Microsoft Will Block .msix and .msixbundle Attachments in New Outlook and Outlook on the Web in November

Microsoft will block .msix and .msixbundle file attachments by default in new Outlook for Windows and Outlook on the web, according to a Microsoft 365 Message Center post (MC1488841). The change applies to Exchange Online, and the rollout is scheduled for early to mid-November 2026. Once the block is active, users of the two affected Outlook clients will no longer be able to download or open attachments with either extension.