Signup forms, lead magnets, webinars and checkout opt-ins are the legitimate methods everyone knows. They are slow, and so there is always a temptation to speed things up with scraping or a purchased list.

That shortcut tends to show up later as bounces, complaints and spam traps, and the damage lands on the domain you need for everything else.

uChecker's article on how to build an email list sorts the methods into legitimate, grey and risky, and explains why validation should be the last step before the first send.