Problem using MailMessage.Fields

I'm a beginner with ASP.NET, but am working through a book to try to build a test e-commerce site.  The book gave me the following code to send an error email; but for some reason Visual Web Developer keeps telling me that "System.Net.Mail.MailMessage does not contain a definition for 'Fields'".  I know there is a property called Fields for Mail Message according to this: http://msdn.microsoft.com/en-us/library/system.web.mail.mailmessage.fields.aspx.

So what am I missing?

using System;
using System.Net.Mail;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.HtmlControls;


/// <summary>
/// Class contains miscellaneous functionality
/// </summary>
public static class Utilities
{
 static Utilities()
 {
&nbs ...