Microsoft plans to block.msix and.msixbundle attachments in new Outlook and Outlook on the web

Neowin reports, citing a Microsoft 365 Message Center announcement, that.msix and.msixbundle attachments will be blocked by default in new Outlook for Windows and Outlook on the web. Microsoft says the rollout will begin in early November 2026 and should finish by mid-November. Admins who need to allow the files can add the extensions to the relevant OWA mailbox policy’s AllowedFileTypes property.