# Outlook will block Windows installer attachments by default

_Published Wednesday, October 7, 2026 at 11:32 PM EDT · Security, Products · Latest · Tier 2 — Notable_

![Outlook will block Windows installer attachments by default — Primary](https://www.bleepstatic.com/content/hl-images/2025/04/29/Outlook.jpg)

Microsoft will block .msix and .msixbundle attachments by default in Outlook on the web and the new Outlook for Windows. The rollout to Exchange Online users starts in early November and is expected to reach general availability by mid-November.

These files install Windows applications, with bundles grouping packages for multiple computer architectures. Microsoft will add them to blocked file lists in default and custom mailbox policies, preventing users from sending, receiving, opening or downloading them. Administrators can allow the file types through their users' mailbox policies when needed.

## Sources

- [BleepingComputer](https://www.bleepingcomputer.com/news/microsoft/microsoft-outlook-to-block-msix-attachments-used-in-attacks/)

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Canonical: https://techandbusiness.org/newswire/Ov011xOGCA9Y1YLsDhJmZz
Published: 2026-10-08T03:32:23.216Z
Story chronology: 2026-10-07T15:44:22.000Z
Retrieved: 2026-10-08T05:43:33.353Z
Publisher: Tech & Business (techandbusiness.org)