There are DNS blocklists for mail servers. It's a slightly different thing, but basically there's if there's an entry it means there's a reputation for the mail sender. Some of them are present/not present and some have a score. Some are against the IP of the sender only, while others are against the domain name or full hostname of the sender. You just set your mail server to do the right lookup against the right DNS server, and incoming mail can be filtered based on the results of the DB someone else is maintaining.