Malachi, whose name means “my messenger,” spoke to the Israelites after their return from exile. The theological message of the book can be summed up in one sentence: The Great King will come not only to judge his people, but also to bless and restore them.
Malachi was the last of the prophets, and is supposed to have prophesied B.C. 420. He reproves the priests and the people for the evil practices into which they had villainy, and invites them to repentance and reformation, with promises of the blessings to be bestowed at the coming of the Messiah. And now that prophecy was to cease, he speaks clearly of the Messiah, as nigh at hand, and directs the people of God to keep in remembrance the law of Moses, while they were in expectation of the gospel of Christ.
Matthew Henry