Final answer:
In Othello, the reason that is NOT believed by Othello to prove Desdemona's unfaithfulness is that she has been receiving secret letters.
Step-by-step explanation:
In Othello, the reason that is NOT believed by Othello to prove Desdemona's unfaithfulness is that she has been receiving secret letters. Othello does believe that Desdemona has been seen with Cassio, she gave Cassio her handkerchief, and she has been acting suspiciously.