Outbound emails are not sent via nginx smtp proxy

All emails ending at the destination note the sender as the actual
backend
and not nginx proxy. How can I force smtp outbound through nginx? I
tried
smtp_bind_address = proxy.ip.addr in main.cf for postfix. And tried
adding
an SNAT rule in iptables to route through the proxy with no luck. Any
advice?

Posted at Nginx Forum: