Suppression list
Also known as: Do-not-email list, Exclusion list
Definition
A suppression list is the set of email addresses a sender is permanently or temporarily barred from contacting, typically comprising hard bounces, spam complaints, and unsubscribes.
Why it matters
Suppression is enforced at send time rather than at list-build time, because a contact can be suppressed between the moment a campaign is composed and the moment it goes out.
The critical property is that there is exactly one authoritative list per sending account. Multiple partial lists across tools is the most common cause of mailing someone who already unsubscribed.