The suppression list protects your reputation: hard bounces, complaints and unsubscribes are removed from future sends.
When a recipient permanently rejects (mailbox doesn't exist, non-existent domain, permanent block), the address is added to the suppression list and every future send to it is blocked at the Target SMTP layer without even attempting delivery. This protects your reputation from repeat bounces.
When a user clicks "Mark as spam" in Gmail/Outlook/Yahoo, we receive a notification via ARF (Abuse Reporting Format). The address is added to the suppression list across all your sends (transactional included, unless explicitly overridden). A complaint rate above 0.3% triggers an automatic alert.
The unsubscribe links automatically injected into marketing emails take the user to a confirmation page. Once confirmed, the address is suppressed for that category (you can keep transactional, block marketing — configurable per category).
From the panel you can: import suppression lists from other providers (crucial during migration), manually remove an address, export the list as CSV/JSON for backup.