写了个发邮件的测试程序，出现如下错误！郁闷，请赐教，谢了


[COMException (0x80040220): “SendUsing”配置值无效。
]

[TargetInvocationException: 调用的目标发生了异常。]
System.RuntimeType.InvokeDispMethod(String name, BindingFlags invokeAttr, Object target, Object[] args, Boolean[] byrefModifiers, Int32 culture, String[] namedParameters) +0
System.RuntimeType.InvokeMember(String name, BindingFlags invokeAttr, Binder binder, Object target, Object[] args, ParameterModifier[] modifiers, CultureInfo culture, String[] namedParameters) +473
System.Type.InvokeMember(String name, BindingFlags invokeAttr, Binder binder, Object target, Object[] args) +29
System.Web.Mail.LateBoundAccessHelper.CallMethod(Type type, Object obj, String methodName, Object[] args)
System.Web.Mail.LateBoundAccessHelper.CallMethod(Object obj, String methodName, Object[] args)

[HttpException (0x80004005): 未能访问“CDO.Message”对象。]
System.Web.Mail.LateBoundAccessHelper.CallMethod(Object obj, String methodName, Object[] args)
System.Web.Mail.CdoSysHelper.Send(MailMessage message)
System.Web.Mail.SmtpMail.Send(MailMessage message)
WebEmail.WebForm1.btnSend_Click(Object sender, EventArgs e) in c:\inetpub\wwwroot\webemail\webform1.aspx.cs:76
System.Web.UI.WebControls.Button.OnClick(EventArgs e)
System.Web.UI.WebControls.Button.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument)
System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument)
System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData)
System.Web.UI.Page.ProcessRequestMain()

错在
SmtpMail.Send(aMail);

请大家帮个忙，多谢了

如下：
SmtpMail.SmtpServer="";
MailMessage aMail=new MailMessage();
aMail.To=txtTo.Text;
aMail.From=txtFrom.Text;
aMail.Subject=txtSubject.Text;
aMail.Priority=(MailPriority)ddPriority.SelectedIndex;
aMail.BodyFormat=(MailFormat)ddBodyFormat.SelectedIndex;
ddPriority.SelectedIndex=1;
ddBodyFormat.SelectedIndex=0;
if (File1.PostedFile.FileName!="")
{
string fileName=File1.PostedFile.FileName;
fileName=fileName.Substring(fileName.LastIndexOf(@"\"));
fileName=Server.MapPath(@"\temp\"+fileName);
File1.PostedFile.SaveAs(fileName);
aMail.Attachments.Add(new MailAttachment(fileName));
}
SmtpMail.Send(aMail);

我猜是不是要配置发送电子邮件的服务器，但是不知道怎么配置，望笨笨姐赐教，谢谢了