John Gill Commentary Jeremiah 29:31

"Send to all them of the captivity, saying, Thus saith Jehovah concerning Shemaiah the Nehelamite: Because that Shemaiah hath prophesied unto you, and I sent him not, and he hath caused you to trust in a lie;" — Jeremiah 29:31 (ASV)

Send to all them of the captivity
Another letter; not to Shemaiah, but to the people, that they might all know that he was a false prophet; and how his lies were resented by the Lord; and what punishment should be inflicted on him and his, on account of them: saying, thus says the Lord concerning Shemaiah the Nehelamite ;
the letter, though written by the prophet, must be sent in the name of the Lord, declaring what he would do with the person mentioned, and the reason of it; which follows: because that Shemaiah has prophesied to you, and I sent him not ,
and he caused you to trust in a lie ;
that they should in a very little time return from their captivity to Jerusalem.