We don’t know when Malachi delivered his message.
- It was probably after the exiles returned to Judah and after the second Temple was built (so after 515 BC) because the text refers to a governor and Temple practices. The text deals with some of the same issues that Ezra and Nehemiah dealt with, but also some more fundamental ones.
- It was probably after Nehemiah’s first stint in Jerusalem, when Nehemiah set up the governance structures for collecting the tithes and offerings and distributing them (after 445 BC) – so there was a system that could be corrupted.
- Nehemiah was a strong, upright leader and he wasn’t named as governor, so some think this means he wasn’t in the position at the time. Perhaps Malachi was supporting Nehemiah’s reforms and governance structures, or perhaps he dealt with their corruption before Nehemiah came back to Jerusalem the second time in about 433 BC.
- Malachi could also have lived after Nehemiah’s rule as governor finished in about 420 BC because there is no evidence that this couldn’t have happened, other than a tendency for God to send prophets in groups, with each complementing and reinforcing the message of the others.
In support of the idea that Malachi was dealing with corruption in Temple practices around the tithing and offering system while Nehemiah was away (so before the final round of reforms in 433–420BC):
Malachi 2:7-9 The words of a priest’s lips should preserve knowledge of God, and people should go to him for instruction, for the priest is the messenger of the Lord of Heaven’s Armies. But you priests have left God’s paths. Your instructions have caused many to stumble into sin. You have corrupted the covenant I made with the Levites … So I have made you despised and humiliated in the eyes of all the people. For you have not obeyed me but have shown favoritism in the way you carry out my instructions.
That Bible blog 