How can I enable DKIM email verification on windows server 2019 for sending DKIM verified email via SMTP service?

The Received: ... with Microsoft SMTPSVC(10.0.17763.1697); suggests you are using IIS SMTP.

• As IIS SMTP is from IIS 6.0 (code name "Duct Tape", included with Windows Server 2003) and not updated since, it does not support DKIM, which has been produced afterwards (released in 2005, first RFC from 2007).

• The outboud TLS encryption (which is not a message but a transit encryption!) is feasible through:

  1. Internet Information Services (IIS) 6.0 Manager
  2. [SMTP Virtual Server #1] Properties
  3. Delivery
  4. Outbound Security...
  5. [x] TLS encryption

  

However, it would be much better to let a fully functional email server like Microsoft Exchange or Postfix handle both DKIM and TLS. Without the intermediate IIS SMTP, using the SmtpClient Class, you could either send the message unencrypted on a local network or utilize STARTTLS from the SmtpClient.EnableSsl Property:

using (SmtpClient smtpClient = new SmtpClient("smtp.example.com", 25))
{
    smtpClient.EnableSsl = true;
    smtpClient.Send(Email);                            
    return "True";
}