Your use of the word "still" is a little misleading. IE _was_ the default email rendering engine until 2007 when Microsoft made the inexplicable decision to move to Word 2007 instead.
By using Word as a viewer the email is broken right on delivery and users now complain "The email you sent me is broken!"
At least it's still better than Notes
IE, and now Edge, is a different business unit.
Not inexplicable at all. NIH doesn't only apply at corporate boundaries.