Silverlight mailto: HyperlinkButton always opens an additional browser window

private void TestLink_Click(object sender, RoutedEventArgs e)
{
    //Only run the click event if in browser because this will not work OOB
    if (Application.Current.IsRunningOutOfBrowser)
        return;

    var cmd = String.Format("location.href='mailto:test@test.com?subject=blah&body=something';",

    HtmlPage.Window.Eval(cmd);
}

/// <summary>
/// This component allows us to display an email address as a hyperlink that the user can click and it will open a new email window
/// in their default email client.  It can handle in browser and out of browser
/// It gets around the issue which we previously were experiencing where a new browser window would open and an email window.
/// </summary>
public class MailToButton : HyperlinkButton
{
    /// <summary>
    /// Email dependency property
    /// </summary>
    public static readonly DependencyProperty EmailProperty = DependencyProperty.Register("Email",
                                                                                          typeof(string),
                                                                                          typeof(
                                                                                              MailToButton
                                                                                              ), null);
    /// <summary>
    /// Email Address
    /// </summary>
    public object Email
    {
        get { return GetValue(EmailProperty); }
        set { SetValue(EmailProperty, Convert.ToString(value)); }
    }

    /// <summary>
    /// Subject dependency property
    /// </summary>
    public static readonly DependencyProperty SubjectProperty = DependencyProperty.Register("Subject",
                                                                                          typeof(string),
                                                                                          typeof(
                                                                                              MailToButton
                                                                                              ), null);
    /// <summary>
    /// Email Subject
    /// </summary>
    public object Subject
    {
        get { return GetValue(SubjectProperty); }
        set { SetValue(SubjectProperty, Convert.ToString(value)); }
    }

    /// <summary>
    /// Body dependency property
    /// </summary>
    public static readonly DependencyProperty BodyProperty = DependencyProperty.Register("Body",
                                                                                          typeof(string),
                                                                                          typeof(
                                                                                              MailToButton
                                                                                              ), null);

    /// <summary>
    /// Hide the NavigateUri property in the base, we will set this manually in code.
    /// </summary>
    public new Uri NavigateUri { get; set; }

    /// <summary>
    /// Email Body
    /// </summary>
    public object Body
    {
        get { return GetValue(BodyProperty); }
        set { SetValue(BodyProperty, Convert.ToString(value)); }
    }

    /// <summary>
    /// Constructor
    /// </summary>
    public MailToButton()
    {
        Loaded += MailToButton_Loaded;
        Click += MailToButtonClicked;

    }

    private void MailToButton_Loaded(object sender, RoutedEventArgs e)
    {
        //Only bind the NavigateUrl when OOB as this method does not work in browser
        if (Application.Current.IsRunningOutOfBrowser)
            base.NavigateUri = new Uri(BuildMailToString());
    }

    private void MailToButtonClicked(object sender, RoutedEventArgs e)
    {
        //Only run the click event if in browser because this will not work OOB
        if (Application.Current.IsRunningOutOfBrowser)
            return;

        if (string.IsNullOrEmpty(Convert.ToString(Email)))
            return;

        HtmlPage.Window.Eval(BuildMailToString());
    }

    /// <summary>
    /// Constructs a MailTo string using the email, subject and body.  It url encodes the email, subject and body
    /// </summary>
    /// <returns>A mailto string</returns>
    private string BuildMailToString()
    {
        var email = HttpUtility.UrlEncode(Convert.ToString(Email));
        var subject = HttpUtility.UrlEncode(Convert.ToString(Subject));
        var body = HttpUtility.UrlEncode(Convert.ToString(Body));

        var sb = new StringBuilder();
        sb.AppendFormat("location.href='mailto:{0}?", email);

        if (!string.IsNullOrEmpty(subject))
            sb.AppendFormat("subject={0}", subject);

        if (!string.IsNullOrEmpty(body))
        {
            if (!string.IsNullOrEmpty(subject))
                sb.Append("&");

            sb.AppendFormat("body={0}", body);
        }
        sb.Append("'");

        return sb.ToString();
    }

}