It may also be that Malachi was referring to the restored gospel as the messenger that would prepare the way before the Lord in addition to mortal messengers, for the Lord declared through the Prophet Joseph Smith:
“I have sent mine everlasting covenant into the world, to be a light to the world, and to be a standard for my people, and for the Gentiles to seek to it, and to be a messenger before my face to prepare the way before me” (D&C 455, italics added).