Microsoft Outlook to block .msix and .msixbundle attachments in November

Microsoft will add .msix and .msixbundle files to the blocked attachment list in Outlook on the web and the new Outlook for Windows starting next month. After Exchange Online policies are updated, users won't be able to send, receive, open, or download them, with general availability expected by mid-November.

• Blocking starts rolling out to Exchange Online in early November
• File types will be added to BlockedFileTypes in all OWA Mailbox policies
• Admins can whitelist them via the AllowedFileTypes property
• Microsoft calls the move part of ongoing Outlook security hardening