Mine works. But if it were to fail it would not be fault of GMX. I went through this before. Let me explain.
Some ISP companies will block all SMTP except for their service. They will tell you it is for everybody's good. Yet they offer no proof that the block was needed.
You can do at least three things.
1) Just resign yourself to use only what they provide.
2) Calling the support and get the details of how to fix this.
3) Go to a service that has more respect for its customer.
Personally, I think this is abusive competition and should be punished. But I don't have the nerve to start a class action suit.