如何在ItemSend事件处理程序中确定邮件项目是回复

我有这样一个类：

using Office = Microsoft.Office.Core;
using Outlook = Microsoft.Office.Interop.Outlook;
public partial class ThisAddIn
{
    private void OnItemSend(object Item, ref bool Cancel)
    {
        Outlook.MailItem mailItem = Item as Outlook.MailItem;
    }
    private void OnNewInspector(Outlook.Inspector Inspector)
    {
        ((Outlook.ItemEvents_10_Event)mailItem).Reply += new Outlook.ItemEvents_10_ReplyEventHandler(OnMailItemReply);
    }
    private void OnMailItemReply(object Response, ref bool Cancel)
    {
        var replyItem = Response as Outlook.MailItem;
        // Maybe I need to add or change in replyItem some property??
    }
}

我需要在OnItemSend事件处理程序中确定mailItem是一条回复消息。此外，我还需要在OnMailItemReply方法中添加一些值。