The suppression list is a special type of subscriber list in Campaign Monitor. It contains email addresses you don't want to send to, to help protect your sending reputation. A subscriber's email address is automatically added to the suppression list if:
- The address hard bounces.
- The subscriber marks one of your emails as spam (unless they are on Gmail).
- The subscriber unsubscribes, or you set their status to unsubscribed.
- It was part of a bulk remove operation, with the "Unsubscribe from all lists" option selected.