Django send_mail with 502 Bad Gateway

I have a contact form on my django based website and the form uses django smtp mail backend with smtp.gmail.com settings.

There was no problem with development server DEBUG=True but with production DEBUG=False, when I try to send contact form, it returns with 502 Bad Gateway error.

sudo tail -F /var/log/nginx/error.log

2022/02/20 17:48:38 [error] 1342#1342: *88 upstream prematurely closed connection while reading response header from upstream, client: 176.42.21.164, server: burcuatak.com, request: "POST /contact/ HTTP/1.1", upstream: "http://unix:/home/user/project/project.sock:/contact/", host: "burcuatak.com", referrer: "https://burcuatak.com/contact/"

/etc/nginx/sites-available/project

server {
        server_name burcuatak.com www.burcuatak.com;
        location = /favicon.ico { access_log off; log_not_found off; }
        location /static/ {
            root /home/user/project;
        }
        location / {
            include proxy_params;
            proxy_pass http://unix:/home/user/project/project.sock;
        }
}
server {
    if ($host = www.burcuatak.com) {
        return 301 https://$host$request_uri;
    } # managed by Certbot


    if ($host = burcuatak.com) {
        return 301 https://$host$request_uri;
    } # managed by Certbot


        listen 80;
    listen [::]:80;
        server_name burcuatak.com www.burcuatak.com;
    return 404; # managed by Certbot