Validate LetsEncrypt certificate on Ubuntu 16.04 server

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When I run this on the .NET Core 3.1 Ubuntu Bionic image, it works fine:

using System;
using MailKit.Net.Smtp;

namespace app {
 class Program {
  static void Main(string[] args) {
   using(var client = new SmtpClient()) {
    client.Connect("dotforward.de", 465, true);
    client.NoOp();
    client.Disconnect(true);
   }
  }
 }
}

When I run the program, I can also see that the .NET Core runtime is reading from /etc/ssl/certs/ca-certificates.crt. Maybe make sure that file exists and is readable.