In Drupal 5 and 6, when a user submits the contact form, and email gets generated and sent to the site maintainer (or whatever address is set in the conact form). This email is sent from the server so clicky reply will send an email back to the server, not the email address the submitter set int he form.
The core hack to contact.pages.inc below fixes this so that reply-to address is the sender.
