Cost depends on the setup. Not everyone is taking the most cost effective approach. Spam reduction is definitely a big consideration. However at the last plac

Cost depends on the setup. Not everyone is taking the most cost effective approach. Spam reduction is definitely a big consideration. However at the last place I worked we would get notifications when people marked the emails as spam, and so we could specifically remove those people from the email list. I don't even know how that part is done - we used one of the big emailer/newsletter websites.