Microsoft reportedly plans to add .msix and .msixbundle to blocked attachment lists for Outlook on the web and the new Outlook for Windows. The Exchange Online rollout is expected to begin in early November 2026 and reach general availability by mid-November.
Which Outlook clients and files are covered?
The planned change applies to Exchange Online users of Outlook on the web and the new Outlook for Windows. Once mailbox policies are updated, the reported block would prevent users of those clients from sending, receiving, opening or downloading .msix and .msixbundle attachments.
An .msix file is a Windows installation package. An .msixbundle groups multiple .msix packages, including packages intended for different computer architectures.
How can administrators allow an extension?
The reported policy change would add both extensions to BlockedFileTypes in default and custom OWA mailbox policies. An administrator can reportedly allow an extension by adding it to AllowedFileTypes.
Microsoft’s Exchange policy documentation says that when an extension appears in both lists, AllowedFileTypes takes precedence over BlockedFileTypes. The settings govern whether file types can be viewed in Outlook on the web or saved locally.
Why is Microsoft planning the block?
Microsoft says the change is intended to strengthen attachment security and protect organizations from potentially unsafe files. It expects most organizations to be unaffected because .msix and .msixbundle files are used infrequently.
