MailMessage mail = new MailMessage();
        mail.From = new MailAddress("x2zn****@yandex.ru");
        mail.To.Add(new MailAddress("s*****@mail.ru"));
        mail.Subject = "Заголовок";
        mail.Body = "Письмо........................";

        SmtpClient client = new SmtpClient();
        client.EnableSsl = true;
        client.Host = "smtp.yandex.ru";
        client.Port = 587;

        client.Credentials = new NetworkCredential("x2zn****@yandex.ru", "*******");
        client.Send(mail);

        Console.ReadKey();