hey Radhika i was getting same error on windows server 2008 R2 when i write belove code i got sollution.. use Gmail smtp client
SmtpClient SmtpServer = new SmtpClient();
SmtpServer.Credentials = new Net.NetworkCredential("xyz@gmail.com", "password");
SmtpServer.Port = 587;
SmtpServer.Host = "smtp.gmail.com";
SmtpServer.EnableSsl = true;
mail = new MailMessage();
mail.DeliveryNotificationOptions = DeliveryNotificationOptions.OnFailure;
mail.ReplyTo = new MailAddress(TextBox1.Text);
SmtpServer.Send(mail);
Hope this will work for u...Kishan Ki Radha jI...