The book of Malachi is 450 years old but the circumstances of that time period are much like ours. At this point in Israel's history, king Cyrus of Persia had allowed Ezra and Nehemiah to return from exile to Jerusalem, to begin rebuilding their temple. Ezekiel had promised the Lord would one day refill the temple with His glory, and Haggai had said the Lord would make this new temple greater than Solomon's had been. When the temple was finally finished, however, nothing special happened, and they waited and waited for decades and decades.
Hope deferred makes the heart sick.
It did for Israel, and it made their lives sick, too. They became bored with the worship of God and morally degenerate. They figured that if God had not come yet, He wasn't going to. We are also in a time of waiting, waiting for Christ's return. In this book we can find help in our time of waiting.