Here’s a nice and simple article explaining how to add Triggers and Actions in Drupal 6. This tutorial will send an email notification to the site administrator when a comment is added to an article. I use it on this site to send an email to the articles author when a comment is added.

http://www.scribbledesigns.co.uk/2008/04/03/howto-configuring-triggers-and-actions-in-drupal-6/