Suppression list
A list of addresses that must never receive mail again — unsubscribes, hard bounces, spam complainants.
A suppression list is the central registry of email addresses that must never receive mail from your domain again. Addresses land on the suppression list through three primary paths: unsubscribe requests (recipient clicked unsubscribe), hard bounces (delivery failed permanently), and abuse reports (recipient marked as spam). Compliant senders never deliver mail to addresses on the suppression list, ever.
Suppression is permanent. Once an address is suppressed, it cannot be re-engaged through any mechanism short of a fresh, explicit re-opt-in initiated by the recipient. "Cleaning" the suppression list — removing old entries to attempt re-engagement — is technically possible but legally precarious and reputationally damaging. CAN-SPAM, GDPR, and CASL all require honoring unsubscribes; some require it perpetually.
ESPs maintain global suppression lists across all customer accounts (Mailgun's "suppression" is platform-wide) which prevents cross-customer abuse. Custom sending stacks must maintain suppression themselves, which is a non-trivial engineering responsibility. The cost of getting suppression wrong is severe — sending to suppressed addresses is one of the fastest paths to reputation damage and regulatory action.