server {
    listen 8443 ssl http2;
    server_name mail.domain.com;

    include /etc/nginx/error.conf;

    location / {
        proxy_set_header Host $host;
        proxy_pass https://<internal ip of mail server>/;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header X-Forwarded-Proto $scheme;
        client_max_body_size 0;
        proxy_connect_timeout 3600;
        proxy_send_timeout 3600;
        proxy_read_timeout 3600;
        send_timeout 3600;
    }
}

<variable name="UseProxiedInfo">1</variable>
<variable name="TrustedProxyAddressGroup">proxy</variable>

<listitem>
  <variable name="Name">proxy</variable>
  <variable name="Value"><nginx ip>/255.255.255.0</variable>
  <variable name="Enabled">1</variable>
  <variable name="Desc"></variable>
  <variable name="Guid">a0ad4250-dcfc-4a76-9336-c6f30df362c1</variable>
  <variable name="GroupGuid">b3b9b745-d659-4cdf-a5a4-069de171d899</variable>
</listitem>