I want to clarify that my checklist doesn't advocate for people to purchase lists. If someone is located in a jurisdiction that permits sending of unsolicited commercial mail, and they purchased a list then they should validate it. Validating also applies to contacts acquired through a single-opt in form (with a consent check) as some users will enter fake email address in order to download an ebook or free product. Though you can always send the download link in the email which forces people to use legitimate email address. But your point is well made, I should add a disclaimer, that users should comply with their countries laws and regulations when sending promotional email.