Deceptive Design is a website that highlights "tricks used in websites and apps that make you do things that you didn't mean to, like buying or signing up for something." Read through their Hall of Shame and you'll see that a lot of them are email related. Hidden/unclear/assumptive opt-in practices, forced newsletter subscriptions, inverted opt-in checkboxes, and more. Don't be on this list, and don't do things that are on this list!
