The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required.

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ASP.NET

Hi
While sending mail I have got this error: The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required.

In my domain also it will not accept and the mail was not send.

Can anyone help me to solve this. Whats the problem in my code?

Here is my code,

string admin_mail = "karthikeyan.marlen@gmail.com";
            try
            {
                MailMessage message = new MailMessage();
                message.From = new MailAddress("karthikeyan.marlen@gmail.com");
                message.To.Add(lmail + "," + admin_mail);
                message.CC.Add("thendral.marlen@gmail.com");
                message.Subject = "UserName_and_Pwd";
                message.SubjectEncoding = System.Text.Encoding.UTF8;
                message.Subject = "Strikeaprice-Registration";
                message.IsBodyHtml = true;

                message.Body = "UserName ='" + un + "' and Pwd='" + pd + "'";
                message.BodyEncoding = System.Text.Encoding.UTF8;
                message.Priority = MailPriority.High;


                SmtpClient mailClient = new SmtpClient();
                mailClient.Credentials = new System.Net.NetworkCredential("username", "password");
                mailClient.UseDefaultCredentials = true;
                mailClient.Port = 587; // Gmail works on this port
                mailClient.Host = "smtp.gmail.com";
                mailClient.EnableSsl = true; //Gmail works on Server Secured Layer 

                try
                {
                    mailClient.Send(message);
                    message.Dispose();
                    return 1;
                }
                catch (Exception s)
                {
                    throw (s);
                }
            }
            catch (Exception e)
            {
                throw (e);
            }