Poison pen letter
A poison pen letter is a letter or note containing unpleasant, abusive, or :wikt:malicious|malicious statements or accusations about the recipient or a third party. It is usually sent anonymously. In the term "poison pen", the word poison is used figuratively, rather than literally. Poison pen letters are usually composed and sent to upset the recipient. They differ from blackmail, which is intended to obtain something from the recipient. In contrast, poison pen letters are purely malicious.
In the United Kingdom, Section 1 of the Malicious Communications Act 1988 covers most cases of poison pen letters.
