Spammers precisely. I'm not famous but with that nobody would be afraid of putting his email address on the net. The idea was already proposed in Bill Gates' The road ahead and nowadays Zuckerberg is making people pay to send him email.
With a small financial incentive people would be motivated to reply to your mail too. You could ask anyone no matter how rich, some questions and be sure to get an answer if when he/she has agreed to get only paid messages he/she can only collect money if he/she actually answers.
