The System.Web.Mail library and System.Net.Mail library are both built-in into .NET 4.0. System.Web.Mail is supported by all versions of .Net Frameworks at the moment. However, in  .Net 2.0, the new System.Net.Mail  namespace was introduced and essentially replaced System.Web.Mail, and for a variety of reasons including stability and reliability, it is recommend over the ‘System.Web.Mail library where-ever possible.