However, both these create a problem when you try and filter. What happens is that whilst you can search the new email field, if you search via order ID, store, or Purchased On - then you are taken to the Dashboard, and when you return to the Order grid, you get an error.
For the sake of example, I am using the following code to make the join in _prepareCollection
$collection->getSelect()->joinLeft( array('myorder'=>'sales_flat_order'), 'myorder.entity_id = main_table.entity_id', array('myorder.customer_email') );
and the following to call it in _prepareColumns:
$this->addColumn('customer_email', array( 'header' => $this->helper('sales')->__('Customer Email'), 'index' => 'customer_email', 'filter_index' => 'myorder.customer_email', ));
Anyone any ideas what is wrong with this code?