_ my /etc/tcp.smtp (the only one I found on my system) apparently only allows localhost to send emails. Or am I wrong? _
You are correct.
_ James: I am not sure if I installed smtp-auth... How can I check? How could I change this configuration now? _
I am not James, but... if you can send email through any client or through a telnet to port 25 without the system asking you for a username/password then you probably don't have it installed. smtp-auth exists as a patch and so you can't just change the configuration. You need to apply it and recompile qmail. Look for any of the several smtp-auth patches that exist at http://www.qmail.org/top.html
_ To: mailto:myprivateemail%yahoo.com@arasis.de _
qmail doesn't relay addresses of this format. If you want to reject even the probe, then patch qmail with the patch located here: http://www.qmail.org/qmail-smtpd-relay-reject
HTH.
