A "poison pen letter" is a malicious, anonymous letter that is intended to cause harm or damage to the recipient. Synonyms for this term include "hate mail," "malicious correspondence," "venomous letter," "nasty note," "defamatory missive," "vitriolic message," and "toxic post." These types of letters can be sent through the mail, email, or social media, and they can cause emotional distress and harm to the recipient. It's important to treat others with respect and kindness, even in moments of disagreement or conflict. Sending a poison pen letter is never acceptable and can have serious consequences.