Outlook will block .msix and .msixbundle files in November

Microsoft will add the .msix and .msixbundle formats to Outlook's list of blocked attachments, according to BleepingComputer, which cites notice MC1488841 in the Microsoft 365 message center. Anyone using Outlook on the web or the new Outlook for Windows will not be able to send, receive, open or download those files once the change reaches their account.

An .msix file is a package for installing apps on Windows, and an .msixbundle combines several packages in one file. This newsroom could not open Microsoft's notice directly because it requires administrator access, but the public index mc.merill.net reproduces it, dated Oct. 5, 2026, with the same content.

The first mailboxes will receive the change in early November, and it should reach everyone by mid-month. It applies to Exchange Online mailboxes, the email service used by organizations, and works by adding the two formats to the BlockedFileTypes list of the OWA mailbox policy, both the default one and any an organization has created. Administrators who do need those files can add them to the AllowedFileTypes property of OwaMailboxPolicy before the rollout.

Microsoft presents the measure as a safeguard against risky attachments and expects little impact, because those formats are rarely used. For IT teams and developers who share MSIX installers by email, the way around it is another delivery method or an exception requested from the administrator. For everyone else, nothing visible changes.

It is not the first such move. BleepingComputer recalls that Outlook has blocked .library-ms and .search-ms files, which were used in phishing campaigns, since June 2025, and that this past October it stopped displaying certain risky SVG images inside email.

It remains unclear whether classic Outlook for Windows is included and whether the change reaches personal Outlook.com accounts: none of the sources consulted mentions either.