I want to change the default SKU field inside the transactional email item field, to a custom attribute.

Therefore I need to edit the following file:

/app/design/frontend/theme/theme/template/email/order/items/order/default.phtml

I want to change the following value:

<?php echo $this->escapeHtml($this->getSku($_item)) ?>

My custom attribute value is "dpn" and it is named "DPN".

How can I change this?

I tried the following but that does not work:

<?php echo $this->escapeHtml($this->getDpn($_item)) ?>