Can you completely block email addresses at gmx?
In the gmx settings I can only find a block list that classifies a specific sender as spam.
But I want to block an address completely – so that nothing from that person reaches me and the person receives a message with the title "MAILER DEMON".
Is that possible?
Such messages are sent only by the mail server itself (the mail server software) when certain error states occur (eg "Email address does not exist on the server").
You will not be able to do this as a GMX customer, so you would have to intervene in the basic behavior of the mail server yourself and set it up there.
And you can't completely block – without "MAILER DEMON"?
You can set up a rule that emails from specific senders are simply marked as read and moved into the recycle bin of the mailbox (so deleted).
Then you can't get any more of them.