Nginx as SMTP Proxy.

nginx.conf

http {
  server {
    listen 80;
    server_name localhost;
    access_log /var/log/nginx/access.log;
    error_log /var/log/nginx/error.log;

    root /var/www;

    location /auth_server {
      add_header Auth-Server 127.0.0.1;
      add_header Auth-Port 2525;
      return 200;
    }
  }
}

mail {
  server_name localhost;
  auth_http localhost/auth_server;

  smtp_auth none;
  xclient off;

  server {
    listen 25;
    protocol smtp;
  }
}