Yes, we'll automatically remove bounce backs from your list and unsubscribes - as long as you are uploading onto the same list in your account. If you're creating new lists in your account, information from old lists will not carry over to any new lists in the account. Why you should add to your old list instead of uploading a new list..
Unsubscribers
MailChimp's list management system automatically removes unsubscribes from your list as soon as recipients click the "Unsubscribe" link in your email.
We also store all Unsubscribes in a separate list that you can download to Microsoft Excel format, so you can submit the list to any global "do not email" systems your organization employs. Lists > View > choose "unsubscribed" from the drop down box
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 immediately. Some email servers penalize you if you keep trying to send messages to an account that they told you was inactive. We store these at Lists > View > 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.