Apparently DOpus doesn't support TLS (which your server is requesting):
% telnet mail.gmx.com 587
Trying 74.208.5.30...
Connected to mail.gmx.com.
Escape character is '^]'.
220 gmx.com (mrgmxus001) Nemesis ESMTP Service ready
EHLO user@example.com
250-gmx.com Hello user@example.com [50.174.211.189]
250-SIZE 69920427
250-AUTH LOGIN PLAIN
250 STARTTLS
Here are some possible options: