How to setup nginx to proxy my smtp mail server

I am having nodejs SMTP server which I made using module “smtp-server”. It is running locally at 127.0.0.1:9001 using pm2 module and I wanted to know how can i configure it with nginx. Currently I am reffering to this site: https://docs.nginx.com/nginx/admin-guide/mail-proxy/mail-proxy/

I ran complete example they gave me which is as follows:

worker_processes auto;

mail {
    server_name mail.example.com;
    auth_http   localhost:9000/cgi-bin/nginxauth.cgi;

    proxy_pass_error_message on;

    ssl                 on;
    ssl_certificate     /etc/ssl/certs/server.crt;
    ssl_certificate_key /etc/ssl/certs/server.key;
    ssl_protocols       TLSv1 TLSv1.1 TLSv1.2;
    ssl_ciphers         HIGH:!aNULL:!MD5;
    ssl_session_cache   shared:SSL:10m;
    ssl_session_timeout 10m;

    server {
        listen     25;
        protocol   smtp;
        smtp_auth  login plain cram-md5;
    }

    server {
        listen    110;
        protocol  pop3;
        pop3_auth plain apop cram-md5;
}

     server {
        listen   143;
        protocol imap;
    }
}

I wanted to know how to connnect my nodejs smtp server to nginx as the code above does not ask anywhere for url to smtp server.