How to resolve exception "the SMTP server requires a secure connection the client was not authenticated. The server response was: 5.7.57 client not authenticated" to send mail in C#?

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C#

SMTP

SmtpClient

I am getting exception while trying to send mail "The SMTP server requires a secure connection The client was not authenticated. The server response was: 5.7.57 Client not authenticated" to send mail in c#. Application is running in .Net 4.8 framework. Mail sending done with SSL Authentication and SSL enabled. Code for sending mail is updated below

What I have tried:

SmtpClient smtpClient = new SmtpClient();
                           MailMessage message = new MailMessage();




                           MailAddress fromAddress = new MailAddress(fromMail, "");

                           // You can specify the host name or ipaddress of your server
                           // Default in IIS will be localhost
                           smtpClient.Host = ConfigurationManager.AppSettings["SMTPServer"];
                           smtpClient.DeliveryMethod = System.Net.Mail.SmtpDeliveryMethod.Network;
                           string SMTPAuthenticationFlag = ConfigurationManager.AppSettings["SMTPAuthenticationFlag"] != null ? Convert.ToString(ConfigurationManager.AppSettings["SMTPAuthenticationFlag"].ToString()) : "0";
                           smtpClient.UseDefaultCredentials = false;
                               smtpClient.Credentials = new System.Net.NetworkCredential(ConfigurationManager.AppSettings["SMTPUserName"], ConfigurationManager.AppSettings["SMTPPassword"]);


                           int SMTPServerPort = 587;
                           smtpClient.Port = SMTPServerPort;
                           //Added by Alex on 20240625 for sending mail with TLS (Mantis Id-1859) Start
                           string SMTPServerSSLFlag = ConfigurationManager.AppSettings["SMTPServerSSLFlag"] != null ? Convert.ToString(ConfigurationManager.AppSettings["SMTPServerSSLFlag"].ToString()) : "0";
                           if (SMTPServerSSLFlag == "1")
                           {
                               smtpClient.EnableSsl = true;

                           }
                           //Added by Alex on 20240625 for sending mail with TLS (Mantis Id-1859) End
                           //From address will be given as a MailAddress Object
                           message.From = fromAddress;

                           // To address collection of MailAddress
                           message.To.Add(toMail);
                           message.Subject = subject;

                           message.IsBodyHtml = true;

                           // Message body content
                           message.Body = mailContent;

                           // Send SMTP mail
                           smtpClient.Send(message);