In the threat explorer, the "Send a message to recipients" option will bring up a new mail window with all of the recipients separated by a comma.

In the Windows Desktop versions of Outlook, recipients must be separated by semicolon. This means that admins must copy the list, replace the comma with a semicolon, and paste it back into the BCC field.

Oddly, the comma separation works fine on the Mac version of Office. Having this feature properly function on Microsoft's Office Suite on Microsoft's Operating System would be nice.