Ok, tested it.
Your reverse (PTR) record:
xxx.xx.xxx.xxx.in-addr.arpa -> static.xxx.xx.xxx.xxx.clients.your-server.de
You have reverse (PTR) records for all your IPs, that is a good thing.
All the records are fine, the only thing that could be the problem (just guessing) is that I'm using CloudFlare so all my traffic is going through CloudFlare and not directly. My servers IP address is not appearing anywhere.. Only the CloudFlare's IP addresses. This one from the reverse DNS is probably from my hosting company, since I rented a server in Germany so they've setup reverse DNS for all their servers.
The reverse DNS IP and the NameServer IP are not the same.
Would that be OK if I tell you my domain so you can check it out yourself? If it isn't against the forum rules..