The manager received a nasty poison-pen letter today.
poison-pen letter
idiomA missive which is malicious, insulting, and/or defamatory toward a person, organization, or point of view, especially one which is unsigned.
The anonymous poison-pen letter accused the local shopkeeper of dishonest business practices, causing a scandal in the small town.
Despite the lack of a signature, the cruel tone of the poison-pen letter left no doubt that the author intended to destroy her professional reputation.