can i be sued for comments made about someone on a forum?

An individual took money to restore our truck and we took our truck back 2 years later because it was sitting in an unlocked compound with no motor in it (motor was left in bed of truck to rust). We found him on a car forum and wrote, basically an open letter, advising these people to watch out for him because he took our money and that he was taking money from new people to fund jobs he had already been paid for. He served us papers this morning saying we defamed his character with this group and is now asking for over $ 250,000 in damages (from a $ 7,000 restoration job). can we really be sued for comments made on the internet in a forum? We received a letter from his lawyer in January advising that if we did not cease and desist from further damaging his reputation, he would

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