypes” list in their OwaMailboxPolicy settings. Microsoft isn’t forcing the update on everyone; it’s just adjusting the defaults for better protection.

Also read: New Outlook for Windows gets .pst support & Copilot upgrades with June 2025 update

The rollout begins in early July, and Microsoft calls this a “major change.” No manual action is needed unless those file types are still part of your workflow. For system admins, the full update is available in the Admin Center—just search for message ID MC1090702.

If you’re curious, which file types are still allowed? Microsoft says popular ones like .pdf, .docx, .xlsx, .jpg, and even .zip are safe. The full list of blocked and allowed extensions is up on the company’s support site.