Madame de Fervques is starting to find Julien's letters interesting. This is because Prince Korasoff intentionally wrote them to get better over time, nurturing love from a little seed.
She is so bored one day that she finally writes a reply letter to Julien. After that, they get into the habit of writing to one another every day. Little does Madame know that all of her letters are tossed into Julien's desk without being read.
Mathilde notices the letters coming to her house from Madame and finally snaps. She grabs one of the letters and confronts Julien about it, accusing him of adultery.
She thinks that because they've had sex, they are now married. She begs Julien to take her back.