I’m not on any of those blacklists, luckily. I guess Proofpoint doesn’t publish theirs. At least iCloud and Gmail both use them. I saw one hint that they may require mail servers to literally have the word “mail” as the subdomain, so I’m working up the courage to mess around with my perfectly working DNS.
I’m currently trying to figure out why my email server got blocked by Proofpoint and they refuse to talk to me. Really about ready to give up on email after self-hosting it for a decade with few problems.
Part of me thinks if I have to pay for a relay service, I should just pay for hosted email. But I’ve definitely been considering it!