An email list is built one person at a time, by giving each of them a reason to hand over an address. There is no shortcut that works, and the shortcuts that exist are actively harmful.
Purchased lists look like an obvious answer and are consistently a mistake:
The damage is not limited to the wasted list. A poisoned sending reputation affects the entire channel afterwards, which is a large cost for something that was supposed to be a shortcut.
"Subscribe to our newsletter" asks for something and offers nothing. It converts poorly because it is not a trade — it is a request.
What works is offering something specific and immediately useful in exchange:
The offer should be closely related to what you do. A giveaway unrelated to your business attracts people who want the giveaway and have no interest in anything else — a larger list that performs worse.
Even keeping the plain newsletter signup, changing the wording from "subscribe to our newsletter" to a specific description of what arrives and how often — "one email a month with a new tutorial, unsubscribe any time" — sets an expectation and gives a reason. It costs nothing to write.
Popups do increase signups, which is why they are everywhere. They also annoy people and, on mobile, an intrusive one appearing immediately can be penalised by search engines as an interstitial that blocks content.
If used at all: trigger on intent or after some engagement rather than instantly, make the close button obvious and large, do not show it again to someone who dismissed it, and never let it cover the content on a small screen. A popup that fails those conditions costs more in goodwill and search visibility than it earns in addresses.
Double opt-in sends a confirmation email that the subscriber must click before being added. It produces a smaller list and a substantially better one: every address is verified, typos are eliminated, and nobody is on it by accident.
It also gives you evidence of consent, which matters if a complaint is ever raised.
List size is a vanity metric. A list of 200 people who open and act on your emails is worth more than 5,000 who ignore them — and the second list actively costs more, since most providers charge by subscriber count and low engagement worsens deliverability for everyone on it.
Removing people who have not opened anything in a long time feels like going backwards and usually improves results.