Double opt-in
Sign-up that is only complete once the subscriber confirms it from their own inbox.
With double opt-in, filling in a form is not enough: a confirmation email goes out, and the address joins the list only when its owner clicks the link. Single opt-in adds the address immediately.
It costs you some sign-ups and buys you a list of addresses that exist, were typed correctly and belong to people who wanted them. That shows up later as fewer bounces, fewer spam complaints and better inbox placement.
It also gives you a record of consent, which matters wherever proof of consent is a legal requirement rather than a courtesy.