jail
IPv4 IP address space is just about used up, as the unused pool of IP addresses waited to be doled out is no more. And practically speaking, almost all email sending lives in IPv4 space. (That’s an oversimplification — a few ISPs do support it, but frankly, I would not call it broad.) I’d even say that most email sending platforms – email service providers (ESPs), customer relationship management (CRM) software, Marketing Clouds, email automation tools and the like, very few of these support sending mail over IPv6. So when it comes to email and email deliverability, the discussion is almost singularly about IPv4. Spammers and scammers need lots of IP addresses. Throughout the modern history of email, most spam filtering and blocking of badness was based on identifying that badness at the IP address level. Meaning bad guys doing bad things with an IP address find it blocked and