NOT receiving any mails to GMX recipients

I have created a newlsetter and wanted to test it with various email adresses of mine.

I had gmail, gmx, hotmail and others.

There was no problem except with the GMX recipients -> the mails where either not delivered or I would receive them like 30 minutes after actually sending the mails.

Anybody who knows whats the problem here and how to fix it?

Thanks guys