Somehow related: My email server uses the NiXSpam DNSBL at smtp level to block spam, which most of the time (but not always) blocks spam attacks by spam networks quite effectively. As I noticed when I started hosting a friend that had been at GMX before, GMX uses a separate smtp server (network, probably) for forwarding emails to other email accounts. It seems they don't do any spam checking in this case, at least these forwarding servers are blocked most of the time, too, if not always.

No problem in this case, fortunately, as my friend didn't care, he had sent his new address to all who were important to him and was receiving more or less spam only via the GMX forward. Interestingly, when I had a closer look at the logs at the beginning, it looked as if for some emails, GMX tried again later from another server that went through. I don't know if it was just coincidence in those cases that the second try was from a server that was currently not listed in NiXspam, or if GMX retries with their normal servers if they think that this particular message was NOT spam. But why deliver the spam in the first place, then?