Definition
Suppression list: A list of addresses a sender must never mail again, such as hard bounces, spam complaints and unsubscribes. It is checked before every send and overrides any later import of the same address.
What belongs on a suppression list
Hard bounces, spam complaints and unsubscribes are the three automatic entries. Add do-not-contact requests from people who emailed or called, addresses tied to data deletion requests, and any address you know to be a trap.
Most platforms keep a global suppression list for the account plus list-level unsubscribes for people who only left one newsletter.
Suppression list vs unsubscribe list
Unsubscribes are a subset of suppressions. Someone who unsubscribes from your weekly newsletter may still want order receipts, so many platforms suppress them per list or per topic.
Bounces and complaints are different: they apply to every send from every list. Under CAN-SPAM in the US, an opt-out must be honored within 10 business days.
How to keep suppression lists in sync across tools
Most teams send from more than one tool: a marketing platform, a CRM, an outreach tool and a transactional service. An address suppressed in one is still live in the others.
Pick one system as the source of truth and sync suppressions out of it, through native integrations, webhooks or a scheduled export and import.
How RocketVerifier handles it
Addresses on your RocketVerifier suppression list come back as risky with sub_status do_not_mail, along with known complainers, so they are caught even when they reappear in a new upload. Connect SendGrid, Mailgun, Amazon SES or Postmark and every hard bounce of an address a job marked deliverable within 30 days is added to your suppression list automatically and refunded, 1 credit each.