Removing Soft Bounces
MailChimp gives "soft bounces" (server overloaded or down at the time you sent to your recipient, etc) 8 chances before cleaning them from your list.
Removing Hard Bounces
When MailChimp receives a "hard bounce," (such as an inactive account, or undeliverable address) we remove the email address from your list. We store these at Lists > View > Subscribers > choose "cleaned" from the drop down box (see screenshot above).
We also factor engagement activity into our bounce cleaning decisions (read about
how MailChimp measures engagement). For example, if we send an email and a receiving server tells us that a recipient “does not exist,” but we have open and click activity in the last 45 days to prove otherwise, we’re not going to blindly clean that recipient from the list. We know they exist, and we know their account works, so we’re going to give them a few more chances than we normally do. If, however, we see that there’s very little (or no) activity by that recipient, we clean them under the same rules we’ve used in the past.
We can't get into specifics about how many stars justifies a “clean vs. a keep,” or exactly how many chances we give hard and soft bounces. The algorithm will surely be adjusted and tweaked over time.