Error On Login And Send Mail ...

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SQL

VS2012

When I Login And Try To Send Mail..
When I run this code it shows the following error:

"Server Does Not Support Secure Connections."

The error comes on "Smtp.Send(Ml)":

protected void btnSend_Click(object sender, EventArgs e)
        {
            MailMessage ml = new MailMessage();
            ml.From = new MailAddress(Session["EmailId"].ToString().Trim());
            ml.To.Add(txtEmaillTo.Text);
            ml.Subject = txtSubject.Text;
            string Bd = txtMessageBody.Text;
            ml.Body = Bd;
            ml.IsBodyHtml = true;
            if (FileUpload1.HasFile)
            {
                ml.Attachments.Add(new Attachment(FileUpload1.PostedFile.InputStream, FileUpload1.FileName));
            }
            SmtpClient smtpcode = new SmtpClient();
            smtpcode.Host = "10.10.10.248";
            smtpcode.Port = 25;
            smtpcode.UseDefaultCredentials = false;                      
            smtpcode.DeliveryMethod = System.Net.Mail.SmtpDeliveryMethod.Network;
            smtpcode.Credentials = new System.Net.NetworkCredential(Session["EmailId"].ToString().Trim(), Session["Password"].ToString().Trim());
            smtpcode.Timeout = 20000;
            smtpcode.EnableSsl = true;             
           
                smtpcode.Send(ml);
                lblmessage.Visible = true;
                lblmessage.ForeColor = Color.Green;
                lblmessage.Text = "Message send successfully";                
         }