having trouble with sending email in C#

Hi ! When I tried to send email , the following error was occured . Can anyone help me ? thanks alot Server Error in '/a' Application. -------------------------------------------------------------------------------- The message could not be sent to the SMTP server. The transport error code was 0x80040217. The server response was not available Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. Exception Details: System.Runtime.InteropServices.COMException: The message could not be sent to the SMTP server. The transport error code was 0x80040217. The server response was not available Source Error: An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below. Stack Trace: [COMException (0x80040211): The message could not be sent to the SMTP server. The transport error code was 0x80040217. The server response was not available ] [TargetInvocationException: Exception has been thrown by the target of an invocation.] 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.Web.Mail.LateBoundAccessHelper.CallMethod(Object obj, String methodName, Object[] args) +58 [HttpException (0x80004005): Could not access 'CDO.Message' object.] System.Web.Mail.LateBoundAccessHelper.CallMethod(Object obj, String methodName, Object[] args) +112 System.Web.Mail.CdoSysHelper.Send(MailMessage message) +1861 System.Web.Mail.SmtpMail.Send(MailMessage message) +153 a.WebForm1.sendMail(String strFilePath) in c:\inetpub\wwwroot\a\a.aspx.cs:104 a.WebForm1.Button1_Click(Object sender, EventArgs e) in c:\inetpub\wwwroot\a\a.aspx.cs:116 System.Web.UI.WebControls.Button.OnClick(EventArgs e) +108 System.Web.UI.WebControls.Button.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) +57 System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) +18 System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) +33 System.Web.UI.Page.ProcessRequestMain() +1292