This way
private void btnSendMail_Click(object sender, EventArgs e)
{
using (MailMessage mailMessage = new MailMessage())
{
mailMessage.From = new MailAddress("sender.com");
mailMessage.Subject = txtSubject.Text.Trim();
string body = "Name: " + txtName.Text.Trim() + Environment.NewLine;
body += "Email: " + txtEmailAddress.Text.Trim() + Environment.NewLine;
body += "Message: " + txtMessage.Text.Trim();
mailMessage.Body = body;
mailMessage.IsBodyHtml = false;
mailMessage.To.Add(new MailAddress(mailMessage.From.Address));
SmtpClient smtp = new SmtpClient();
smtp.Host = "smtp.gmail.com";
smtp.EnableSsl = true;
System.Net.NetworkCredential NetworkCred = new System.Net.NetworkCredential();
NetworkCred.UserName = mailMessage.From.Address;
NetworkCred.Password = "<Password>";
smtp.UseDefaultCredentials = true;
smtp.Credentials = NetworkCred;
smtp.Port = 587;
smtp.Send(mailMessage);
}
}