Because this is a systematic problem _and_ I don't wan't to change the default SA scores for dnswl for some reasons seems the only way to fight against this special problem is to write a local rule. This rule should check if mail from mail.mobile.de has been originated by them itself (then it is never spam and the DNSWL_HI negative score is ok). But if mail is received from mail.mobile.de by external sources this negative score should be neutralized. It's a little bit hard to detect, but I'll archive some more examples to find a rule that will do this.
It makes no sense to point this to dnswl - mobile.de itself is not a spam source itself.
