This is my code (added the above line) but still getting same error:
protected void Button1_Click(object sender, EventArgs e)
{
// Create a message and set up the recipients.
SmtpClient smtpClient = new SmtpClient();
MailMessage message = new MailMessage();
message.To.Add("mymail@domain.com");
message.Subject = "Form - ";
message.IsBodyHtml = true;
message.Priority = MailPriority.High;
message.Body = "<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.0 Transitional//EN\">";
message.Body += "<HTML><HEAD><META http-equiv=Content-Type content=\"text/html; charset=iso-8859-1\">";
message.Body += "</HEAD><BODY>";
message.Body += "<table border='1'><tr><td>Textbox1.Text</td><td>Textbox2.Text</td></tr><tr><td>Textbox3.Text</td><td>Textbox4.Text</td></tr></table>";
message.Body += "</BODY></HTML>";
// Send the message.
smtpClient.Port = 587;
smtpClient.UseDefaultCredentials = false;
smtpClient.Credentials = new System.Net.NetworkCredential("mygmailusername@gmail.com", "password");
smtpClient.DeliveryMethod = SmtpDeliveryMethod.Network;
smtpClient.EnableSsl = true;
smtpClient.Send(message);
EmailSentForm.Visible = Visible;
}