MailChimp's John Foreman summarizes it thusly:
If we look at campaign performance versus the percentage of a mailing list that’s purchased or scraped, we find that positive engagement falls off a cliff as purchased correlation increases. Since most folks have to open an email to unsubscribe, unsubscribes die off too. The only thing that does go up? Complaints.Click on through to see the pretty graphs. Great stuff!
H/T: Matt Vernhout.