Dynamically resolving smtp upstream hostnames

[kay]

I'm using a new "proxy_smtp_auth on;" option in my mail proxy server inside
k8s cluster, therefore I just need to proxy_pass smtp requests to the
backend. Since the backend must be set using the "auth_http" directive, I
use the code below and provide the hostname instead of IP address:

  server {
    listen 127.0.0.1:8025;
    server_name _;
    access_log off;

    location / {
      add_header "Auth-Server" "smtp-hostname.local";
      add_header "Auth-Port" "25";
      return 200;
    }
  }

Unfortunately nginx even with the resolver enabled in mail section doesn't
resolve the hostname:

auth http server 127.0.0.1:8025 sent invalid server
address:"smtp-hostname.local" while in http auth state

It is also not clear how to resolve the "smtp-hostname.local" hostname to an
IP address in the "127.0.0.1:8025" listener. Using LUA is an overkill. Any
suggestions on how to dynamically resolve the smtp upstream hostname?

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,291890,291890#msg-291890