My guess would be that the casino went out of business and sold the emails to one person. At this point one would send a piece of mail asking them if they would like to receive a newsletter or whatever one wants to send. If they do, they can respond and the resulting number of adresses will be the real list.
Any other scenario would either be criminal or a rip-off. I think the professor is much too smart to be pulled into a rip-off (the same poor adresses being sold many times over) or to do something illegal.