[chrisretusn]

I use Claws-Mail these are my settings.

SMTP server: mail.gmx.com
Use STARTTLS command to start encrypted session
Port: 587

Here is the session log, sanitized of course

* Account 'GMX': Connecting to SMTP server: mail.gmx.com:587...
[15:55:23] SMTP< 220 gmx.com (mrgmx103) Nemesis ESMTP Service ready
[15:55:23] ESMTP> EHLO redacted
[15:55:24] ESMTP< 250-gmx.com Hello redacted
[15:55:24] ESMTP< 250-AUTH LOGIN PLAIN
[15:55:24] ESMTP< 250-SIZE 69920427
[15:55:24] ESMTP< 250 STARTTLS
[15:55:24] ESMTP> STARTTLS
[15:55:24] ESMTP< 220 OK
[15:55:27] ESMTP> EHLO redacted
[15:55:28] ESMTP< 250-gmx.com Hello redacted
[15:55:28] ESMTP< 250-AUTH LOGIN PLAIN
[15:55:28] ESMTP< 250 SIZE 69920427
[15:55:28] ESMTP> AUTH LOGIN
[15:55:28] ESMTP< 334 redacted
[15:55:28] ESMTP> [USERID]
[15:55:29] ESMTP< 334 redacted
[15:55:29] ESMTP> [PASSWORD]
[15:55:29] ESMTP< 235 Authentication succeeded
[15:55:29] ESMTP> MAIL FROM:<redacted> SIZE=396
[15:55:30] SMTP< 250 Requested mail action okay, completed
[15:55:30] SMTP> RCPT TO:<redacted>
[15:55:30] SMTP< 250 OK
[15:55:30] SMTP> DATA
[15:55:31] SMTP< 354 Start mail input; end with <CRLF>.<CRLF>
[15:55:31] SMTP> . (EOM)
[15:55:32] SMTP< 250 Requested mail action okay, completed: id=redacted
* Mail sent successfully.
[15:55:32] SMTP> QUIT
[15:55:33] SMTP< 221 gmx.com Service closing transmission channel