Where to find system.web.dll in visual studio 2010?

I tried to create a simple web application to run mail from my company address to an external mailbox. By doing this, I learned about the system.web.dll namespace. I could not find system.web.dll in the add link. Where can I find? or is there any other namespace that can be used to start mail? This is the control page I checked.

If there are other pages that can teach me basic asp.net startup concepts, post a link here. It will be very helpful.