Is it legal to redefine wp_password_change_notification in a mu plugin?

I am trying to add some logic to this function of when to alert the admin to a password change but no email is being sent at all. The codex says it needs to be declared in a plugin, so is this legal from a mu plugin or is there only specific locations this can be redeclared?

As it stands this is roughly my code but the mail never goes out:

if(!function_exists('wp_password_change_notification')){
    //die('I can get here');
    function wp_password_change_notification($user){
        //die('but never get here');
        //...code...
        wp_mail(get_option('admin_email'), sprintf(__('[%s] Password Lost/Changed'), $blogname), $message);
    }
}