How to pass connection's real IP through Nginx smtp proxy to Postfix/postscreen backend?

[cyang]

I run Postfix 3.3.1 & Nginx 1.15.0

Both work great.

I'm beginning to experiment with putting Postfix (and eventually other) server 
behind Nginx (v 1.15.0) setup as a mail (SMTP) proxy.

Without the proxy, Postfix logs show an inbound connection to my real IP

        Jun 21 12:12:31 mailprox postfix/postscreen[55634]: CONNECT from 
[74.125.142.27]:43757 to [192.0.2.1]:25

The way nginx gets configured for smtp proxy, even if I'm *NOT* doing any auth 
is to direct the connection to a "fake" auth_http destination,

        mail {
                ...
            auth_http 127.0.0.1:33001/dummy.php;
                ...
        }
        http {
                ...
                server {
                listen 127.0.0.1:33001;
                ...
                location ~ .php$ {
                        add_header Auth-Server 127.0.0.1;
                        add_header Auth-Port 33025;
                        return 200;
                }
                ...
        }

Switching over, the proxy is set up to listen on the real IP

        [192.0.2.1]:25

and passes to Postfix's postscreen which using the config above is listening on

        [127.0.0.1]:33025

What I see in the Postfix log is

        Jun 21 12:10:12 mailprox postfix/postscreen[55329]: CONNECT from 
[127.0.0.1]:31460 to [127.0.0.1]:33025
        Jun 21 12:10:12 mailprox postfix/postscreen[55329]: WHITELISTED 
[127.0.0.1]:31460

Mail does get delivered but postscreen is whitelisting the IP of the proxy, 
127.0.0.1, and not using the real IP.

I need to somehow pass the Real-IP through to postscreen, and anything further 
downstream that'll need it.

For web server proxying I'd pass something like

        X-Forwarded-For

or

        X-Real-IP

to a downstream webserver listener.

What do I need for Postfix/Postscreen to correctly 'see' the Real IP?

A header added to the nginx config?  Some additional code in the auth_http? 
Something else?

Cheers!

Cy