Poison-pen letters

• Abusive correspondence
• Venomous letters
• Fiery feedback
• Letters that are hard to read?
• Form of intimidation
• It may begin: "Dear Fuckface"
• Nastygrams
• Angry letter in response to perceived crudity or politics in a crossword puzzle, say
• Sour notes?

Wiktionary Word definitions in Wiktionary
n. (alternative form of hate mail English)