MailMessage mail = new MailMessage();
mail.From = new System.Net.Mail.MailAddress("[email protected]");
// The important part -- configuring the SMTP client
SmtpClient smtp = new SmtpClient();
smtp.Port = 587; // [1] You can try with 465 also, I always used 587 and got success
smtp.EnableSsl = true;
smtp.DeliveryMethod = SmtpDeliveryMethod.Network; // [2] Added this
smtp.UseDefaultCredentials = false; // [3] Changed this
smtp.Credentials = new NetworkCredential(mail.From, "password_here"); // [4] Added this. Note, first parameter is NOT string.
smtp.Host = "smtp.gmail.com";
//recipient address
mail.To.Add(new MailAddress("[email protected]"));
//Formatted mail body
mail.IsBodyHtml = true;
string st = "Test";
mail.Body = st;
smtp.Send(mail);