Different strokes
Lots of admins on here defending Spamhaus and saying it's easy to get off the lists, and as a sysadmin (but not responsible for email) I understand that, however as a user I have very much been collateral damage as a result of a single one of my email provider's servers being put on their list due to relatively spurious reasons.
I will not forget the arrogant and unhelpful attitude of Spamhaus at the time. They just did not care about collateral damage and there wasn't a thing I could do about it.
Of course, the problem was made immeasurably worse by my Dad's email provider mostly not sending a bounce, or occasionally sending one after 48 hours.
Either way, absolutely nightmare of a problem to troubleshoot as an end user, the problem effectively manifested itself as "random" depending on where my provider's load balancer directed me - to the flagged server, my mail wouldn't get through, or to an unflagged server and my mail would get through.
I don't know how the problem was eventually resolved, but it was months later, well after I had started using another provider just to keep in touch with my Dad.