Yes, we'll automatically clean bouncebacks from your list, and remove 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..

Unsubscribes:
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 seperate 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 Tab ==> View List ==> choose "unsubscribes" from the drop down box

Removing Soft Bounces:
MailChimp gives "soft bounces" (such as "I'm on vacation" auto-replies from your recipients) five 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 Tab ==> View List ==> choose "cleaned" from the drop down box (see screenshot above)