SMTP服务器需要一个安全连接，或者客户端没有经过身份验证-仍然不能工作

我得到上述错误。我看了网上的解决方案，我似乎已经做了所有必要的，但我仍然得到它。我加入了Enablessl = true;传递方式是Network，我已经提供了凭据。我调试了一下，用户名和密码似乎是正确的。

using System;
using System.Net;
using System.Net.Mail;
using System.Windows.Forms;
namespace SendMail
{
    public partial class MainWindow : Form
    {
        #region Private variables
        private MailMessage _message = new MailMessage();
        private EmailSender sender = EmailSender.GetInstance();
        private SmtpClient _smtpClient = new SmtpClient();
        #endregion
        public MainWindow()
        {
            InitializeComponent();
        }
        private void PrepareMailMessage()
        {
            // Set the FROM address
            _message.From = new MailAddress(tbFromAddr.ToString().Trim());
            // Set the TO address
            _message.To.Add(new MailAddress(tbToAddr.ToString().Trim()));
            // Set the SUBJECT
            _message.Subject = tbSubject.ToString();
            _message.IsBodyHtml = false;
            _message.DeliveryNotificationOptions = DeliveryNotificationOptions.OnFailure;
            _message.Priority = MailPriority.High;
            _message.SubjectEncoding = System.Text.Encoding.UTF8;
            _message.BodyEncoding = System.Text.Encoding.UTF8;
        }
        private void PrepareServer()
        {
            _smtpClient.Host = "smtp.gmail.com";
            _smtpClient.Port = 587;
            _smtpClient.DeliveryMethod = SmtpDeliveryMethod.Network;
            _smtpClient.UseDefaultCredentials = false;
            _smtpClient.Credentials = new NetworkCredential(tbFromAddr.ToString().Trim(), tbPassword.ToString().Trim());
            _smtpClient.EnableSsl = true;
        }
        private void PrepareSender()
        {
            sender.Message = _message;
            sender.smtpClient = _smtpClient;
        }
        private void btnSend_Click(object sender, EventArgs e)
        {
            PrepareMailMessage();
            PrepareServer();
            PrepareSender();
            //this.sender.Send();
            _smtpClient.Send(_message);
        }
    }
}