The email deliverability issues we have had as a legitimate business are insane. Moving to an ESP years ago has helped, but it's far from perfect. I've wondered how a small business without the tech resources could manage this.
For instance, several months back some of our account holders suddenly stopped receiving important account info as well as newsletters from us. Tracked it down to a third party filtering service used by various email providers. Contacted them and they indicated that we were sending to honeypot addresses and/or doing other things that landed us on their list. We refuted all of their assertions (all of our recipients are double out-in, etc.) But , they insisted, as if their processes were infallible and refused to remove us.
Here they were, a third party with whom we had no agreement, yet they were interfering with our ability to do business (and profiting from it). We also had paid advertising in the newsletters per agreements with advertisers. Consulted counsel and the next step was to send a cease and desist, followed by an injunction on the grounds of tortious interference. Then, just as suddenly, our emails started going through again.
So, all of this, just to send email.
Yet, I still have to deal with a ton of spam personally when wading through my quarantine folder for routine false positives. Current processes are harder on the good guys than the spammers.