Clarification/Follow-up by Jim.McGinness on 08/26/05 4:09 pm:
One of the problems with a preview window is not that it triggers the "return receipt" mechanism -- the problem arises because the preview window will interpret and render mail with HTML content. The person sending the mail can embed links in the HTML content that, when interpreted, will fetch images (typically) and, piggybacked on the request, can include identifying information that lets the originator of the mail know that your particular email address is a "live one".