Some of you are near the bottom of our checklist.  There was an article about disabling the mail app in Windows 10. This is so users will automatically use Outlook instead of the mail app.  I tried to find the original but could not.  Try

"GPO to disable mail app Windows 10"

https://social.technet.microsoft.com/Forums/windows/en-US/53dbf5a2-69b7-4ba0-b030-28aeee8c94ec/windows-10-mail-disable-via-gpo-not-working?forum=win10itprosetup