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What Is a Suppression List?
TL;DR
A suppression list is a list of contacts a sender permanently excludes from all future sends — typically people who unsubscribed, marked a message as spam, or requested removal — checked automatically before every campaign so those contacts are never re-emailed.
What goes on a suppression list
Explicit unsubscribes, spam complaints, hard bounces from invalid addresses, and manual do-not-contact requests. Some organizations also suppress contacts who submitted a data-deletion request under GDPR or a similar regulation.
Why suppression is checked automatically, not manually
At any real sending volume, manually cross-referencing every new list against every past opt-out is not reliable. A missed suppression means re-emailing someone who explicitly opted out, which is both a compliance risk and a strong negative deliverability signal through repeat spam complaints.
Suppression lists across multiple lists or campaigns
A suppression list should apply account-wide, not per campaign — someone who unsubscribed from one campaign should not receive a different campaign from the same sender afterward, since from the recipient perspective it is the same company emailing them again.
