How do I send email with extra variables / fields from the "orders" table?

I created an extra field in the orders table to store information about the desired delivery date (I called it order_delivery_date).

I managed to get the extra field in the front-end during checkout shipping, and in the back-end invoice/packinglist.
However so far I did not succeed in sending variables via email.
I am only able send the text labels (which are stored in the language files).

How can I include this field in the emails from Zen Cart
(which are sent after after checkout and after change of status from the admin back-end)?
Can anyone point me in the right direction (e.g. recommend me a forum topic about this)?

Background info (about how I added the extra field) is available in:
http://www.zen-cart.com/forum/showthread.php?t=88550