Double Opt-In
A signup process where the subscriber must click a confirmation link in an email before being added to the list, proving the address is real and theirs.
With double opt-in, submitting the popup form is only step one. The email service sends a confirmation message, and the address joins the list only after the recipient clicks the link inside it. Single opt-in skips that step and subscribes the address immediately.
The cost is real: some people who submitted never confirm, because the email lands in spam, arrives too late, or the intent has already faded. The gain is list quality. Typos, fake addresses, bot submissions, and people who entered someone else's email are filtered out before they can hurt deliverability, and the confirmation click is documented consent, which matters under GDPR in the EU and similar rules elsewhere.
If you use double opt-in, say so in the popup's success state and tell the visitor to check their inbox, then make the confirmation email arrive fast and deliver the lead magnet in it or right after. Most major email platforms offer double opt-in as a list setting, so the popup only needs to collect the address and pass it on.
See it in practice