Double opt-in
Lists default to double opt-in. A new subscriber is pending until they confirm. Confirmation mail is transactional, so it is not blocked by the marketing postal-address gate.
What the subscriber sees
After they submit the form they are asked to check their inbox. The confirmation message has a button that sends a POST. Inbox scanners that only open the link do not confirm the subscription. Tokens are hashed, single-use, and expire after 72 hours. An expired link shows an explanation and does not subscribe them.
If they already subscribed
The public form returns the same success path for a new or existing address, so the page cannot be used to probe who is on the list. Confirmed subscribers can change lists later in the preference center.