They are using "PTR resource record (PTR-RR) test" for receiving e-mails. If your sender has e-mail account on server with badly configured PTR-RR you will never receive this e-mail on gmx account. If you send first e-mail with question to unknown person you will never receive answer and you will be thinking that this person is ignoring you. And the best thing: With any other e-mail provider there's no problem with badly configured PTR-RR, you will receive e-mail.
Effect: You can't rely on gmx account. And technical support can't see problem. They are proud that they have good security policy. I wish them keep it up :)