Double opt-in
With double opt-in, a sign-up only takes effect once the recipient confirms it themselves through a link in a confirmation email. Only then does the address enter the list.
The reason is simple: without confirmation anyone can enter somebody else's address. The second step proves that the owner of the address agreed — and in a dispute that proof is exactly what counts.
In practice: sign-up, immediate confirmation email, one click, done. Whoever does not click stays out. A proper implementation also records the time and origin of the confirmation and sets up the unsubscribe path at the same time.
One distinction matters: the procedure applies to the newsletter list. An enquiry through a contact form is something else — you answer that, you do not add it to a marketing list.