Big spammers own multiple ranges of IPs just as easily as they own multiple domains. When you block only 1 of their range, they can re-use their same domains to

Big spammers own multiple ranges of IPs just as easily as they own multiple domains. When you block only 1 of their range, they can re-use their same domains to spam you from other ranges. If you blocked by domain, this would be effective across all their ranges of IPs. So, in your example, blocking by IP isn't necessarily superior to blocking by domain (and vice versa.) Once again: it's possible to have the tools and infrastructure manage domain reputation and do a job as good as (and arguably better than) IP reputation. Also to consider: IPs can be acquired by spammers for nearly $0 (eg: destroy+recreate VPS to get a new IP for free), whereas domain names have a fixed cost of ~$10 USD per year per domain. So if you have a system that manages domain reputation and has 1 million domain in the blacklist, you know for a fact that you are burning ~$10M/year of spammers' money.