smtp proxy with postfix

[Cugar15]

Hello,

  I'd like to build a smtp Proxy with nginx (v1.8.0) and postfix (v2.9.6) on
Debian7. 

Somehow I'm stuck with the following problem:

1) Configuration1:
    smtp_auth         login plain cram-md5;
    xclient               on;

    ==> a) My mail client can authenticate (IP:yy.yyy.yy.yy), send email and
receive email (imap) - even with tls
                mail.log: 
                 connect from nginx_prox.de[xx.xxx.xx.xx]
                 client=unknown[yy.yyy.yy.yy], sasl_method=XCLIENT,
sasl_username=my_username

            b) But no emails from others are received - obviously everybody
has to authenticate!!
          
2) Configuration2:
    smtp_auth         none;
    xclient               on;

    ==> creates an open relay!

In Postfix, I have set: smtpd_authorized_xclient_hosts = xx.xxx.xx.xx  

What I'd like to achive is the current postfix behaviour:
1) Receive emails from every Sender
2) Only authorized users can send emails from outside the Network

Help is appreciated... I found bits and pieces in the Forum and other places
- but nothing seems to be consistent...

Thanks,
Norbert

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,263559,263559#msg-263559