The truth? You have far less control over the recipient’s interpretation of and reaction to your message. A letter leaves room between the lines for misunderstanding. Worse, it leaves a permanent record of your anger – a record that can be saved, shared, or distributed for the world to see.
If you are really that angry, go talk to the person. If you can’t do so face to face, at least pick up the phone. You will reach an understanding much quicker if you hash out your opinions in dialogue.
By all means, write the letter. It will make you feel better. But never send it. Abraham Lincoln died with a desk drawer full of nasty letters. Be the better man or woman. Be brave. Do not hide behind a letter.