Honest and non-rhetorical question here: Have any of your customers had an e-mail they've tried to send not arrive because the recipient's system was using a black-hole list that, for some erroneous reason, had you blocked? If so, were you able to successfully communicate with and/or reasonably work through whatever issue got you black-holed?
I haven't administered e-mail servers for 20 years, but back when I did, this started to be a problem that eventually became insurmountable. I used to manage a small business oriented ISP. We were multi-homed with a /18 that we used for everything. I had a customer that was a reasonably sized organization that dealt with tourism and conventions for a major city. On one of their websites, (which we hosted with IPs that came out of the same /18 as their mail server,) they had a directory of vendors who were associated with them. ONE of those members had a website that had been hacked/defaced. This got our entire /18 on a blac-khole list. They had an employee that was trying to send e-mail to someone on a system that was using this black-hole server to filter spam.
When we explained to them what the problem was, we got glassy-eyed stares back at us and a, "just fix it." I told them that, they would need to remove the link to their partner's site from their website in order to get them AND all of our other customers using numbers inside our /18 de-listed from this particular black-hole. They asked, "We have hundreds of partners who pay for membership in our organization and being listed on our website is one of the benefits. How can we possibly police every one of those websites every day to make sure there's no defacement or serving of any problematic material from any URL in any of those domains?" That's a decent argument in my opinion. And I tried to explain that different black-holes have different policies and no black-hole is demanding that anyone use their system for filtering. I tired contacting the organization that was using that black-hole to explain the situation to them, but they weren't interested in discussing it. As far as they were concerned it was our problem to deal with.
This kind of problem happened dozens of times with varying degrees of severity but with increasing regularity and it was one of the primary reasons we quit hosting e-mail and started re-selling another vendor's solution. That was a long time ago, and maybe black-hole lists aren't a thing anymore.