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.
Read next
More explainers from the same section of the Learn index.
Frequently asked questions
Explicit unsubscribes, spam complaints, hard bounces, and manual do-not-contact or data-deletion requests.