At what stage is it reasonable to tell a journalist to whom you have been speaking or emailing that it's all confidential and not for use?

A Guardian writer was recently seeking an interview with a well-known author via email. In his first email the journalist identified himself and explained why he wanted to talk to her. Although she was reluctant at first, there was an email exchange between her and the journalist. Towards the end of the third email the author wrote: "These emails, btw, are confidential… just so's you know."