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What Microsoft is reportedly changing
According to Neowin’s October 6, 2026 report, Microsoft plans to add .msix and .msixbundle to the BlockedFileTypes list in OWA Mailbox Policy. The report names New Outlook for Windows and Outlook on the web as the affected clients.
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The report says rollout is expected to start in early November 2026 and finish by mid-November in Worldwide, GCC, GCC High, and DoD environments. These dates and environments are reported details attributed to MC1488841, not independently confirmed from Microsoft’s notice. Treat the change as planned, not as proof that the extensions are already blocked. It does not establish that every Outlook client or every email service is affected.
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What this means if you need to send an MSIX package
If Outlook rejects an MSIX file, the simplest supported alternative is to upload it to OneDrive or SharePoint and send the recipient a link. Microsoft’s blocked-attachments guidance recommends sharing a OneDrive link and also identifies a secure network share such as SharePoint as an option.
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Before sharing, confirm that the intended recipient can access the location and that the link’s permissions match your organization’s security rules. For instructions on attaching files or sharing files from OneDrive in New Outlook and Outlook on the web, see Microsoft’s email attachment guidance.
What administrators should check
Organizations that routinely exchange MSIX packages should identify the affected mailboxes and review the relevant OWA mailbox policies before the reported rollout window. Neowin says administrators can add the extensions to the AllowedFileTypes property of relevant OWA mailbox policies, but the current procedure and policy scope should be checked against the tenant’s Message Center notice and Microsoft’s current administrative documentation before making a change.
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Microsoft says Exchange administrators may be able to adjust blocked-file behavior for some file types. Any exception should be evaluated against the organization’s security requirements rather than applied broadly for convenience.
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This is a file-type block, not a ban on all attachments
The reported change concerns two installer-package extensions in named newer Outlook clients. It is not evidence that Outlook is blocking every attachment or that MSIX packages are blocked across all Microsoft mail products. Microsoft’s general attachment guidance explains that certain file types are blocked to help protect users from malicious files.
Outlook’s blocked-file list has been extended before: a June 2025 report described additions of .library-ms and .search-ms for Outlook on the web and New Outlook, and both appear on Microsoft’s current blocked-file list. That earlier change is separate from the reported MSIX plan.
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