With a lot of people using free email services, most have some report button. What this does is flag the specific email as potential spam that you specifically do not want to see. With enough people doing that, the probability of the email and subsequently the source domain being spam and spam generators goes up. High probability means the emails may end up in the spam folder without hitting your inbox.

There’s a bit of fine tuning email marketing can do to mitigate that, like not sending emails too frequently. But that’s not a passive thing they can do, which is why there are teams devoted to email marketing specifically at some companies.

The worst thing for a marketeer is to be dumped in spam. No one will ever see it or any future emails.