nginx reverse proxy error: The page you are looking for is temporarily unavailable

Question

I have a Nginx reverse proxy. I config it like this :

 server {
        listen       80 default_server;
        listen       [::]:80 default_server;
        server_name miro.behinwebsite.com;
        #root         /usr/share/nginx/html;

        # Load configuration files for the default server block.
        #include /etc/nginx/default.d/*.conf;

        location / {
            proxy_pass https://mail.behinwebsite.com;
        }

        error_page 404 /404.html;
            location = /40x.html {
        }

    error_page 500 502 503 504 /50x.html;

            location = /50x.html {
        }
    }

but i get an error when i browse http://miro.behinwebsite.com in browser nginx error! The page you are looking for is temporarily unavailable. Please try again later.