Outlook will block MSIX attachments from early November
The change will stop .msix and .msixbundle files from being sent, received, opened or downloaded by default in Outlook on the web and new Outlook for Windows.
Microsoft will start blocking .msix and .msixbundle attachments in Outlook on the web and the new Outlook for Windows, a change aimed at reducing a file type that can be abused in attacks. The rollout begins for Exchange Online users in early November and should reach general availability by mid-November, after which these files cannot be sent, received, opened, or downloaded by default. Microsoft will add the two extensions to the BlockedFileTypes list in default and custom OWA Mailbox policies, though administrators can still allow them explicitly if needed. The move follows earlier Outlook restrictions on other risky file types, including .library-ms and .search-ms in June 2025 and inline SVG blocking announced in October 2025.
Why it matters
For people using Outlook on the web and the new Outlook for Windows, these attachments will no longer pass through by default once the rollout completes. That reduces the risk from files Microsoft says can be abused in attacks, while still leaving administrators a way to permit them through policy when needed.
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Sources
- BleepingComputer