When a sales order is authorised and the product is backordered, it would be great for an automatic email to be sent to the customer notifying them that their order has been backordered.
The notification would be sent to the email address associated with that customer.
I'm surprised this wasn't built into the software from the beginning as it's fairly basic functionality in regards to order processing.
Adding on to this, there may as well be email notifications added for other things like when an order is shipped. The customer should receive a notification with the carrier and tracking info.