It's better now, but in the early days of organized blacklists (more than 20 years ago) it was somewhat chaotic. Many large ISPs ran their own blacklists and s

It's better now, but in the early days of organized blacklists (more than 20 years ago) it was somewhat chaotic. Many large ISPs ran their own blacklists and some were poorly managed. AOL was the worst of them all. Their admin staff was unprofessional and unresponsive when I provided a PoC for their defective spam control system. There are a few sites where you can plug in an IP address to see if it's on any blacklists. A handy thing to do before setting up a new server is to work with your provider to find a clean IP address beforehand. Here's one that I have used: https://mxtoolbox.com/blacklists.aspx https://mxtoolbox.com/blacklists.aspx