Microsoft will block .msix and .msixbundle attachments in Outlook Web and the new Outlook for Windows starting in November. The change removes a legitimate installer format from those clients’ attachment handling, so email can no longer be used to move MSIX packages through that path.
MSIX is Windows’ application packaging format, so the break is in delivery, not in the installer itself. If a team has been sending software, updates, or support bundles as email attachments, Outlook will turn that workflow into a dead end for recipients using those clients.
For organizations that route packaging through Outlook, the exposure is operational: the package still exists, but the email handoff no longer does. Teams that depend on attachment-based rollout or support flows will see those processes stop only where these Outlook clients sit in the path.
Microsoft Outlook to block MSIX attachments starting November
Microsoft announced that it will add .msix and .msixbundle attachments to the list of blocked attachments in Outlook Web and the new Outlook Windows client starting next month.