Below is one of the warnings that you may receive. You’ll need to acknowledge the warning but won’t need to undo your import (and Mailchimp won’t suspend sending of your campaign emails).
Mailchimp’s artificial intelligence system, Omnivore, automatically scanned your recent import. Omnivore flagged the list for having a large number of addresses that may cause complaints, high bounce rates, or blacklistings. Our research shows that this can lead to increased abuse and complaint rates. This is known as a ‘stale’ list. Stale lists are often older lists, and may not have been sent to in some time.
Sending to a stale list will generate undeliverable mail, known as bounces. Stale lists also typically contain spam trap addresses. Sending to spam traps hurts the reputation of the servers used to deliver your content, and can cause further blocks against sending.
Internet Service Providers and email services set limits on the number of emails that report abuse, send to spam traps, and generate high bounce rates. When those limits are exceeded, the sender of the emails can be blocked from further sending.
You may still send to the list without making any changes, however to avoid exceeding thresholds, we strongly recommend re-confirming the list through double opt-in methods to ensure that all subscribers on the list expect to receive the content you send. If you would like to remove the new addresses, you can undo recent imports from the list’s import history page.