Well, here we go again... somehow, I'm not getting this smtp proxy to work with nginx.
I moved to haproxy, and this combination works ok. Creating a tcp connection passes over to postfix
and the postfix prompt is seen using a telnet connection - and all works just fine.

However, I'd like to stick with nginx if possible....actually if possible at all!

Here are my findings - and maybe somebody can help to confirm or disagree:

1) Xclient = on will basically bypass sals authorithation in postfix.
Postfix/Sasl will assume that the message is already authenticated.
All the auth login commands are basically exectuted

2) Xclient = off will not trigger any sals authentication in postfix.
Somehow, it seems, that the credentials are not forwarded to postfix

Is this really the expected behaviour?
IMAP behaviour is completely different. Here the authentication works just fine...

Comments appreciated,
Cugar15