NullReferenceException from Exchange.WebServices.Managed.Net5

Question

Triggering mail from ASP.net Core MVC application, using Exchange.WebServices.Managed.Net5(https://www.nuget.org/packages/Exchange.WebServices.Managed.Net5/2.2.0).Intermittently getting the System.NullReferenceException: Object reference not set to an instance of an object., Sometimes mail gets send even after the exception. Why this error and how can it be solve?

public static void CreateMessage(string username, string To, string Subject, string Msg, string Smtp, int Port, string Password, bool useSsl, string ToCc = "")
       {
           try
           {

               ExchangeService service = new ExchangeService(ExchangeVersion.Exchange2007_SP1);

               service.Url = new Uri("https://" + Smtp + "/ews/Exchange.asmx");
               ServicePointManager.ServerCertificateValidationCallback = (sender, certificate, chain, sslPolicyErrors) => true;
               service.UseDefaultCredentials = false;
               service.Credentials = new WebCredentials(username, Password);


               EmailMessage message = new EmailMessage(service);
               message.Subject = Subject;
               message.Body = Msg;
               message.ToRecipients.Add(To);
               if (!string.IsNullOrEmpty(ToCc))
                   message.CcRecipients.Add(ToCc);
               message.Save();

               message.SendAndSaveCopy();
           }
           catch (Exception ex)
           {
               throw new Exception("Email sending error:" + ex.ToString());
           }
       }