时钟控件跨线程调用不起作用,怎么办?

我是在做邮件的收与发,在接收的时候,用个进度条来表示它的接收进度,当进度等于一百的时候,启动一个Timer控件,进行3秒钟倒计时,然后自动关闭这个接收窗体(我这是用WinForm搞的)!可是就是没有效果!更诡异的是上边那句被注掉的MessageBox.Show("接收完毕!"); 一写上,让对话框弹出来,这个时钟控件就起作用,当把对话框窗体上的"确定"一单击,时钟控件马上就停了,如果没有那句代码,时钟控件压根就不起作用.这是为什么?有哪个高手帮帮忙!急! 还有,上边的代码是写在线程调用的方法里的                  
                    string StrEmailReceive = ReceiveState(); 
                    lbReceiveMessage.Text = StrEmailReceive; 
                    //切割接收到的信息 
                    string[] StrCountReceive = StrEmailReceive.Split(' '); 
                    //获取邮件一共有多少封 
                    int CountEmail = Convert.ToInt32(StrCountReceive[1]); 
                    //判断邮件一共有多少封 
                    if (CountEmail > 0) 
                    { 
                        panelProgressBar.Visible = true; 
                        //总进度条的最小值 
                        progressBarCount.Minimum = 0; 
                        //总进度条的最大值 
                        progressBarCount.Maximum = CountEmail; 
                        for (int i = 1; i <= CountEmail; i++) 
                        { 
                            //向接收邮件服务器发送命令，查看邮件 
                            SendMessage("RETR " + i); 
                            //定义变量用于接收POP3邮件服务器返回的信息 
                            string MyReceiveMessage = ""; 
                            //接收POP3邮件服务器返回的信息 
                            MyReceiveMessage = ReceiveState();                             //以\r\n.\r\n做标识判断是否将邮件内容接收完毕，没有接收完则循环接收 
                            while (MyReceiveMessage.IndexOf("\r\n.\r\n") < 0) 
                            { 
                                MyReceiveMessage += ReceiveState(); 
                            } 
                            //总进度的当前值 
                            progressBarCount.Value = i; 
                            //总进度的百分数 
                            lbProgressBar.Text = (progressBarCount.Value * 100) / CountEmail + "%"; 
                            if (lbProgressBar.Text == "100%") 
                            {                                 lbTime.Visible = true; 
                                lbCutTime.Visible = true; 
                                timer1.Enabled = true; 
                                //MessageBox.Show("接收完毕!"); 
                            }                           } 
                        } 
        #region 计时器 
        int i = 3; 
        private void timer1_Tick(object sender, EventArgs e) 
        { 
            int k = i--; 
            lbTime.Text = k.ToString(); 
            if (lbTime.Text == "0") 
            { 
                timer1.Enabled = false; 
                this.Close(); 
            } 
        } 
        #endregion