MailMessage mm = new MailMessage("xyz@gmail.com", TextBoxEmail.Text.Trim());
mm.Subject = "Password Recovery";
mm.Body = string.Format("Hi ,<br /><br />Your password is .<br /><br />Thank You.");
mm.IsBodyHtml = true;
SmtpClient smtp = new SmtpClient();
smtp.Host = "smtp.gmail.com";
smtp.EnableSsl = true;
NetworkCredential NetworkCred = new NetworkCredential();
NetworkCred.UserName = "xyz@gmail.com";
NetworkCred.Password = "xyz";
smtp.UseDefaultCredentials = true;
smtp.Credentials = NetworkCred;
smtp.Port = 587;
smtp.Send(mm);
message = "Registration successful. Activation email has been sent.";
ClientScript.RegisterStartupScript(GetType(), "alert", "alert('" + message + "');", true);

using (MailMessage mail = new MailMessage())
                {
                    mail.From = new MailAddress("xyz@gmail.com");
                    mail.Subject = "mailSubject";
                    mail.Body = "mailBody";
                    mail.IsBodyHtml = true;

                    mail.To.Add("xyz@gmail.com");



                    using (SmtpClient smtp = new SmtpClient("smtp.gmail.com", 587))//2nd parameter is PORT No.
                    {
                        smtp.Credentials = new System.Net.NetworkCredential("xyz@gmail.com", "xyz");
                        smtp.EnableSsl = true;//set this as your Host Name properties, for gmail,its true
                        smtp.Send(mail);//actual sending operation here
                    }
                }

using System.Net;
using System.Net.Mail;

var fromAddress = new MailAddress("from@gmail.com", "From Name");
var toAddress = new MailAddress("to@example.com", "To Name");
const string fromPassword = "fromPassword";
const string subject = "Subject";
const string body = "Body";

var smtp = new SmtpClient
{
    Host = "smtp.gmail.com",
    Port = 587,
    EnableSsl = true,
    DeliveryMethod = SmtpDeliveryMethod.Network,
    UseDefaultCredentials = false,
    Credentials = new NetworkCredential(fromAddress.Address, fromPassword)
};
using (var message = new MailMessage(fromAddress, toAddress)
{
    Subject = subject,
    Body = body
})
{
    smtp.Send(message);
}