SmtpClientで送信時に画面が固まらないようにする方法

SmtpClientで送信時に画面が固まらないようにする方法
(SendAsyncを利用して送信することで非同期送信が可能になります。）
VB.NET

Dim sc As System.Net.Mail.SmtpClient = Nothing

Private Sub btn1_Click(ByVal sender As Object, ByVal e As EventArgs) _
        Handles btn1.Click
    btn1.Enabled = False
    btn2.Enabled = True

    Dim msg As New System.Net.Mail.MailMessage( _
        "sender@xxx.xxx", "recipient@xxx.xxx", "題名", "本文")

    If sc Is Nothing Then
        sc = New System.Net.Mail.SmtpClient()
        AddHandler sc.SendCompleted, AddressOf sc_SendCompleted
    End If
    sc.Host = "localhost"
    sc.Port = 25
    sc.DeliveryMethod = System.Net.Mail.SmtpDeliveryMethod.Network
    sc.SendAsync(msg, msg)
End Sub

Private Sub btn2_Click(ByVal sender As Object, ByVal e As EventArgs) _
        Handles btn2.Click
    If Not (sc Is Nothing) Then
        sc.SendAsyncCancel()
    End If
End Sub

Private Sub sc_SendCompleted(ByVal sender As Object, _
        ByVal e As System.ComponentModel.AsyncCompletedEventArgs)
    Dim msg As System.Net.Mail.MailMessage = _
        CType(e.UserState, System.Net.Mail.MailMessage)

    If e.Cancelled Then
        Console.WriteLine("[{0}]の送信はキャンセルされました。", msg.Subject)
    Else
        If Not (e.Error Is Nothing) Then
            Console.WriteLine("[{0}]の送信でエラーが発生しました。", msg.Subject)
            Console.WriteLine(e.Error.Message)
        Else
            Console.WriteLine("[{0}]の送信が完了しました。", msg.Subject)
        End If
    End If
    msg.Dispose()

    btn1.Enabled = True
    btn2.Enabled = False
End Sub

C#

System.Net.Mail.SmtpClient sc = null;

private void btn1_Click(object sender, EventArgs e)
{
    btn1.Enabled = false;
    btn2.Enabled = true;

    System.Net.Mail.MailMessage msg = new System.Net.Mail.MailMessage(
        "sender@xxx.xxx", "recipient@xxx.xxx", "題名", "本文");

    if (sc == null)
    {
        sc = new System.Net.Mail.SmtpClient();
        sc.SendCompleted +=
            new System.Net.Mail.SendCompletedEventHandler(sc_SendCompleted);
    }
    sc.Host = "localhost";
    sc.Port = 25;
    sc.DeliveryMethod = System.Net.Mail.SmtpDeliveryMethod.Network;
    sc.SendAsync(msg, msg);
}

private void btn2_Click(object sender, EventArgs e)
{
    if (sc != null)
        sc.SendAsyncCancel();
}

private void sc_SendCompleted(object sender,
       System.ComponentModel.AsyncCompletedEventArgs e)
{
    System.Net.Mail.MailMessage msg = (System.Net.Mail.MailMessage)e.UserState;

    if (e.Cancelled)
    {
        Console.WriteLine("[{0}]の送信はキャンセルされました。", msg.Subject);
    }
    else if (e.Error != null)
    {
        Console.WriteLine("[{0}]の送信でエラーが発生しました。", msg.Subject);
        Console.WriteLine(e.Error.Message);
    }
    else
    {
        Console.WriteLine("[{0}]の送信が完了しました。", msg.Subject);
    }

    msg.Dispose();

    btn1.Enabled = true;
    btn2.Enabled = false;
}