Marc Kasowitz. Yuri Gripas/Reuters

Marc Kasowitz, head of President Donald Trump's legal team for all Russia-related matters, apologized Thursday to a stranger he berated in a string of emails Wednesday night, saying that the exchange happened "at the end of a very long day."

The exchange was a result of a person emailing Kasowitz with the subject line: "Resign Now."

Kasowitz, through his spokesperson Mike Sitrick, said the person he went after was within his right to send the email, and said he would be apologizing to the emailer.

"Mr. Kasowitz, who is tied up with client matters, said he intends to apologize to the writer of the email referenced in today's ProPublica story," Sitrick said in the statement. "While no excuse, the email came at the end of a very long day that at 10 p.m. was not yet over."

Sitrick attached a comment from Kasowitz.

"The person sending that email is entitled to his opinion and I should not have responded in that inappropriate manner," Kasowitz said. "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."

The apology came shortly after ProPublica published the profanity-laden emails.

The person who sparked the exchange is a retired public relations professional in the western US, according to ProPublica. He sent the "Resign Now" email after reading ProPublica's story this week on Kasowitz.

Kasowitz fired back with replies sent between 9:30 p.m. and 10 p.m. ET.

"I'm on you now," one read. " You are f------ with me now Let's see who you are Watch your back , b----."

"Call me," Kasowitz wrote in another email. "Don't be afraid, you piece of s---. Stand up. If you don't call, you're just afraid."

Later, he added: "I already know where you live, I'm on you. You might as well call me. You will see me. I promise. Bro."

ProPublica was able to confirm the man's identity. He did not want to be identified in the publication's story.