C# - Send Method Of SMTP Is Always Throwing An Exception

The send method of SMTP is always throwing an exception. The message is : Failure sending mail Here is my code:

MailMessage mm = new MailMessage();
mm.To.Add("Yuvraj.dhot@xxxxx.co.in");
mm.From = new MailAddress("Kumar.Chaudhari@xxxxx.co.in");
mm.Subject = "Ant Subject";
mm.Body = "Body Cha MEssag here ";
SmtpClient ss = new SmtpClient("localhost");
ss.EnableSsl = false;
try
{
**ss.Send(mm);**
Result.Text = "Message Sent";
Result.ForeColor = System.Drawing.Color.Green;
}
catch (SmtpException ex)
{
Result.Text = "Message Not Sent :
" + ex.Message;
Result.ForeColor = System.Drawing.Color.Red;
}

I also tried using

ss.DeliveryMethod = SmtpDeliveryMethod.PickupDirectoryFromIis;

Now it doesn't throw any exception, it executes fine but receiver does not receive any mail in his inbox. How can I fix this? Edit - This is the stack trace im getting.................