Microsoft extends the Outlook naughty step with two more file types

Microsoft is adding two extra file types to its Outlook block list to strengthen security. The file types are.msix and.msixbundle, used for Windows application packages and bundles. The change affects New Outlook for Windows and Outlook on the Web in Exchange Online. By default, users of the affected clients will no longer be able to download or open attachments with these extensions, which is no bad thing because blindly installing a malicious.msix package could compromise a device. That said, although Microsoft noted that the file types were "infrequently used," there are legitimate reasons for their presence in emails. Administrators who need to permit these attachments can add the extensions to the AllowedFileTypes property of the relevant OwaMailboxPolicy before the rollout, scheduled for early to mid-November 2026. "This update is part of our ongoing efforts to strengthen security and help protect organizations from potentially unsafe file attachments," Microsoft said. …