auth http server 127.0.0.1:9000 did not send server or port while in http auth state- mail proxy nginx

I am absolute newbie to nginx, What i am trying to do is :

1. Setup and configure nginx (from source v1.16.1) on AWS (nginx as mail proxy)- *server status ok..
2. Using stunnel(v5.56)to upstream to G-Suit API, during login authentication from mail client (eg:Thunderbird), the server disconnects and give an error message.

/nginx/error.log: " auth http server 127.0.0.1:9000 did not send server or port while in http auth state, client: xxx.xxx.xxx.xxx, server: 0.0.0.0:993, login: "[email protected]"

Here is /nginx.conf

user nginx; worker_processes 1; pid /var/run/nginx.pid; load_module /usr/lib/nginx/modules/ngx_mail_module.so; error_log /var/log/nginx/error.log info; events { worker_connections 1024; multi_accept on; } mail { server_name example.com; auth_http localhost:9000/api/mail/proxyauth; #ssl on; ssl_prefer_server_ciphers 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 995 ssl; protocol pop3; proxy on; proxy_pass_error_message on; } server { listen 993 ssl; protocol imap; proxy on; proxy_pass_error_message on; imap_auth login plain; } server { listen 465 ssl; protocol smtp; proxy on; proxy_pass_error_message on; xclient off; smtp_auth login; } } 

And /stunnel.conf

[https] accept = 9000 connect = gsuit1domain.com:443 delay = yes 

What must be possible cause of the error?