by Sergio Tapia.
Thank you, that helped a lot. I found this in the lang/en/message.php file:
$string['emailtagline'] = 'This is a copy of a message sent to you at "{$a->sitename}". Go to {$a->url} to reply.';
I can understand the concept that the link is being dynamically genereted by a script somewhere. The '{$a->url}' bit.
Where should I look for if I want to change the generated link? I want to append a certain url GET parameter into it. Thanks!