From an N.H. S. Ct. decision earlier this year in Appeal of Hoppock; I thought the letter involved was likely too aggressive, but I'm surprised that the court found it to be a violation of the professional rules:

Hoppock represented a party with whom the complainant was involved in a dispute over the registration of a trade name. In June 2019, the complainant filed a grievance with the Attorney Discipline Office (ADO) regarding a letter, dated May 20, 2019, that she received from Hoppock…. The letter stated that the complainant had defamed Hoppock's client on social media and warned, "As you will soon find out, legal consequences shall befall you because of your reckless defamatory conduct." The letter continued, in pertinent part, to state:

[L]et me be very clear. I will not get into it

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