I think you are missing that a particular newsletter is not intrinsically ham or spam. It is ham if the user has subscribed, and spam if they have not affirmatively subscribed.
I have seen the very same content arrive at my mailserver for 2 users. For one it is ham and the other it is spam. Therefore the determination of ham/spam has to be done based upon per-user data, and from that user's classification (or records of subscription, in theory, but that's unrealistic).
