Here is an update:

Configuration2:
smtp_auth none;
xclient on;

Does not create an open relay. I have tested a telnet connection on my proxy host - which is different to my postfix host.
Out of what ever reason, I don't seem to need authentication when sending email from this host...

If I'm testing from a different host, authentication is required - though not adverticed bz EHLO....

One more thing is still strange:
My email client (Outlook) connected via a different ISP to the Internet does not seem to have to authenticate when testing the settings.
First, an imap Connection is established - here, authentication is done and successfull.
Next smtp is tested. When I set, that the outgoing mail sever (smtp) requires authentication, Outlook reports, that it cannot find a proper authentication method...
If I disable that the outgoing mail server authentication, everything runs ok...

Any idea why this is?

Thanks,
Cugar15