Posted inExchange Online

MC1488841: Outlook on the Web and New Outlook for Windows Add .msix and .msixbundle to Blocked File Types for Enhanced Security

3 minutesPosted inExchange Online
MWPRO IMPACT SCORE
OPERATIONAL IMPACT
53
0 25 50 75 100
HIGH IMPACT • REVIEW RECOMMENDED
Recommended Action:
Review the update and plan any required actions before rollout.
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Primary Audience

Exchange AdminsMicrosoft 365 AdminsSecurity TeamsIT ManagersService Owners
Why this score?
AI Confidence
HIGH
Enough detail is available to trust this assessment.
Assessment Reasoning
The update adds .msix and .msixbundle file types to the blocked list in OwaMailboxPolicy, preventing users from sending or receiving these attachments after rollout. Admins need to review and adjust allowed file type policies if these extensions are required. The change enhances security but may affect a small number of users who use these formats. Preparation is mainly through policy review and adjustment before rollout.
55
🛡️ Admin Impact
HIGH
40
👥 User Impact
MEDIUM
60
⚡ Urgency
HIGH
45
🔧 Effort
MEDIUM
ℹ️ WHAT YOU NEED TO KNOW
📌

AT A GLANCE

Outlook on the web and new Outlook for Windows will block .msix and .msixbundle attachments by default to improve security.
👥

END USERS

Users won’t be able to open or download .msix or .msixbundle attachments.
🛡️

IT ADMINS

If your organisation uses these file types, add them to the AllowedFileTypes list before rollout.
📅

ROLLOUT TIMELINE

Upcoming:
November 2026

📢 Official Microsoft Message Center Announcement

Microsoft Outlook: Update to default blocked file types in OwaMailboxPolicy
Message ID: MC1488841

[What and why]

To enhance security in Outlook on the web and new Outlook for Windows, we are updating the default list of blocked file types in OwaMailboxPolicy. As part of this update, the .msix and .msixbundle file types will be added to the BlockedFileTypes list in the default OWA Mailbox policy and any custom policies created in your tenant. Most organizations are not expected to be affected by this update because these file types are infrequently used. This update is part of our ongoing efforts to strengthen security and help protect organizations from potentially unsafe file attachments.

[Rollout schedule]

  • General Availability (Worldwide, GCC, GCC High, DoD): Rollout begins in early November 2026 and is expected to complete by mid-November 2026.

[Impact on your organization]

Who is affected

  • Exchange Online administrators who manage OWA mailbox policies
  • Users who send or receive .msix or .msixbundle attachments in Outlook on the web or new Outlook for Windows

Platforms and services

  • Outlook for the web
  • New Outlook for Windows
  • Exchange Online

What will happen

  • The .msix and .msixbundle file types will be added to the BlockedFileTypes list in all OWA Mailbox policies in your organization, including the default policy and any custom policies created in your tenant.
  • Users who send or receive .msix or .msixbundle attachments in Outlook on the web or new Outlook for Windows will no longer be able to open or download them.

[Action required and recommendations]

No action is required if your organization does not rely on the .msix or .msixbundle file types. If your organization relies on these file types, we recommend that you add them to the AllowedFileTypes property of your users’ OwaMailboxPolicy objects prior to rollout.

[Learn more]

[Compliance considerations]

No compliance considerations identified. Review as appropriate for your organization.

Source: Microsoft Message Center • Analysed by MWPro

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