How to resolve the exception "authentication failed because the remote party has closed the transport stream" on sending mail in C#?

private bool SendMail(string fromMail, string senderName, string toMail, string mailContent, string subject)
    {
        bool result = true;
        SmtpClient smtpClient = new SmtpClient();
        MailMessage message = new MailMessage();
        try
        {
            MailAddress fromAddress = new MailAddress(fromMail, senderName);

            // You can specify the host name or ipaddress of your server
            // Default in IIS will be localhost 
            smtpClient.Host = ConfigurationManager.AppSettings["SMTPServer"];

            //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 
            //Default port will be 25
            smtpClient.Port = 25;

            //From address will be given as a MailAddress Object
            message.From = fromAddress;

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

            // CC and BCC optional
            // MailAddressCollection class is used to send the email to various users
            // You can specify Address as new MailAddress("admin1@yoursite.com")
            //message.CC.Add("admin1@yoursite.com");
            //message.CC.Add("admin2@yoursite.com");

            // You can specify Address directly as string
            //message.Bcc.Add(new MailAddress("admin3@yoursite.com"));
            //message.Bcc.Add(new MailAddress("admin4@yoursite.com"));

            //Body can be Html or text format
            //Specify true if it  is html message
            message.IsBodyHtml = true;

            // Message body content
            message.Body = mailContent;

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

            //Common.showMessage("e-Mail successfully sent.", this);
        }
        catch (Exception ex)
        {
            result = false;
        }
        return result;
    }