This is a salutation, a title. The only time I use this in English any more is when I sign up to present at a conference. Some organizations insist I pick a title. But now on to the crux of the matter.
Why not use Mx for everything? I didn’t realize that sex-based titles have only been around since the mid-1800’s. Go figure. Before that they were mostly about class and status. Read more about this over at Language: A Feminist Guide.