In Post v. Jones, the people making the improper threat were the captains of the Elizabeth Frith and the Panama. They were parties to the contract.
What if the threat is made by someone who is not a party to the contract? Suppose Brian offers to buy Brianna's car for $15,000, and Brianna accepts. She accepts, however, because Rex blackmailed her by threatening to publish compromising photos of Brianna's private life on the Internet if she did not accept the offer.
Is Rex's threat improper?