What I find infuriating is when an admin gets the bright idea to block all connections from IP's that are in mail dnsbl's. It's a great way to alienate people from using your services. I can't remember which company was doing it, but a customers API calls to a vendor failed because the vendor blocked the IP, which was blacklisted somewhere.
Here's the kicker: The server wasn't even used to send mail and hadn't been for a long time. So we had to apply for a delisting from a mail blacklist for a server that didn't send mail so that a customer could use an API.
The admin thought they were being clever, but instead they were just being difficult.
https://www.dnsbl.info/