protected void Application_Error(object sender,EventArgs e){
  // An error has occured on a .Net page.
  var serverError = Server.GetLastError() as HttpException;

  if (null != serverError){
    int errorCode = serverError.GetHttpCode();

    if (404 == errorCode){
      Server.ClearError();
      Server.Transfer("/Errors/404.aspx");
    }
  }
}

在错误页面中，您应该确保正确设置状态代码：

// If you're running under IIS 7 in Integrated mode set use this line to override
// IIS errors:
Response.TrySkipIisCustomErrors = true;

// Set status code and message; you could also use the HttpStatusCode enum:
// System.Net.HttpStatusCode.NotFound
Response.StatusCode = 404;
Response.StatusDescription = "Page not found";

你也可以在这里很好地处理各种其他错误代码。

Google一般会遵循302，然后遵守404状态代码，因此您需要确保在错误页面上返回该状态代码。