Think in terms of a mailing list for a business, like a newsletter.
It costs money to send out emails. If 50% of your users aren't reading them, you can trim the list of those who don't open the email.
It costs money to send out emails. If 50% of your users aren't reading them, you can trim the list of those who don't open the email.
Staying on the good side of spam blockers is a much bigger deal.
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.