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Microsoft is reportedly planning to block .msix and .msixbundle email attachments in New Outlook for Windows and Outlook on the web, starting in early November 2026. The change is attributed to Microsoft 365 Message Center notice MC1488841; the notice itself was not independently verified, so the schedule and scope may change. If Outlook blocks one of these installer packages, Microsoft recommends sharing it through OneDrive or SharePoint instead.
What Outlook attachment change is planned?
A Neowin report published October 6, 2026 says Microsoft will add .msix and .msixbundle to the BlockedFileTypes list in the OWA Mailbox Policy. The reported change applies to New Outlook for Windows and Outlook on the web. It is a planned block on these file types, not a statement that all email attachments will be blocked.
The report attributes the details to Microsoft 365 Message Center notice MC1488841. Because the notice was not independently accessible, the policy details and dates below should be treated as reported rather than confirmed directly in public Microsoft documentation. Read Neowin’s report.
Reported rollout window and environments
The report says rollout is expected to begin in early November 2026 and finish by mid-November 2026 for Worldwide, GCC, GCC High, and DoD environments. These are reported dates, not a verified guarantee; organizations should check Message Center notice MC1488841 in their Microsoft 365 tenant for current status.
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Why can’t I send an MSIX file in Outlook?
Outlook blocks certain attachment types as a security measure because files can carry malicious content. Microsoft’s support guidance explains that Outlook may prevent sending or receiving some file types and recommends sharing a blocked file through OneDrive; SharePoint or another secure network share may also be appropriate. Microsoft’s guidance on blocked Outlook attachments.
If the reported policy change takes effect for your account, an MSIX installer might be blocked even when it is legitimate. The block concerns the attachment’s file type, not a determination that a particular package is malicious.
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How to share an MSIX package if Outlook blocks it
- Upload the package to OneDrive or SharePoint. Use a location approved by your organization and ensure the intended recipient can access it.
- Set sharing permissions deliberately. Choose the narrowest access that works for the recipient, following your organization’s policy.
- Send a link in the email. New Outlook and Outlook on the web support attaching files and uploading locally saved files to OneDrive to share as links. Microsoft’s instructions for adding pictures or attaching files in Outlook.
- Confirm access with the recipient. If they cannot open the link, check the sharing scope and whether their account or organization is permitted to access the file.
Microsoft’s support page also describes compression utilities and file-extension changes as possible ways users may encounter blocked-attachment restrictions. Renaming an installer’s extension is not a safe recommended bypass: it can obscure what the file is and does not make the package safer. Prefer an authorized cloud link or an administrator-approved delivery method.
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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesWhat should Microsoft 365 administrators do?
Administrators whose organizations distribute MSIX packages by email should check whether the reported block applies to their users and review the tenant’s Message Center notice. The Neowin report says administrators can add the extensions to the AllowedFileTypes property of relevant OWA mailbox policies before rollout, but it does not provide an independently verified procedure or establish that this exception is appropriate for every organization. Confirm the current steps, policy scope, and security implications against the tenant notice and current Microsoft documentation before changing policy.
- Identify whether users send or receive
.msixor.msixbundlefiles through New Outlook or Outlook on the web. - Decide whether approved cloud sharing is sufficient for those workflows.
- If an exception is necessary, validate the current policy procedure and apply it only to the relevant users and file types, consistent with organizational security rules.
Microsoft says Exchange administrators may be able to adjust some blocked-file behavior, but that general guidance is not confirmation of a specific exception for this reported OWA policy change.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Is this the first expansion of Outlook’s blocked-file list?
No. Microsoft’s current general blocked-file list includes .library-ms and .search-ms. A June 10, 2025 BleepingComputer report described those types being added for Outlook on the web and New Outlook beginning in July 2025. That earlier change is precedent for additions to the list; it is separate from the reported 2026 plan for MSIX files. Read BleepingComputer’s 2025 report.
Microsoft’s blocked-attachments support page lists the file types covered by its general guidance and explains suggested workarounds. Check Microsoft’s current blocked-attachment list.
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