RestClient client = new RestClient();
   client.BaseUrl = new Uri("https://api.mailgun.net/v3");
   client.Authenticator =
           new HttpBasicAuthenticator("api",
                                      "YOUR_API_KEY");
   RestRequest request = new RestRequest();
   request.AddParameter("domain",
                        "YOUR_DOMAIN_NAME", ParameterType.UrlSegment);
   request.Resource = "{domain}/messages";
   request.AddParameter("from", "Excited User <mailgun@YOUR_DOMAIN_NAME>");
   request.AddParameter("to", "bar@example.com");