Lỗi gửi Gmail: The message could not be sent to the SMTP server asp.net c#

HI mọi người, mình sử dụng chức năng gửi email trong web asp.net, nhưng khi gửi nó báo lỗi : System.Web.HttpException (0x80004005): The message could not be sent to the SMTP server. The transport error code was 0x80040217. The server response was not available. Chi tiết lỗi ở dưới đó các bạn.
Code mình viết gửi email như sau:

Mã:

 
mail.Subject = strSubject;
mail.Body = strBody;
mail.BodyFormat = MailFormat.Html;
mail.BodyEncoding = System.Text.Encoding.UTF8;
mail.Priority = MailPriority.Normal;
if (strReturn == "")
{
  SmtpMail.SmtpServer = nội dung....;
  mail.Fields.Add("http://schemas.microsoft.com/cdo/configuration/smtpserver", "smtp.gmail.com");
  mail.Fields.Add("http://schemas.microsoft.com/cdo/configuration/smtpserverport", "465");
  mail.Fields.Add("http://schemas.microsoft.com/cdo/configuration/sendusing", 2);
  mail.Fields.Add("http://schemas.microsoft.com/cdo/configuration/smtpauthenticate", 1);
  mail.Fields.Add("http://schemas.microsoft.com/cdo/configuration/sendusername", "123@gmail.com");
  mail.Fields.Add("http://schemas.microsoft.com/cdo/configuration/sendpassword", "123@123");                
  mail.Fields.Add("http://schemas.microsoft.com/cdo/configuration/smtpusessl", "true");
  SmtpMail.Send(mail);
}

Bị báo lỗi mỗi khi gửi email tới như sau.

Mã:

System.Web.HttpException (0x80004005): The message could not be sent to the SMTP server.
The transport error code was 0x80040217. The server response was not available
---> System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation.
---> System.Runtime.InteropServices.COMException: The message could not be sent to the SMTP server.
The transport error code was 0x80040217. The server response was not available  
--- End of inner exception stack trace
--- at System.RuntimeType.InvokeDispMethod(String name, BindingFlags invokeAttr, Object target, Object[] args, Boolean[] byrefModifiers, Int32 culture, String[] namedParameters) 
at System.RuntimeType.InvokeMember(String name, BindingFlags bindingFlags, Binder binder, Object target,
Object[] providedArgs, ParameterModifier[] modifiers, CultureInfo culture, String[] namedParams) 
at System.Web.Mail.SmtpMail.LateBoundAccessHelper.CallMethod(Object obj, String methodName, Object[] args)
at System.Web.Mail.SmtpMail.LateBoundAccessHelper.CallMethod(Object obj, String methodName, Object[] args)
at System.Web.Mail.SmtpMail.CdoSysHelper.Send(MailMessage message)   at System.Web.Mail.SmtpMail.Send(MailMessage message) 
at WebCode.Common.SendMail(String strFrom, String strTo, String strCC, String strBCC, String strSubject, String strBody) in E:\C...\Common.cs:line 2788