Microsoft announced that it will add .msix and .msixbundle attachments to the list of blocked attachments in Outlook Web and the new Outlook Windows client starting next month.
.msix files are modern Windows installation packages tailored for specific computer architectures or configurations, while .msixbundle is a container that groups multiple .msix packages into a single file compatible with multiple computer architectures.
The change will begin rolling out to Exchange Online users in early November, when the new file types will be added to the BlockedFileTypes list in all OWA Mailbox policies, and is expected to reach general availability by mid-November.
After the policies are updated, .msix or .msixbundle attachments will be blocked by default, and users of Outlook on the web and new Outlook for Windows will no longer be able to send, receive, open, or download them.
"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," Microsoft said in a Microsoft 365 message center update.
"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."