Changing Drupal notifications e-mail address

I am using built in Drupal 6 user module, fore user registration, forgot-your-password-emails and all that stuff. When a notification is sent by e-mail for confirming reistration, resending passwords, etc, these are sent from a different e-mail address to the one I want. I dont remember where I configured this setting, and cannot find where to change it again. Can you help me? Thanks!