Typically the average web surfer visits hundreds of websites a day. And unsuspecting users may have their machine used by spammers or harvesters, as they are called in the digital world.
For example, try accessing this website http://visualwalkthroughs.com/(they are a partner of the HoneyPot prevention project, and if you are already blacklisted you will get the screenshot below).
Project Honey Pot is a distributed network of decoy web pages website administrators can include on their sites in order to gather information about robots, crawlers, and spiders. Most of the times, our IP address is used as a SPAM Server, by a Dictionary attacker and/or several different types of attackers. Project Honey Pot collates data on harvesters, spammers, dictionary attackers, and comment spammers and makes this data available to their users in order for them to protect their websites and inboxes.