When you unsubscribe from a Mailchimp email, they ask the recipient why they are unsubscribing. If the recipient marks "I never subscribed" or "this email is spam," the threshold is probably much lower than 20 percent. The software tries to decide if your emails were malicious.

If your account is locked, then you have to call MailChimp and explain that your intentions weren't malicious and that you won't send anything else to those emails from your account.

I had a small email list that I had not sent anything in about a year and had my account locked after sending a campaign. People just forgot they were on the list, because it had been so long. Their support team unlocked it and suggested that I send an email at least every 6 months.