It’s a serious federal crime to use the mail to threaten a person. It’s one
of more than 200 postal crimes that Postal Inspectors investigate. These mailings
may threaten a person's reputation or involve blackmail or extortion. Or it may be a
direct act of coercion where the action is proposed to elicit a negative or fearful
response. Threats are communicated attempts to inflict harm, fear, or some form of
loss on another individual.
Postal Inspectors investigate extortion and blackmail when demands for ransoms or rewards are sent through the U.S. Mail. Inspectors also strictly enforce laws prohibiting mail that contains threats of kidnapping, physical injury, or injury to the property or reputations of others.
The United States defines and classifies threats based on how they are delivered and the likelihood that they will be acted upon. For instance, Federal law criminalizes specific true threats that are transmitted via the U.S. mail or in interstate commerce.
Some states, for example, criminalize cyber-bullying, which is any action transmitted through a computer or cell phone that elicits a harmful action.
Or the mailing might be a hoax meant to scare the recipient. Either way, report such mailings to Postal Inspectors and keep the letters as evidence
Fortunately, these threats don’t happen very often.