root@localhost:~# telnet smtp.gmx.com 587
Trying 212.227.17.184...
Connected to smtp.gmx.com.
Escape character is '^]'.
220 gmx.com (mrgmx002) Nemesis ESMTP Service ready
ehlo gmx.com
250-gmx.com Hello gmx.com [181.95.231.27]
250-SIZE 69920427
250-AUTH LOGIN PLAIN
250 STARTTLS
auth login
334 VXNlcm5hbWU6
bmlvY29kQGdteC5lcwo=
334 UGFzc3dvcmQ6
azhzNDIxMDkxMg==
235 Authentication succeeded
mail from: niocod@gmx.com
550-Requested action not taken: mailbox unavailable
550 Sender address is not allowed.