I can receive E-Mails but I cannot send E-Mails

[leon1912]

Hello,
today I have setup Roundcube.
Everything works fine.
When I tested the SMTP Settings, this error appears: "(Connection failed: Failed to connect socket: fsockopen(): unable to connect to ssl://mail.gmx.net:587 (Unknown error))"
What did I made wrong?
I can receive E-Mails but I cannot send E-Mails.
Thanks in advanced.

Best regards
leon1912