Microsoft to block .msix and .msixbundle email attachments in Outlook
The change hits New Outlook for Windows and Outlook on the Web, with general rollout starting in early November 2026.
Microsoft will add .msix and .msixbundle to the BlockedFileTypes list in the default OWA Mailbox policy and in custom tenant policies for New Outlook for Windows and Outlook on the Web, according to techradar.com. The change stops users from downloading .msix and .msixbundle files sent as email attachments.
The general rollout begins in early November 2026 and is expected to complete by mid-November, giving administrators a short window to prepare.
Organizations that rely on these file types must add them to the AllowedFileTypes property before rollout. Without that step, the blocked extensions will apply automatically to affected mailboxes when the change reaches their tenant.
Earlier attachment restrictions
Microsoft disabled the ms-appinstaller protocol handler by default in December 2023 after it was abused to distribute malware. Since then, the company has also blocked .py, .ps1, and .cab file types.
Quick answers
Which Outlook clients are affected by the .msix and .msixbundle block?
The change applies to New Outlook for Windows and Outlook on the Web, through the default OWA Mailbox policy and custom tenant policies.
When does Microsoft start blocking these attachments?
General rollout begins in early November 2026 and is expected to complete by mid-November.
How can an organization keep receiving .msix and .msixbundle attachments?
Administrators must add those file types to the AllowedFileTypes property before rollout.