Good advice, if you are like an overwhelming percentage of people in this world, who have no control over their emotions and little if any ability to think critically.
Often when I'm compelled to write letters it is when I'm complaining about something that has made me angry. Not doing so because I'm angry would be defeatist and disempowering. If you can remain calm while being angry, I think the results are likely to be better than if you don't remain calm. It can also allow you to get on with your own life and do the things you need to do in order to progress while still harboring a long-term plan for revenge!