If you have "All others" blocked in your MSN Messenger (Tools\Options\Privacy), though people who are not on your list or are blocked can open conversation window to you (indeed), they won't see you online; that is, they don't get response from your Messenger, all the messages they send to you will be neglected..etc. And thus they can't tell if you are really offline or not. It is Messenger protocol that have something to do with all the matters