GoKawiilMicrosoft to block .msix and .msixbundle email attachments in Outlook
Microsoft will add .msix and .msixbundle files to Outlook's default blocked attachment list starting early November, rolling out to Exchange Online users with general availability expected by mid-November. Once applied, Outlook on the web and the new Outlook for Windows will block users from sending, receiving, opening or downloading these file types by default. Admins can whitelist the formats via the AllowedFileTypes property if their organization needs them.