I worked with a guy who would take every piece of spam we got, dig into the mail headers, and then add the source IP to a custom blacklist on our mail server. I

I worked with a guy who would take every piece of spam we got, dig into the mail headers, and then add the source IP to a custom blacklist on our mail server. It took a few hours to figure out why we weren't getting email from a customer and that's how I found the list. He had manually added over 20,000 IP addresses to the list.