在哪里可以找到包含。visual studio 2010中的dll ?

I tried to create a simple web application to trigger mail from my company domail address to the outside mailbox. While doing so, i came to know about system.web.dll namespace. I couldn't find the system.web.dll in the add reference. Where can i find? or Is there any other namespace that can be used for mail triggering? I tried to create a simple web application to t