C#发送邮件失败命令顺序不正确need EHLO and AUTH first解决方法

System.Net.Mail.SmtpException:“命令顺序不正确。 服务器响应为:Error: need EHLO and AUTH first !”

//using System.Net;
//using System.Net.Mail;
//using System.Text;

//电子邮件
MailMessage mailMessage = new MailMessage();
mailMessage.From = new MailAddress("111111@qq.com", "张三");//发件人
mailMessage.To.Add(new MailAddress("222222@qq.com", "李四"));//收件人
mailMessage.CC.Add(new MailAddress("333333@qq.com", "王五"));//抄送人
mailMessage.Priority = MailPriority.Normal;//优先级
mailMessage.SubjectEncoding = Encoding.UTF8;
mailMessage.Subject = "Happy New Year!";//标题
mailMessage.BodyEncoding = Encoding.UTF8;
mailMessage.IsBodyHtml = true;
mailMessage.Body = "<font color=red>Hello,Happy New Year!</font>";//内容
mailMessage.Attachments.Add(new Attachment(@"F:\mzwucom.jpg"));//附件

//邮件服务器
SmtpClient smtpClient = new SmtpClient();
smtpClient.Host = "smtp.qq.com";
[color=Red]smtpClient.EnableSsl = true;
smtpClient.UseDefaultCredentials = false;[/color]
smtpClient.Credentials = new NetworkCredential("用户名", "[color=Red]nsatogkujrazbiac[/color]");  //这边用授权码,不能使用用户密码
smtpClient.DeliveryMethod = SmtpDeliveryMethod.Network;
smtpClient.Send(mailMessage);