It may begin: "Dear Fuckface"

• It's angry and often anonymous
• Poison-pen letters
• Abusive correspondence
• Nastygrams
• Poison pen letters
• Angry letter in response to perceived crudity or politics in a crossword puzzle, say
• Not-so-special delivery
• Angry letters
• Nasty letters

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