First Add namespace :
using System.Net.Mail;
MailMessage mail = new MailMessage();
mail.To.Add( Gmail-id of sender );
mail.To.Add(Email Id of Receiver);
mail.From = new MailAddress(Gmail-id of sender);
mail.Subject = "Subject body";
string Body ="Message Body";
mail.Body = Body;
mail.IsBodyHtml = true;
SmtpClient smtp = new SmtpClient();
smtp.Host = "smtp.gmail.com"; //Or Your SMTP Server Address
smtp.Credentials = new System.Net.NetworkCredential
(Gmail-id of sender,Gmail-id Password of sender);
//Or your Smtp Email ID and Password
smtp.EnableSsl = true;
smtp.Send(mail);
MessageBox.Show("your mail successfully send");
By:Ravi Chaudhari