错误SMTP服务器需要安全连接，或者客户端未通过身份验证.服务器响应为:5.5.1需要身份验证??

这是我的代码

private void sendEmail()
    {
            var fromAddress = new MailAddress("gmail email", "name");
            var toAddress = new MailAddress("email", "name");
            const string fromPassword = "password";
            const string subject = "ERROR";
            var smtp = new SmtpClient
            {
                Host = "smtp.gmail.com",
                Port = 587,
                EnableSsl = true,
                DeliveryMethod = SmtpDeliveryMethod.Network,
                Credentials = new NetworkCredential(fromAddress.Address, fromPassword),
                UseDefaultCredentials = false,
                Timeout = 20000
            };
            using (var message = new MailMessage(fromAddress, toAddress)
            {
                Subject = subject,
                Body = "this is an example",
            })
            {
                smtp.Send(message);
            }
    }

当我调试应用程序时，它将在smtp处停止。发送（消息）并显示错误消息"SMTP服务器需要安全连接或客户端未通过身份验证。服务器响应为：5.5.1是否需要身份验证？"

我该怎么办？有时有效，有时无效。