Some time ago when some server admins got fed up with all the junk mail they received, they started to track the IP addresses of the servers that sent them spam, and put them on “blacklists.” Any letter received from a server on their blacklist was deleted immediately. It worked pretty well. So they started to share their blacklists with other server admins. And other admins started to add on to those blacklists.
The blacklists got very, very big soon. Large ISPs started to sync up with them. If you send email that’s very “spammy” you could end up on one of these blacklists (whether the recipients are opt-in or not). Once you got on a blacklist, good luck getting off.
This is why you need to make sure you only send to recipients who gave you explicit permission to email them, and you need to have proof that each one of them opted in to your list (such as through a double opt-in system).