I’m using a GMX account across three different platforms.
On all of them I have the SMTP server as:
Name: mail.gmx.com
Port: 587
Security: SSL or TLS/SSL (not STARTTLS)
The one setting not mentioned so far is “Authentication” - use “Normal password”.
Scroll down on this page, browse through the various client set-ups and you’ll see a mix of SMTP settings offered
http://support.gmx.com/pop-imap/imap/index.html.