What constitutes a threatening letter?
I know it's illegal to send a threatening letter through the mail. However, is it legal to send a confronting but firm letter to someone telling them the way they treated you was rude and why you think it was wrong?
A threat of what? Harm to a person or property? A threat to expose someone for something (akin to blackmail)? A threat of criminal charges? There are many types of threats in the legal system but I do not think that what you want to do is in any way illegal as song as you do not step over the line with your letter. Telling someone that you thought that there actions and treatment were rude is not threatening. Advising them that you are aware of their behaviour and giving your opinion that their actions were wrong in your eyes is not threatening. It is your opinion of their behaviour and you are not indicating or intending to retaliate against them in any way, correct? But may I suggest that you consider letting go of the idea of putting anything in writing to anyone about these type of things as they could come back to haunt you in the end. You seem to be a better person than they and you do not want to stopp to their level. Good luck.