When you want to merge newsletters in MailChimp like I do and tell your subscribers to sign up at the new list and unsubscribe from the old to not receive any notifications – then you're setting yourself up for trouble!
Because it turns out that mass-migrations from list A to list B mostly register as mass unsubscribes from list A. And an unsubscribe rate of 9% or higher (I don't know the actual minimum value, but 9.23% triggered it for my account) will automatically up the risk level of your whole MailChimp account.
Now double opt-in is enforced on my account (which isn't bad, because I had this enabled anyway), and I'm honestly getting a bit nervous about the account being closed. It's not a big deal if I lost the emails, mind you, but I reached out to so many people and asked for a subscription renewal that it's highly unprofessional to screw everything up a week later.
Of course MailChimp offers instructions: The "Reconfirm Addresses" help page tell you to unsubscribe everyone on old lists, reach out via email, not via MailChimp, and have them reconfirm their subscription.
Should've done that, but I didn't look that page up in advance.
So if you want to merge newsletter lists or "Audiences" on MailChimp, listen to the help page to avoid trouble!
Also, if you are affected by this, I'm terribly, terribly sorry for getting on your nerves with this.
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