MC1488841 Plan for changeMajor changeFeature updateUser impactAdmin impact

Microsoft Outlook: Update to default blocked file types in OwaMailboxPolicy

Exchange Online Microsoft 365 admin center

Versions captured by this deployment. Earlier versions and notices may be missing; first observed is not first published.

Ends: 18 Dec 2026

Rollout dates are often only in the text. When Microsoft moves them, the change shows under "What changed".

Microsoft's start date: 5 Oct 2026
Last changed by Microsoft: 5 Oct 2026
First seen here: 19 Oct 2026
Versions seen: 1

What the notice says

Microsoft's text, with its sections, lists and tables; source markup removed

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.

What changed

Between the versions this installation saw, newest first

This installation has seen one version of this notice, first on 19 Oct 2026. Later changes will show here.

Linked documentation

Microsoft pages the notice links to

The notice links to no Microsoft Learn or Support page that this installation reads.

Not confirmed here

What this page cannot tell you

Whether your tenant is affected: this is Microsoft's notice, not a check of your settings.
Versions before 19 Oct 2026: this installation first saw the notice after Microsoft posted it.