The idea would be that when you send an email, you give it a category, like politics. Then at the bottom of each person's email, there are two links: 1) Unsubscribe from person X's emails about category Y, and 2) Unsubscribe from all of person X's emails from this system

So if that person clicked link #1, the token in the link is verified that it was created for that email address, and then it's noted in a database. In the future, if person X sends another email for that category, it won't go to that person. I'm thinking the sender wouldn't even know that the person unsubscribed