What is Suppression list (blocklist)?
Addresses and domains that must never be emailed — unsubscribes, complaints, bounces, competitors — enforced at import, enrolment and send time.
A suppression list is only useful if every path into a campaign checks it: CSV import, enrolment from an existing list, manual adds, and the send itself. Unsubscribes and hard bounces should be added automatically; competitors, existing customers and do-not-contact requests are added by hand. A blocked domain should cover everyone at that company.