Posted inExchange Online

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

3 minutesPosted inExchange Online
MWPRO IMPACT SCORE
OPERATIONAL IMPACT
47
0 25 50 75 100
MODERATE IMPACT • ASSESS BUSINESS IMPACT
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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
This update pushes back the rollout of the new blocked file types for Outlook on the web and new Outlook for Windows by roughly one month. Admins managing OWA mailbox policies should confirm whether .msix or .msixbundle files are needed and adjust the AllowedFileTypes settings if required. The impact for most users is minimal as these file types are uncommon, but security and Exchange admins should prepare and communicate if exceptions are necessary.
55
🛡️ Admin Impact
HIGH
35
👥 User Impact
MEDIUM
45
⚡ Urgency
MEDIUM
40
🔧 Effort
MEDIUM
ℹ️ WHAT YOU NEED TO KNOW
📌

AT A GLANCE

Outlook on the web and new Outlook for Windows will soon 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 users rely on these file types, add them to the AllowedFileTypes list before rollout.
📅

ROLLOUT TIMELINE

Upcoming:
Early–Mid Dec 2026

📢 Official Microsoft Message Center Announcement

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

Updated November 10, 2026: We have updated the timeline. Thank you for your patience. 

[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 December 2026 (previously early November) and is expected to complete by mid-December 2026 (previously mid-November).

[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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