Can't find System.Net.Mail in VS 2010, Silverlight 5, C#

Hi,

I am unable to add System.Net.Mail

Writing using System.Net.Mail give the error message - The namespace Mail does not exist in System.Net. Why it is searching it in System.Net when it is part of System dll

I have already added both System.Net and System references in my project

My purpose: I want to use MailAddress to validate email format. For this System.Net.Mail must be added

Thank you