The exchange began when the unidentified man, described as a retired public relations worker, saw a ProPublica story on MSNBC’s “Rachel Maddow Show” that discussed how Kasowitz had made the unusual decision not to seek a security clearance to handle the Russia case.

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After the segment aired Wednesday night, the man sent Kasowitz an email with the subject line, “Resign Now.” According to ProPublica, the message read in part: “I believe it is in your interest and the long-term interest of your firm for you to resign from your position advising the President re. pending federal legal matters. No good can come from this.”

Kasowitz responded with a flurry of messages sent between 9:30 p.m. and 10 p.m. Eastern Time. ProPublica published the thread in full, saying it had confirmed the man’s identity and verified that the emails had come from Kasowitz’s firm.

The exchange comes amid ongoing turmoil within Trump’s legal team, which has struggled to rein in a client notorious for his unfiltered early-morning tweet storms, off-the-cuff speaking style and general lack of verbal restraint.

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As the investigations into Russian meddling in the 2016 election have intensified, Trump has defied his attorneys’ instructions to avoid talking about the matter in his tweets and private conversations with others under scrutiny, including Jared Kushner, his son-in-law and senior adviser, as The Washington Post reported Thursday.

Earlier this week, the New York Times reported that Trump’s relationship with Kasowitz, his personal lawyer and head of the legal team, was showing strain and that the president was growing disillusioned by his strategy. Kasowitz, in turn, had become “deeply frustrated” by the president, the Times reported.

After receiving the stranger’s email Wednesday night, Kasowitz wrote a message back saying “f‑‑‑ you,” then followed up 15 minutes later with a longer response, according to ProPublica.

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“How dare you send me an email like that,” he wrote. “I’m on you now. You are f‑‑‑ing with me now Let’s see who you are Watch your back, bitch.”

“Call me. Don’t be afraid, you piece of s‑‑‑,” he wrote in another message. “Stand up. If you don’t call, you’re just afraid.”

A later email read: “I already know where you live, I’m on you. You might as well call me. You will see me. I promise. Bro.”

The man sent only one response, thanking Kasowitz for his “kind reply” and saying he may get in touch with him “as appropriate.” He later told ProPublica he was so disturbed by the conversation that he forwarded it to the FBI.

Kasowitz said through his spokesman Thursday: “The person sending that email is entitled to his opinion and I should not have responded in that inappropriate manner. I intend to send him an email stating just that. This is one of those times where one wishes he could reverse the clock, but of course I can’t.”

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In the story that prompted the exchange, ProPublica’s Justin Elliott and Jesse Eisinger reported that Kasowitz did not expect to apply for a security clearance, even though the Russia investigations would involve reviewing classified materials.

Even if Kasowitz applied for a security clearance, ProPublica suggested he will likely have trouble getting one because of personal issues which were disputed by a Kasowitz spokesman as “false and defamatory.” The Post has not independently confirmed ProPublica’s allegations.

Kasowitz’s spokesman told the publication he did not require a security clearance to represent the president.

Trump retained Kasowitz to help him navigate the Russia investigations in May, shortly after former FBI director Robert S. Mueller III was appointed special counsel in charge of the Department of Justice’s probe into Russian meddling in the 2016 election and possible collusion by the Trump campaign.

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Kasowitz, who has represented Trump in a variety of cases over the years, is known in New York law circles for his aggressive style in and out of the courtroom. In a recent profile, Bloomberg called him a “bare-knuckled litigator” and a “pit bull loyal to the boss.”

A look at President Trump’s first six months in office share Share View Photos View Photos Next Image U.S. President Donald Trump, center, signs an executive order at the Department of Homeland Security (DHS) in Washington, D.C. U.S., on Wednesday, Jan. 25, 2017. Trump acted on two of the most fundamental -- and controversial -- elements of his presidential campaign, building a wall on the border with Mexico and greatly tightening restrictions on who can enter the U.S. Photographer: Chip Somodevilla/Pool via Bloomberg (Chip Somodevilla/Bloomberg)