how to send an email to outlook to approve, reject, onhold

If you have an idea on how to work with handlers, it helps you to solve your problem.

A handler is something that handle the request from an URL. Here in your case Accept. Shoukd be a hyperlink that actually has a URL associated to it. Now you need to write a handler to handle that Url.

Here is some of the info more about your question

http://www.c-sharpcorner.com/interview-question/what-are-http-handlers-in-asp-net

and here is an example of what you need to do

http://www.c-sharpcorner.com/uploadfile/37db1d/create-your-first-http-handler-in-Asp-Net-3-5/

Please do accept this as anser, if it is helpful :)