Your recipients can report emails as spam in their email program. You've probably seen this option in your Gmail or Yahoo accounts. If an ISP has what is called a Feedback Loop (most of them do), a recipient who marks one of your emails as spam gets registered as an abuse complaint in your MailChimp account. How this is handled depends on the email service provider, but in MailChimp, we immediately move them off of your active list and into the abuse complaints area.
I got an abuse alert in my account
At first, you should take an abuse alert as a "heads up" that people are complaining about your campaigns. If you get more complaints than a small handful, you should be concerned and we highly recommended to re-evaluate your email marketing process to prevent further complaints.
Once abuse complaints exceed our threshold you will receive a warning notice. If the complaint rate is far beyond that threshold, a suspension notice will arrive in your inbox from our abuse team. They will ask about your list collection process and ask for other details about the health and life of your list. We'll need that information because by then ISPs and anti-spam organizations are threatening to blacklist us unless we explain why your list generated so many complaints. A full investigation will proceed once we have the requested information from you.
Even if you're a legitimate marketer who does everything "by the book" and only uses opt-in lists, you can still get reported for spamming. Even if someone has signed up to receive emails from you, they can click the spam button in their email application if they think it's spam or junk mail. Some people just think the button is an easy way to unsubscribe from your list. Some people are just too lazy to scroll down and click your opt-out link. That's why a couple of complaints are understood, but several gets into sticky territory.
Accidental Spam Reports
If someone accidentally clicks that "report spam" button and is noted as an abuse complaint the only way to have them get back on your list is if they use the signup forms provided by MailChimp in your account so that we have absolute proof of opt-in.