When does letter writing cross the line into harment?

There are some people that I'd like to tell how I feel. They did me dirty. I know that I have the right to free speech. But I don't want to end up in jail either. Some people can be vindictive. I want to write a letter to these people telling them how I feel. Am I legally allowed to do this? They cannot get me on harment can they? I plan on writing one letter to each of the people but I cannot guarantee that I won't let a curse word or two slip into that letter. I feel I need to do this for closure. They need to know the truth and I need to get this off my chest. One of the people works for a jail so I am paranoid.