If you are even a little known (published a book, a business leader, write a popular blog, etc.), you probably get a lot of unsolicited emails. It would be good to have something in return for all the time you waste on those emails.
An email service that allows whitelisted people (your contact list) to send you an email just as they do today. Anyone else who sends you an email, before you receive it, gets an automated reply saying something like this (for instance if you just had your book published): “Hi. You are not on my white list, and since I receive a lot of emails from people I don’t know, I need to know this is beneficial for both of us. I ask anyone who wants my attention to do me a small favor and give me their attention first. I just published my new book, and it will help me if you take some time and help me promote it. Click on any of the following buttons to help me promote it. The more you help me by giving your attention to me and sharing on multiple platforms the more attention I can give your email. Thanks.” You can set a threshold for how much they need to do for their email to get to your inbox.