Outlook to Block MSIX Attachments Starting in November

Microsoft will add .msix and .msixbundle files to the blocked attachment list in Outlook on the web and the new Outlook for Windows, beginning in November. The change applies to Exchange Online users.

The rollout is set to begin in early November, when the file types will be added to the BlockedFileTypes list in all OWA Mailbox policies. General availability is expected by mid-November. After the policies are updated, users of the two Outlook clients will be unable to send, receive, open, or download these attachments by default.

Microsoft said the update is intended to strengthen security and help protect organizations from potentially unsafe attachments. It said most organizations are not expected to be affected because the file types are infrequently used. Administrators whose organizations need them can allow the file types by adding them to the AllowedFileTypes property of users’ OwaMailboxPolicy objects.

A .msix file is a Windows installation package for a specific computer architecture or configuration. A .msixbundle groups multiple .msix packages in one file for use across multiple computer architectures.

The change follows other Outlook restrictions. In June 2025, Outlook began blocking .library-ms and .search-ms files, which had been exploited in phishing and malware attacks since at least June 2022. In October 2025, Microsoft announced that Outlook on the web and the new Outlook for Windows would stop displaying risky inline SVG images.