Thank you for your reply.
I do not know what a Greylist is or where to find the setting.
Upon further investigation, it appears that the problem is on the GMX end.
GMX appears to have a filter in their server and check the incoming email for being spam. If there are more than a specific number of spams from an IP address, it is blocked from further email receipts temporarily.
I suspect that the emails I am trying to receive are misjudged for being spam.
I am now communicating with GMX to clear up this issue.