A suppression list is checked before every send, and any matching recipient is skipped, regardless of what triggered the campaign. It's the mechanical enforcement layer behind unsubscribe, hard-bounce, and spam-complaint handling.
Suppression is different from a subscriber preference (which topics someone wants) and different from a simple "unsubscribed" flag on one list; a robust suppression list applies globally across all sending on a domain, not just the specific campaign or list someone opted out of.
PulsePigeon suppresses hard bounces and complaints automatically and immediately; suppression events are also exposed through the API so customers can keep their own systems in sync rather than re-adding a suppressed address by accident.