Insights Security & Privacy
Microsoft will block MSIX installer attachments in Outlook from November
The short version
Microsoft is adding .msix and .msixbundle, two Windows app installer file types, to the attachments Outlook on the web and the new Outlook for Windows block. Rollout starts in early November 2026 and should finish by mid-November. Most people will not notice, but anyone who emails such installers will.
What happened
Microsoft has announced that .msix and .msixbundle files will be added to the list of attachment types blocked in Outlook on the web and the new Outlook for Windows. MSIX is a package format for installing apps on Windows. An .msixbundle file groups several of these packages together, for example versions for different types of processor.
The change starts rolling out to Exchange Online users in early November 2026 and is expected to be complete by mid-November. Once it applies, users of these two Outlook versions will no longer be able to send, receive, open or download such attachments. The block covers the default mailbox policy and any custom policies an organisation has created.
Microsoft says most organisations will not be affected because these file types are rarely used. Administrators who do need them can add the extensions to an allowed list in their mailbox policy. Microsoft describes the change as part of its work to protect people from unsafe attachments.
Why it matters to you
Installer files are not harmful in themselves, but they can contain malware, so blocking them in email closes one possible route for unsafe files. Microsoft has taken similar steps before, for example by blocking two other file types in Outlook, .library-ms and .search-ms, in June 2025.
For a Namibian business, the effect depends on habit. If you or your IT supplier email software installers to staff or customers using Outlook on the web or the new Outlook for Windows, those messages will stop working once the change reaches your account. Most businesses will see no difference.
What to do
Ask whoever manages your Microsoft 365 account whether anyone sends or receives .msix or .msixbundle files by email. If not, no action is needed. If so, either ask your administrator to allow the extensions before the change reaches your account, or share installers through another route, such as a trusted download link. Treat any unexpected installer attachment with suspicion and confirm with the sender first.
Sources
- Microsoft Outlook to block MSIX attachments starting November BleepingComputer (opens in a new tab)
- Microsoft Will Block .msix and .msixbundle Attachments in New Outlook and Outlook on the Web in November gHacks (opens in a new tab)
- Microsoft blocks MSIX files in Outlook heise online (opens in a new tab)
Researched and written by our automated news system.