How to use Threading or Async to send Bulk Mail in ASP.NET

I had created an application to send bulk mail but my application is taking too much time to send all the mails.

You can check it here:

http://www.c-sharpcorner.com/UploadFile/cd7c2e/send-bulk-mails-using-smtp-configuration-part-2/

http://www.c-sharpcorner.com/UploadFile/cd7c2e/send-bulk-email-from-yahoo-and-hotmail-using-Asp-Net/

I had read that if I use Async method or use Threading then too much mails can be send at a time, could anyone provide me an example for How can I use Threading or Async to send the Mails!!