Adam Clarke on Jeremiah 29:1

Now these are the words of the letter - This transaction took place in the first or second year of Zedekiah. It appears that the prophet had been informed that the Jews who had already been carried into captivity had, through the instigations of false prophets, been led to believe that they were to be brought out of their captivity speedily. Jeremiah, fearing that this delusion might induce them to take some hasty steps, ill comporting with their present state, wrote a letter to them, which he entrusted to an embassy which Zedekiah had sent on some political concerns to Nebuchadnezzar. The letter was directed to the elders, priests, prophets, and people who had been carried away captives to Babylon.