For he has sent to us in Babylon, claiming: Since the exile will be lengthy, build houses and settle down; plant gardens and eat their produce.’”
Shemaiah objects because Jeremiah has told the exiles the captivity will be long and that they should build and plant in Babylon.
This verse shows that the real offense is Jeremiah's truthful long-term message.
24You are to tell Shemaiah the Nehelamite that 25this is what the LORD of Hosts, the God of Israel, says: “In your own name you have sent out letters to all the people of Jerusalem, to the priest Zephaniah son of Maaseiah, and to all the priests. You said to Zephaniah: 26‘The LORD has appointed you priest in place of Jehoiada, to be the chief officer in the house of the LORD, responsible for any madman who acts like a prophet — you must put him in stocks and neck irons. 27So now, why have you not rebuked Jeremiah of Anathoth, who poses as a prophet among you? 28For he has sent to us in Babylon, claiming: Since the exile will be lengthy, build houses and settle down; plant gardens and eat their produce.’” 29(Zephaniah the priest, however, had read this letter to Jeremiah the prophet.) 30Then the word of the LORD came to Jeremiah: 31“Send a message telling all the exiles what the LORD says concerning Shemaiah the Nehelamite. Because Shemaiah has prophesied to you— though I did not send him— and has made you trust in a lie, 32this is what the LORD says: ‘I will surely punish Shemaiah the Nehelamite and his descendants. He will have no one left among this people, nor will he see the good that I will bring to My people, declares the LORD, for he has preached rebellion against the LORD.’”