phishersfritz
Spamhaus PBL is build based on your ISP telling Spamhaus which IPs are dynamic and which IPs should not send email. Your ISPs seemed to be nice enough to allow you delisting from it, which not all ISPs will allow.
Abusix has a similar list, but its completely build based on dynamic looking or no PTRs. You can create an account and delist without any issues.
Never the less there is way more services than GMX that block based on dynamic or no PTR. A lot of smaller solutions have this option checked by default. And it actually has been a best practice for decades to have a proper PTR.
If you can, I'd set one and be done with it.
A blacklist is not directly interfering with your business. They just provide a list that contains IPs, your IP, and say we have seen spam traffic from it in the last x hours. The mail receiver, who trusts and uses the list might be interfering, but it is his right to pick and choose who he is accepting email from. Same right you have to pick and choose who you let into your bar, apartment, club, house, ... what so ever.