Set custom email template for custom order status
If you define a custom order status, the customer should be notified a specific email depending on the status optionally if active.
So an order email can have a custom message, rather than just processed or shipped. This might include delivery delay, items on backorder, other, and the email will have the information in the template.
Amanda N commented
I agree with this suggestion - it's nice to create customer statuses and the ability for an auto-generated email should be available for the custom status.