Moini, do you mean the gmx.de server admin?
Our host (mikey) tells me it's a common spam domain. So maybe they can't confirm us because we've got some kind of block related to spam?? I'm not sure. We've concluded that the problem is on their side, but I didn't think of trying to contact them.
If we were going to remain as the official forum, I guess it would be worth tying to contact them. We'll just have to be mindful of this, when we set up the new forum. Maybe the new host won't have any problems, who knows.