M365 Changelog: Outlook to block new attachment file types to improve security

MC395211 — As Microsoft continually assess to improve security, the company is adding additional attachment file types to the list of BlockedFileTypes.

When this will happen:

Microsoft will begin rolling out early July and expect to complete by late July.

How this will affect your organization:

The newly blocked file types are rarely used, so most organizations will not be affected by the change. However, if your users are sending and receiving affected attachments, they will report that they are no longer able to download them.

What you need to do to prepare:

There are several ways to send and receive a blocked file. You can save the file to the cloud and send a link to the file, use a file compression utility like WinZip, or even rename the file with another extension, then have your recipient change the name back to the original name after they receive the file.

What file extensions will be added to the BlockedFileTypes list with this change?

Outlook Win32: “.cab”,

Outlook on Web: “application”, “bgi”, “img”, “.iso”, “cab”

New Outlook on Mac, Outlook Android, Outlook iOS: “.appref-ms”, “.cdxml”, “.mht”, .mhtml”, “.pssc”, “.udl”, “.wsb” “.cab”

Where is the fu​ll list of Outlook client block file types?

Outlook Win32: Blocked attachments in Outlook (microsoft.com)

Outlook on Web: Changes to File Types Blocked in Outlook on the web – Microsoft Tech Community

New Outlook on Mac, Outlook Android, Outlook iOS