What is an email suppression list?
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You've probably encountered this scenario: someone unsubscribes from your emails, but a week later, you accidentally mail them again. They get annoyed. You potentially violate regulations. That's where a suppression list comes in.
A suppression list is essentially your "do not send" registry. It contains addresses you should never mail again. Hard bounces (mailboxes that don't exist), unsubscribes, complaints, and other refusals go on this list. Once an address is suppressed, your email service provider won't let it receive mail from you.
Why does this matter? Legal compliance is the biggest reason. Under regulations like CAN-SPAM (U.S.) and GDPR (Europe), you're legally required to honor unsubscribe requests and bounce signals. Send to someone after they've asked out, and you're looking at fines or worse. Suppression lists also protect your reputation. ISPs track whether you're respecting refusals. Ignore your suppressions and mailbox providers penalize you with lower deliverability rates.
Think of it this way: your suppression list is the guardrail between sending volume and sender reputation. You could send to everyone, but that kills trust and violates the law. Suppressions let you send only to people who actually want your mail.
Next step: Learn which address types belong in your suppression list, then check if your ESP is automatically removing suppressions during sends.
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