Bounces are emails that are returned to the sender because they cannot be delivered for a given reason. At first, you should take the warnings as a "heads up" that some of the email addresses you sent to are no longer valid or some other delivery error is occurring. ISPs closely monitor how many bounces occur. If an ISP sees too many of these bounces, it can lead to deliverability issues for your account and other MailChimp users' accounts. We've found that 100% double opt-in lists contain less than 5% bounced email addresses. If you get more than that, you should be concerned. It's highly recommended to re-evaluate your email marketing process to prevent further bounces. Here are some helpful tips.
You should be very concerned, if you see more than a small handful of these warnings. Once bounce complaints exceed our 5% threshold (5.01% is over the limit), 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 compliance team. They will ask about your list collection process and ask for other details about the health and life of your list.