No one likes spam. While most people think of it as being unwanted email, spam in the form of unwanted and bogus comments on a website is equally annoying. But on your Drupal site, you want users to comment and interact with your content, but you want to ensure that those comments are legit.
To thwart spam, email providers use aggregated blacklists to determine whether to deliver an email to the inbox or the spam folder, or reject it completely. The blacklist is a reference point for IP addresses identified as sending spam. The list is compiled from IP address traffic patterns and user feedback. Some providers allow their customers to participate in activities that increase the likelihood of winding up on a blacklist, like sending large volumes of email or repeatedly sending unsolicited email for marketing purposes.