Glossary
Double opt-in
Confirming a subscription via a follow-up verification step.
Double opt-in adds a confirmation step after a form submission: the address is added to a list only after the person clicks a verification link sent to that address. A single opt-in adds them immediately on submit, with no proof they own the address.
It protects list quality and deliverability. It blocks typos and malicious sign-ups using someone else's address, and it produces a timestamped record that the owner of the inbox genuinely asked to join — useful evidence for GDPR consent.
The cost is some shrinkage: a fraction of people never click the confirmation. That is usually a net gain, because an engaged, verified list outperforms a larger unverified one on every downstream metric and keeps your sending reputation intact.