5.5.4 Error: send AUTH command first

MailAddress from = new MailAddress("lady.occur@yandex.ru", "Tom");
        MailAddress to = new MailAddress("vasilisa.tcheremushkina@yandex.ru");
        MailMessage m = new MailMessage(from, to);
        m.Subject = "Тест";
        m.Body = "<h2>Письмо-тест работы smtp-клиента</h2>";
        m.IsBodyHtml = true;
        SmtpClient smtp = new SmtpClient("smtp.yandex.ru", 25);
        smtp.EnableSsl = true;
        smtp.Credentials = new NetworkCredential("lady.occur@yandex.ru", "X29Fgbf2nnwKCRaR");
        smtp.Send(m);
        MessageBox.Show("СМС ОТПРАВЛЕН");