マルチスレッド群発メールの例

   //       
   private static void SendEmailsUsingThreadPool(List recipients)
    {
      //   
      var coreCount = Environment.ProcessorCount;
      //   
      var itemCount = recipients.Count;
      //         
      var batchSize = itemCount / coreCount;

      var pending = coreCount;
      using (var mre = new ManualResetEvent(false))
      {
        for (int batchCount = 0; batchCount < coreCount; batchCount++)
        {
          var lower = batchCount * batchSize;
          var upper = (batchCount == coreCount - 1) ? itemCount : lower + batchSize;
          ThreadPool.QueueUserWorkItem(st =>
          {
            for (int i = lower; i < upper; i++)
              SendEmail(recipients[i]);
            if (Interlocked.Decrement(ref pending) == 0)
              mre.Set();
          });
        }
        mre.WaitOne();
      }      
    }
    //      
    private static void SendEmail(Recipient recipient)
    {
      //Send your Emails here
    }
  }
  //    
  class Recipient
  {
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string EmailAddress { get; set; }
  }