I think what "spam" means to most of us is automated mail that we don't want to see. I remember in the days before gmail I saw people set up auto-responders with captcha-like replies. Even that tiny extra validation step was too much friction. So I guess the people who will find this useful are those that are getting too much mail that an actual human is spending to time to write? Definitely not for me, I could see a business mail, HR/recruiting or something similar but you will probably lose them all unfortunately as soon as they see "bitcoin".