What is a suppression list?

Entries arrive from four sources: someone unsubscribed, someone reported you as spam, an address hard bounced, or a person asked you directly. Each has to be permanent.

Suppression is a subtraction that runs after every other rule. A segment that matches a suppressed address must still exclude it, and that ordering belongs in code rather than in a checklist.

Under GDPR the record is also your compliance evidence. Deleting someone entirely means losing the proof you were told to stop, so an opt-out record is one of the things you keep rather than erase.

Export it whenever you change platform, and import it before the first send on the new one. That single step is what stops a migration from re-mailing everyone who ever left.

Say you're moving 84,000 contacts from one sending platform to another. Over four years the old platform has quietly collected 6,200 suppressions: 3,900 unsubscribes, 1,850 hard bounces and 450 spam complaints. Your CRM export contains all 84,000, because the CRM never knew about most of those events.

Import the CRM file alone and your first campaign reaches 3,900 people who asked you to stop, plus 450 who've already reported you once. At a 0.3% complaint ceiling, a send of 84,000 can absorb about 250 complaints before providers react. Those 450 earlier complainers could use up that allowance by themselves.

The safe order is fixed. Export suppressions from the old system with the reason and date for each, load them into the new one as suppressions, and only then import contacts. Afterwards, search the new platform for five addresses you know opted out and confirm each shows as blocked.

Seed it. Add two addresses you control to the suppression list, then include them in every import and segment you build. If either one ever receives a campaign, your subtraction step is broken, and you've found out from your own inbox instead of from a regulator.

Match on a normalized form of the address. Someone who unsubscribed as Dana.Reyes@Acme.example should still be blocked when a trade show list brings her back in lowercase. Lowercase and trim both sides before comparing.

Know the scope of each entry too. Many platforms keep a per-list unsubscribe and a separate account-wide block, and an opt-out recorded against one list won't stop a send from another. Hard bounces and complaints belong at the account-wide level every time.

Apply the same subtraction before you verify a file. There's no reason to spend a credit on an address you're never going to mail.