While we dont' use MailChimp as our email provider I find this posting in the MailChimp Knowledge base and found it very helpful. Note this is a cut and pasted from the MailChimp site - here's the original link. Also there's a link at the bottom to Is My List Okay To Use In MailChimp? which is somewhat the same but has more information for different scenarios to help you understand and mitigate possible issues.
From the MailChimp site:
Your list is okay
Your list is NOT okay
People who have opted in to your list on your website
People who have opted in to receive emails from you offline
Your customers who have a prior business relationship with you (but if you haven't contacted them in ages, be sure to send a re-introduction email)
People who opted in to receive emails from you at a co-registration site
You're sending on behalf of your customer, and they comply with the above.
People who purchased one of your products, or attended an event, and gave you their email address so they could receive updates from you.
Your internal employee list (such as for company notices)
Press releases sent to reporters who signed up at your website
Lists that you purchased from a 3rd party (even if it's "opt-in")
Even if Chuck Norris or GOD HIMSELF sold you the list, it's not okay to use in MailChimp.