what is solution of this exception System.IO.IOException: Unable to read data from the transport connection: The connection was closed. in c# email

I am using the .net core 3.1 console application for sending auto-mail to different Email Addresses after sending 4 successful emails It shows Exception

Message: Email sending Failure.

Inner Exception: System.IO.IOException: Unable to read data from the transport connection: The connection was closed.

I have tried all available solutions on stack overflow

My code :

public email
{
            Thread.Sleep(10000);

            count++;
            if (count % 18 == 0)
            {
                Thread.Sleep(3600 * 1000);
            }
            try
                {
                //           System.Net.ServicePointManager.SecurityProtocol = System.Net.SecurityProtocolType.Tls12;
                int tls12 = 3072; // Tls12 is not defined in the SecurityProtocolType enum in CLI/C++ / ToolsVersion="4.0"  
                                  //  System::Net::ServicePointManager::SecurityProtocol = (SecurityProtocolType)tls12;
                                   System.Net.ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12;
                                  //System.Net.ServicePointManager.SecurityProtocol |= SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12;
                                  //shubham .....
                string toEmailId = sEmailAddress;

                var fromAddress = new MailAddress(From_Email);
                var toAddress = new MailAddress(toEmailId);


               


                var smtp = new SmtpClient
                {
                    Host = "smtp.gmail.com",
                    Port = 587,
                    //587
                    EnableSsl = true,
                    DeliveryMethod = SmtpDeliveryMethod.Network,
                    UseDefaultCredentials = false,
                    Credentials = new NetworkCredential(fromAddress.Address, Passwd)
                };
                string messge = MessageBody.ToString();
                var message = new MailMessage(fromAddress, toAddress)
                {

                    Subject = subject,
                    Body = messge,
                    IsBodyHtml = true

                };
                if (cc != null)
                {
                    foreach (string s in cc)
                        message.CC.Add(s);
                  message.Bcc.Add("email Id");
                }

                {



                    ServicePointManager.ServerCertificateValidationCallback =
                          delegate (object s, X509Certificate certificate,
                            X509Chain chain, SslPolicyErrors sslPolicyErrors)
                          { return true; };

                    smtp.Send(message);
                    General.WriteToLogFile(sEmailAddress.ToString(), mFolderPath, "Mailsend.txt");
                }
            }

            catch (Exception ex)
            {
                General.WriteToLogFile(sEmailAddress.ToString(), mFolderPath, "Exception.txt");
                General.WriteToLogFile(ex.Message.ToString(), mFolderPath, "Exception.txt");
                General.WriteToLogFile(ex.InnerException.ToString(), mFolderPath, "InnerException.txt");

            }

        }
}