What happened
Microsoft announced that .msix and .msixbundle files will be added to Outlook’s default blocked‑attachment list. The change rolls out to Exchange Online users in early November and reaches general availability by mid‑November. After the update, Outlook on the web and the new Outlook Windows client will prevent sending, receiving, opening, or downloading these files unless an admin explicitly whitelists them.
Why it matters for trust and compliance
- The update illustrates why organizations must maintain up‑to‑date attachment‑filter policies and retain evidence of enforcement to satisfy control‑assurance requirements across frameworks.
- Documented attachment‑blocking policy provides verifiable evidence for data‑loss‑prevention controls.
- Continuous monitoring of blocked file logs supports audit readiness and demonstrates due‑diligence.
Who is affected
Enterprises using Microsoft 365 SaaS providers leveraging Exchange Online
Recommended actions
- Review OwaMailboxPolicy to confirm allowed file types align with business needs
- Log and retain blocked attachment events for audit evidence
- Update your control‑mapping inventory to reflect the new blocked file type