Get on your Server:

ssh root@example.org

Install Sogo Software

apt install sogo

Edit the Configuration File to point to your mailserver:

{
  SOGoSMTPServer = yourdomain.xyz:Port;
  SOGoIMAPServer = "imaps://yourdomain.xyz:Port/?tls=YES";
# SOGoSieveServer = "sieve://localhost:4190/?tls=YES";

}

Now change your Nginx File:

server {
    listen 443 ssl;
    server_name example.xyz;

    ssl_certificate /etc/letsencrypt/live/example.xyz/fullchain.pem;
    ssl_certificate_key /etc/letsencrypt/live/example.xyz/privkey.pem;

    root /usr/lib/GNUstep/SOGo;
    index index.html;

    location /SOGo {
        proxy_pass http://127.0.0.1:20000/SOGo;
        proxy_buffering off;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header X-Forwarded-Proto https;
        proxy_redirect off;
    }