To be safe, Julien tells Madame de Rênal that he won't be able to visit her at night. If the anonymous letter written to Monsieur de Rênal says what Julien thinks it does, it would be crazy to try and sleep with Madame.
The next day, Madame sends him a secret note asking whether he loves her anymore. She can't believe that he has stood her up. She also says that it doesn't matter if her husband has received a letter about her. It's not the first attempt to question her honor.
Madame claims that she's going to convince her husband that the anonymous letter came from Monsieur Valenod (which it did) and that Monsieur is just jealous that Madame has never given in to his sexual advances.
Madame writes out a letter to her husband that she wants Julien to create by cutting words out of papers and magazines. The letter basically makes it obvious that Mr. Valenod is trying to steal Madame away from her husband.