crank letter
Also, crank call. An irrational, fanatical, or hostile letter or telephone call. For example, The office was flooded with mail, including a lot of crank letters, or Harriet was upset enough by the crank calls to notify the police. This expression employs crank in the sense of "irrational person." The first term dates from the mid-1900s, the variant from the 1960s.