After someone submits a form, the system sends a confirmation email. The person is added to the list only after clicking the link. This confirms the address belongs to them and that they want to receive messages.
Why it matters
Double opt-in produces cleaner lists, fewer typos and spam complaints, and a clear record of consent. The trade-off is that some people never confirm, so the list grows more slowly.
Common pitfalls and tips
It is most common for newsletters and consumer sign-ups. For B2B outreach to contacts who have not signed up, it does not apply, so other opt-out practices matter more.
Example
A visitor signs up for a newsletter and receives a message asking to confirm. Once they click, they are added to the list and receive the welcome email.