How to delete a GMX account?
The description of the service:
GMX Mail is a free advertising-supported email service provided by GMX (Global Mail eXchange, in Germany: Global Message eXchange). Users may access GMX Mail via webmail as well as via POP3 and IMAP4 protocols.
You can't delete your account, but you can deactivate it. (Start → Mein Account → Account stilllegen)