Lou DiBella was an executive responsible for programming boxing shows on HBO cable network. DiBella signed Bernard Hopkins, the then-middleweight world boxing champion, to participate in a fight televised by HBO. After DiBella's departure from the network, he and Hopkins entered into an agreement in which Hopkins paid $50,000 for DiBella's promotional services. Months later, Hopkins publicly accused DiBella of taking bribes and "selling" spots in HBO fights, calling him greedy, filthy, and unethical. DiBella sued Hopkins for libel. What did DiBella have to prove to be successful in his claim?