[LinuxBender]

In some of the examples you provided they the email campaign providers will create a unique SNAT with a unique FCrDNS for the customer if they are big enough so they don't get in the polluted SNAT pool of smaller companies. In those cases they can still be blocked by regex or sub-domain name.

For the generic SNAT pools of the smaller customers one can at least rate limit those providers and also tag them as Possible Spam or adjust their spam score in tools like Spam-Assassin. It's not perfect but tagging and rate-limiting can help.