MailMessage mail = new MailMessage();
mail.From = new System.Net.Mail.MailAddress("apps@xxxx.com");

// 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("yyyy@xxxx.com"));

//Formatted mail body
mail.IsBodyHtml = true;
string st = "Test";

mail.Body = st;
smtp.Send(mail);