From early November, Outlook on the web and the new Outlook for Windows will stop users opening or downloading MSIX and MSIXBUNDLE attachments. These are installer files for Windows apps, and Microsoft is blocking them as a precaution.
- What it is: Microsoft is adding .msix and .msixbundle files to the list of attachment types Outlook blocks by default, alongside other installer and program files.
- Who is affected: Microsoft 365 businesses using Exchange Online. Classic Outlook for Windows is not covered.
- When: Rollout starts in early November 2026 and should finish by mid-November. Emails are still delivered, but the attachment cannot be opened.
- The risk: Most businesses will not notice. Anyone who regularly emails app installers to staff or clients may find that process stops working.
- What to do: Check whether anyone sends MSIX files by email. If so, we can add an exception for the people who need it before the change, or move to a safer way of sharing software.
Blocking installer files by email also reduces the chance of staff installing something harmful, so we recommend most businesses leave the default in place.