MailMessage objMailMessage = new MailMessage(); 
                objMailMessage.From = new MailAddress("Xin_Yin@intra.pegatroncorp.com"); 
                //objMailMessage.To.Add(new MailAddress(To[i])); 
                objMailMessage.To.Add(new MailAddress("kylin_liu@pegatroncorp.com")); 
                objMailMessage.To.Add(new MailAddress("azai_wang@intra.pegatroncorp.com")); 
                objMailMessage.CC.Add(new MailAddress("xin_yin@intra.pegatroncorp.com")); 
                objMailMessage.Subject = message.Subject; 
                objMailMessage.Body = body; 
                objMailMessage.IsBodyHtml = true; 
                System.String strFileName8 = v_Img1; 
                Attachment attached8 = new Attachment(strFileName8); 
                attached8.ContentId = "Img1"; 
                objMailMessage.Attachments.Add(attached8); 
                System.String strFileName5 = v_Img2; 
                Attachment attached5 = new Attachment(strFileName5); 
                attached5.ContentId = "Img2"; 
                objMailMessage.Attachments.Add(attached5); 
                System.String strFileName6 = v_Img3; 
                Attachment attached6 = new Attachment(strFileName6); 
                attached6.ContentId = "Img3"; 
                objMailMessage.Attachments.Add(attached6); 
                Attachment attached7 = new Attachment(_Xls); 
                objMailMessage.Attachments.Add(attached7); 
                SmtpClient sender = new SmtpClient(); 
                sender.Host = "relay-b.sz.pegatroncorp.com"; 
                sender.Port = 25; 
                sender.Credentials = new NetworkCredential("mail賬號", "密碼", "網域"); 
                sender.DeliveryMethod = SmtpDeliveryMethod.Network; 
                //sender.EnableSsl = true; 
                sender.Send(objMailMessage);

------解决方案--------------------

从3年前的blog里翻出来的：

客户端用javascript操作outlook(包括自动加附件)的代码：

   var outlookApp = new ActiveXObject("Outlook.Application");
　　var nameSpace = outlookApp.getNameSpace("MAPI");
　　mailFolder = nameSpace.getDefaultFolder(6);
　　mailItem = mailFolder.Items.add('IPM.Note.FormA');
　　mailItem.Subject = "主题";
　　mailItem.To = "abc@abc.com";
　　mailItem.HTMLBody = "邮件内容";
　　var fso = new ActiveXObject("Scripting.FileSystemObject");
　　var tempfile = "c:\\temp\\t1.txt"
　　var addr = "http://abc.com/test.aspx?filename=test.pdf";//这个aspx的后台代码比较简单,就是利用Response.ContentType = "application/pdf"; Response.WriteFile(fileName)的方式返回pdf文件
　　var oHttp = new ActiveXObject("Microsoft.XMLHTTP");//用ajax方法获得pdf文件,本来想用jquery做,但查了一下,说是jquery不直接支持传二进制文件,虽然有别人写的base64的扩充函数,但觉得比较麻烦.
　　oHttp.open("GET", addr, false);
　　oHttp.send();
　　var ostream = new ActiveXObject("ADODB.Stream");//Scripting.FileSystemObject说是不支持二进制文件,只能用这个
　　with (ostream) {
　　type = 1; //binary
　　mode = 3; //readwrite
　　open();
　　write(oHttp.responseBody);
　　savetofile(tempfile, 1); //savecreatenotexist
　　close();
　　mailItem.Attachments.Add(tempfile );
　　fso.DeleteFile(tempfile);//delete tempfile
　　mailItem.display(0);
　}
　　ostream = null;
　　oHttp = null;
　　fso = null;
这个方法的主要缺点，就是必须修改浏览器的安全性设置，ie下：（其他浏览器没有测试)
1.把服务器加到受信任的站点
2.点击自定义级别,这两项必须都设为"启用":
　　1)对没有标记为安全的ActiveX控件进行初始化和脚本运行
　　2)下载未签名的ActiveX控件

另外,savetofile函数只接受绝对路径.
所以，这种方法的实用性很低。
结论：要自动添加附件，只能在服务器端做。