Suppression list

A suppression list is a do-not-email list. It usually holds people who unsubscribed, addresses that hard bounced, people who complained, and anyone who asked not to be contacted. Many teams also add current customers, partners and competitors so they never get a cold email by accident.

The list should work across every campaign. If it only applies to one campaign, the same person can be mailed again from the next one. It should hold domains as well as single addresses, because some companies ask you to stop for the whole organization.

Keep the list when you move to a new tool, and import it first. Losing it is a common way to email people who already said no, which causes complaints and can break the law in many places. Check each new list against it before you launch, and never remove an address from it just to send one more email.