title: The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required
body: <p>I have a Problem, when i m sending a mail through this code then error is occured that "The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required. "</p>
  
  <p>and my code is :</p>
  
  <pre><code>protected void ImageButton1_Click(object sender, ImageClickEventArgs e)
  {
  
      MailMessage mail = new MailMessage();
      mail.To.Add("info@msslindia.com");
      mail.From = new MailAddress("info@msslindia.com");
      string body = "&lt;table&gt;&lt;tr&gt;&lt;td&gt;Company Name:&lt;/td&gt;&lt;td&gt;" + txt_cname.Text + "&lt;/td&gt;&lt;/tr&gt;&lt;tr&gt;&lt;td&gt;Address With No.:&lt;/td&gt;&lt;td&gt;" + txt_addwithno.Text + "&lt;/td&gt;&lt;/tr&gt;&lt;tr&gt;&lt;td&gt;Contact Person:&lt;/td&gt;&lt;td&gt;" + txt_conperson.Text + "&lt;/td&gt;&lt;/tr&gt;&lt;tr&gt;&lt;td&gt;Email Id&lt;/td&gt;&lt;td&gt;" + txt_email.Text + "&lt;/td&gt;&lt;/tr&gt;&lt;tr&gt;&lt;td&gt;Description&lt;/td&gt;&lt;td&gt;" + txt_description.Text + "&lt;/td&gt;&lt;/tr&gt;&lt;/table&gt;";
      mail.Body = body;
      mail.IsBodyHtml = true;
      SmtpClient smtp = new SmtpClient();
      smtp.Host = "smtp.gmail.com";
      smtp.Port = 25;
      smtp.UseDefaultCredentials = true;
      smtp.Credentials = new System.Net.NetworkCredential("info@msslindia.com", "xyz");
      smtp.EnableSsl = true;
      smtp.Send(mail);
  
  
      txt_cname.Focus();
      txt_cname.Text = "";
      txt_addwithno.Text = "";
      txt_conperson.Text = "";
      txt_email.Text = "";
      txt_description.Text = "";
  
  }
  </code></pre>