SMTP Server Exception

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C#

ASP.NET

C#4.0

SMTP

Following is my code:-

string to, sub, mess;
            to = "info@sdmmedicalcollege.org";
            sub ="Weekly Clinical Quiz";
            mess ="Quiz Title: " + dr["Quiz_Title"].ToString() + "<br>Name: " + txtName.Text + "<br> Email: " + txtEmail.Text + "<br>Clinical Diagnosis: " + txtContent.Text;
            mess = mess + "<br> Occupation : " + txtOccupation.Text + "<br>Institution: " + txtInst.Text;
            //  tel = "99";

            //MailMessage mg = new MailMessage();
            //MailMessage mg = new MailMessage(txtEmail.Text, "info@sdmmedicalcollege.org");
            MailMessage mg = new MailMessage(txtEmail.Text, dr["Quiz_Email"].ToString());  
            mg.Subject = sub;
            mg.Body = mess;
            mg.IsBodyHtml = true;
            SmtpClient st = new SmtpClient();
            st.EnableSsl = true;
            st.Host = "smtp.secureserver.net";
            st.Port = 25;
            st.Credentials = new System.Net.NetworkCredential("info@sdmmedicalcollege.org", "hospital");
            st.UseDefaultCredentials = true;
            st.DeliveryMethod = SmtpDeliveryMethod.Network;
            st.Send(mg);

When am trying to run this on server,am getting following exception,

The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.0 Must issue a STARTTLS command first. ua5sm10078511igb.10

I tried with the different port numbers like 457,565 but it didnt work out for me.

Can anyone please help me??