I would recommend contacting gmx again, and ask them specifically about the "server reputation" from the Groundspeak servers at the following addresses  63.251.163.224, 63.251.163.225, 63.251.163.228, 63.251.163.229. While the problem may be 'with Groundspeak', it could be that as a result, your ISP is bouncing or throttling messages from one or more of those IP addresses, which would explain why you do not receive them.   I do not know for certain whether t