I have got the following Exception :

System.Reflection.TargetInvocationException:
    Exception has been thrown by the target of an invocation. -

 System.Runtime.InteropServices.COMException (0x8004020F):
 The server rejected one or more recipient addresses. The server response was: 550 5.7.1 Unable to relay for [email protected]  


--- End of inner exception stack trace --- 
at System.RuntimeType.InvokeDispMethod(String name, BindingFlags invokeAttr, Object target, Object[] args, Boolean[] byrefModifiers, Int32 culture, String[] namedParameters) at System.RuntimeType.InvokeMember(String name, BindingFlags bindingFlags, Binder binder, Object target, Object[] providedArgs, ParameterModifier[] modifiers, CultureInfo culture, String[] namedParams) at System.Type.InvokeMember(String name, BindingFlags, invokeAttr, Binder binder, Object target, Object[] args, CultureInfo culture) 
at System.Web.Mail.SmtpMail.LateBoundAccessHelper.CallMethod(Type type, Object obj, String methodName, Object[] args) at 

System.Web.Mail.SmtpMail.LateBoundAccessHelper.CallMethod(Object obj, String methodName, Object[] args)

public static void SendEmail(string _FromEmail, string _ToEmail, string _Subject, string _EmailBody)
{

    // setup email header . 
    SmtpMail.SmtpServer = "127.0.0.1";
    MailMessage _MailMessage = new MailMessage();

    _MailMessage.From = _FromEmail;
    _MailMessage.To = _ToEmail;
    _MailMessage.Subject = _Subject;
    _MailMessage.Body = _EmailBody;

    try
    {
        SmtpMail.Send(_MailMessage);
    }
    catch (Exception ex)
    {
        if (ex.InnerException != null)
        {
            String str = ex.InnerException.ToString();

        }
    }
}