Microsoft has announced plans to restrict the use of specific file formats within its email services. Starting in November 2026, the new Outlook for Windows and the web-based version of Outlook will block .msix and .msixbundle attachments by default.
The company will add these file types to the BlockedFileTypes list within the OwaMailboxPolicy. Once the policy update takes effect, users will be unable to send, receive, open, or download files with these extensions.
Microsoft stated that this change is intended to enhance security by protecting organizations from potentially unsafe attachments. The company noted that these file formats are rarely used in daily office workflows, meaning the impact on most organizations will be minimal.
Administrators who require the use of these files can bypass the restriction. By adding .msix and .msixbundle to the AllowedFileTypes attribute of the OwaMailboxPolicy object, they can whitelist these formats for their users.
