Obscenidades (1986)

A bourgeois housewife's life is shaken by a series of anonymous letters that she periodically receives, increasingly obscene. At first, she believes the sender to be a complete stranger, then she begins to suspect her husband's friends. But she doesn't dare talk to her husband about the letters or their contents: she remains anxious, thinking about suicide, and always waiting for the postman.

Unfortunately the movie Obscenidades is not yet available on HBO Max.