poison-pen letter

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A usually anonymous written communication that makes abusive or malicious statements or accusations about the recipient or another person.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun idiomatic A missive which is malicious, insulting, and/or defamatory toward a person, organization, or point of view, especially one which is unsigned.