SMTP-Authentification with GMX - I am getting crazy...

[Friedericksen Hope]

Dear all,

since one hour I am trying to set up gnus to send mail via my gmx account. I 
always get this error (*trace of SMTP session to mail.gmx.net* buffer):

>
> Process SMTP deleted
> 220 mail.gmx.net GMX Mailservices ESMTP {mp020}

> EHLO friedericksen-desktop

> 250-mail.gmx.net GMX Mailservices

> 250-8BITMIME

> 250-ENHANCEDSTATUSCODES

> 250-SIZE

> 250-AUTH=LOGIN PLAIN

> 250-AUTH LOGIN PLAIN

> 250 STARTTLS

> MAIL FROM:<mymailaddress@gmx.de> SIZE=287

> 550 5.7.0 Need to authenticate via SMTP-AUTH-Login {mp020}

> QUIT

> 221 2.0.0 GMX Mailservices {mp020}


I googled around and found some threads and messages, but unfortunately nothing 
helped. What can I do to find out where the problem is? Or maybe someone of you 
sends successfully mail with gmx and can
help me.

I am under ubuntu 10.10 with GNU Emacs 23.1.1 (x86_64-pc-linux-gnu, GTK+ 
Version 2.22.0) of 2010-11-26 on crested, modified by Debian
Gnus is version v5.13.

My configuration in .emacs:

> ;;;;; sending
> (setq starttls-use-gnutls t
>       starttls-gnutls-program "gnutls-cli"
>       starttls-extra-arguments nil
>       send-mail-function 'smtpmail-send-it ; not for Gnus
>       message-send-mail-function 'smtpmail-send-it ; for Gnus
>       smtp-mail-local-domain nil
>       smtp-starttls-credentials '(("mail.gmx.net" 25 nil nil))
>       smtp-auth-credentials '(("mail.gmx.net" 25 "mymailaddress@gmx.de" 
> "mypassword"))
>       smtpmail-default-smtp-server "mail.gmx.net"
>       smtpmail-smtp-server "mail.gmx.net"
>       smtp-smtp-service 25
>       smtpmail-debug-info t
>       smtpmail-debug-verb t) ; only to debug problems

gnutls-cli is installed:

>  which gnutls-cli
> /usr/bin/gnutls-cli

I would greatly appreciate any help!

Best,
Friedericksen