You actually shouldnt makebit 25 cents to send, just make it so you need to deposit 25 cents for every mail you want to send for a 7 day period and make it trivially easy for me as the recipient to cause ypu to lose that deposit in the 7 days. For example embed a url in the email that i can click to open a webpage that asks me if the email is legit or not with a big red no button and a big green yes button. Money doesnt even have to flow to me. The whole thing could be a non profit that invests the forfitted money in medical research or saving puppies or something else suitably unoffensive.