Outlook to Block MSIX Attachments Starting in November
Summary
Microsoft will block .msix and .msixbundle attachments by default in Outlook on the web and the new Outlook for Windows, with the Exchange Online rollout beginning in early November 2026.
Key points
- The change adds .msix and .msixbundle to the blocked file types in Outlook on the web mailbox policies and the new Outlook for Windows.
- The Exchange Online rollout is scheduled to begin in early November, with general availability expected by mid-November 2026.
- Once enabled, users will be unable to send, receive, open, or download these attachments through the affected Outlook clients.
- Admins do not need to act unless their organization uses these file types; they can allow them through the OwaMailboxPolicy AllowedFileTypes property.
- Microsoft says the change is intended to protect organizations from potentially unsafe attachments and that these file types are infrequently used.
- Outlook previously blocked .library-ms and .search-ms files and stopped displaying risky inline SVG images as part of efforts to limit file types abused in attacks.
Article Details
- Event Type
- Email attachment security policy update
- Impact
- Exchange Online users of Outlook on the web and new Outlook for Windows will be unable to send, receive, open, or download .msix and .msixbundle attachments by default. Administrators can allow the file types through OwaMailboxPolicy if needed.
Vendors
Products
Exchange OnlineThe change will begin rolling out to Exchange Online users in early November, when the new file types will be added to the BlockedFileTypes list in all OWA Mailbox policies, and is expected to reach general availabilitynew Outlook for WindowsAfter the policies are updated, .msix or .msixbundle attachments will be blocked by default, and users of Outlook on the web and new Outlook for Windows will no longer be able to send, receive, open, or download them.Outlook on the webAfter the policies are updated, .msix or .msixbundle attachments will be blocked by default, and users of Outlook on the web and new Outlook for Windows will no longer be able to send, receive, open, or download them.