Everything is in a state of disarray at Anna's house due to the packing, when Karenin's messenger comes up with his letter for Anna. He says he has been instructed to bring back a reply.
Anna is completely horrified by the letter, which, if you'll recall, demands that she continue their lives as a married couple as it has gone on before. If she doesn't comply, he'll use legal means to take Seryozha away from Anna.
Anna's greatest fury at the letter arises from the fact that she knows that everyone on the outside of their marriage will see Karenin's decision as the decision of an ethical, honorable man.
Anna, on the other hand, exclaims that he's truly vile, that people who see him as generous towards her in this matter have no idea that he has smothered her for the past eight years, never thinking of her as the lively, vibrant woman that she truly is. The birth of her son is the only thing that justifies her settling for Karenin.
She goes to write a response, but then realizes that she actually lacks the strength to abandon her former social position as Karenin's wife, no matter how false it is. She can't bear the idea of exchanging her current life for an even more criminal one on the outskirts of society, as a woman who has abandoned her husband and son in favor of her lover.
She bursts into tears, feeling shameful and hopeless. She sees her future as a criminal wife.
The footman comes and tells Anna that the messenger is awaiting a reply.
Anna feels desperate to see Vronsky. She decides to go to Betsy's.
Her reply to her husband is merely that she received his letter.
She tells her maid that they are not leaving for Moscow that evening, but not to unpack. Anna decides to go to Princess Betsy's home.