SendUsing" configuration value is invalid. error in sending mail in asp.net

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SendUsing" configuration value is invalid. error in sending mail in asp.net website.

my url:vjcet.ac.in

in this site hava an srror SendUsing" configuration value is invalid.during send mail

.
my code is

MailMessage mail1 = new MailMessage();
mail1.From = txtEmail.Text;
// ds = FillDataset.ReadXml(Server.MapPath("~/Admin/XML/feedbackmail.xml"));
DataSet ds = new DataSet();
ds.ReadXml(Server.MapPath("~/Admin/xml/feedbackmail.xml"));
string ToAddress = ds.Tables[0].Rows[0][0].ToString();

// mail1.To = "feedback@stpius.ac.in";
mail1.To = ToAddress;
mail1.Subject = txtComments.Text;

mail1.Body = "Sender : " + txtEmail.Text + "\n" + "Name : " + txtname.Text + "\n" + "Address : " + txtCity.Text + "\n" + "Phone No:  : " + txtFax.Text + "\n" + "Email:" + txtEmail.Text + "\n" + "Message :" + txtCountry.Text + "\n" + "Body :" + txtComments.Text;
SmtpMail.SmtpServer.Insert(0, "27.251.157.141");

SmtpMail.Send(mail1);

I think my ip address is not currect.But in command prompt i checked ping vjcet.ac.in now i got this same is 27.251.157.141

any one please help me