The *simplest* way is to add it to the existing EMAIL_DISCLAIMER definition, as that requires no rewriting of the order class or email templates.
I'd suggest this, in your email_extras.php file, around line 27:
Code:
if (!defined('OFFICE_IP_TO_HOST_ADDRESS')) define('OFFICE_IP_TO_HOST_ADDRESS', 'OFF');
define('EMAIL_UK_DSR_REGULATIONS_TEXT', "Put your text here.");
// email disclaimer
define('EMAIL_DISCLAIMER', EMAIL_UK_DSR_REGULATIONS_TEXT . "\n\n" . 'This email address was given to us by you or by one of our customers. If you feel that you have received this email in error, please send an email to %s ');
define('EMAIL_SPAM_DISCLAIMER' ...
Remember, the "Put your text here." part is enclosed with double-quotes, so if you actual text that you enter contains any double-quotes, then write them as \" (add a backslash before them) else you'll get PHP syntax errors.
And if you need to add any line-breaks, use \n