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#TIL Never delete a contact on Mailchimp – not even when you’re just testing a sign-up form, unless (see below). As per Mailchimp support:
Deleting a contact is a permanent action that is intended to prevent the address from being re-added in the future.
You will not be able to re-add the same email address to the audience manually, import it, nor re-subscribe through a third-party signup form. The only way for a deleted contact to be re-added is if they sign up through a Mailchimp-created signup form.
So if you’re testing sign-up forms, always use aliases. (Here’s how to do that in Gmail). In other words, don’t test your form with name@ – use something like name+{yyyymmdd}@.
Also means people who unsubscribe can never subscribe again with that email address. Not exactly customer friendly. People have been known to change their mind…