bool mailThreadStarted = false;

protected virtual void Application_Start(object sender, EventArgs e)
{
   Application.Lock();

   if(!mailThreadStarted)
   {
    Thread th = new Thread(SendEmailWorker);
        th.IsBackground = true;
        th.Start();
        mailThreadStarted = true;
   }

   Application.UnLock();
}

private void SendEmailWorker()
{
  while(true)
  {
    ..sending email code
    Thread.Sleep(...);
  }
}

------解决方案--------------------
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C# code
bool mailThreadStarted = false;

protected virtual void Application_Start(object sender, EventArgs e)
{
   Application.Lock();

   if(!mailThreadStarted)
   {
    Thread th = new Thread(SendEmailWorker);
        th.IsBackground = true;
        th.Start();
        mailThreadStart…