System.Web.Mail (SWM) is the .NET namespace used to send email in .NET Framework applications. SWM contains three classes:
MailMessage – used for creating and manipulating the mail message contents.
MailAttachments – used for creating a mail attachment to be added to the mail message.
SmtpMail – used for sending email to the relay mail server.

More information on the System.Web.Mail Namespace can be found on MSDN here: http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpref/html/frlrfsystemwebmail.asp