poison pen letter 
noun
An anonymous letter that is libellous, abusive, or malicious.The idea that the great principle of protecting journalistic sources was not designed to apply to the political equivalent of a poison pen letter never seems to occur to them....
- Police have traced the author of a poison pen letter which embroiled a private school in computer porn allegations - but have decided not to prosecute.
- The odds of being shot dead by a ruminant must be on a par with being killed by a real poison pen letter.