You read the letter to understand why she is sad. So, don't talk about the smoking unless she brings it up. My guess is that she will. When she does, keep it short and encouraging. She probably knows your feelings about smoking anyway. You might gently mention that after the initial rush, smoking adds to depression or sadness because of its physical effect on the body.
Usually the best way to re-establish trust is to be open and honest. Teens often don't want to talk with their parents, but parents have to continue talking to their teens.