This is a spectacular article in The New York Times. How much things have changed since email, texting, and the digital age we live in. "Back in the day" people separated by distance mostly communicated by letter. Once you sealed the letter and sent it on its way you had days or weeks to think about what you wrote; and maybe regret a thing or two.
Here is an excerpt from Maria Konnikova on "The Lost Art of the Unsent Angry Letter":
WHENEVER Abraham Lincoln felt the urge to tell someone off, he would compose what he called a “hot letter.” [emphasis added] He’d pile all of his anger into a note, “put it aside until his emotions cooled down,” Doris Kearns Goodwin once explained on NPR, “and then write: ‘Never sent. Never signed.’ ” Which meant that Gen. George G. Meade, for one, would never hear from his commander in chief that Lincoln blamed him for letting Robert E. Lee escape after Gettysburg.
Lincoln was hardly unique. Among public figures who need to think twice about their choice of words, the unsent angry letter has a venerable tradition. Its purpose is twofold. It serves as a type of emotional catharsis, a way to let it all out without the repercussions of true engagement. And it acts as a strategic catharsis, an exercise in saying what you really think, which Mark Twain (himself a notable non-sender of correspondence) believed provided “unallowable frankness & freedom.”