ASP 4.0 Email ReplyToList build error

Hi all,   I am learning ASP and have run into a build error I'm trying to figure out.  I am using the WROX book ASP 4.5 as a guide but am running on VS 2010 which I think uses ASP 4.0.  Anyway, I have coded up a form which "works" except that if I add the code to add an email address to the ReplyToList of the email message there is a build error.  I'm very much a newbie on this and am having trouble resolving the error.  The code is in a file named ControlForm.ascx.cs and looks like this:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.IO; // Provides access to the File class for reading the file
using System.Net.Mail; // Provides access to the various mail related classes

public partial class Controls_ContactForm : System.Web.UI.UserControl
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void CustomValidator1_ServerValidate(object source, ServerValidateEventArgs args)
    {
        if (!string.IsNullOrEmpty(PhoneHome.Text) ||
        !string.IsNullOrEmpty(PhoneBusiness.Text))
        {
            args.IsValid = true;
        }
        else
        {
            args.IsValid = false;
        }
    }
    protected void SendButton_Click(object sender, EventArgs e)
    {
        if (Page.IsValid)
        {
            string fileName = Server.MapPath("~/App_Data/ContactForm.txt");
            string mailBody = File.ReadAllText(fileName);
            mailBody = mailBody.Replace("##Name##", Name.Text);
            mailBody = mailBody.Replace("##Email##", EmailAddress.Text);
            mailBody = mailBody.Replace("##HomePhone##", PhoneHome.Text);
            mailBody = mailBody.Replace("##BusinessPhone##", PhoneBusiness.Text);
            mailBody = mailBody.Replace("##Comments##", Comments.Text);
            MailMessage myMessage = new MailMessage();
            myMessage.Subject = "Response from web site";
            myMessage.Body = mailBody;
            myMessage.From = new MailAddress("Richard.Zore@OneSource.com", "Rich at work");
            myMessage.To.Add(new MailAddress("Richard.Zore@onesource.com", "Rich at home"));
            myMessage.ReplyToList.Add(new MailAddress(EmailAddress.Text));
            SmtpClient mySmtpClient = new SmtpClient();
            mySmtpClient.Send(myMessage);
            Message.Visible = true;
            FormTable.Visible = false;
            //System.Threading.Thread.Sleep(5000);
        }
    }
}

The build error I am getting is this:
Error      1      'System.Net.Mail.MailMessage' does not contain a definition for 'ReplyToList' and no extension method 'ReplyToList' accepting a first argument of type 'System.Net.Mail.MailMessage' could be found (are you missing a using directive or an assembly reference?)      C:\BegASPNET\Site\Controls\ContactForm.ascx.cs      44      23      C:\BegASPNET\Site\


As I say, if I comment out the bolded line in the code the error goes away (naturally).