I had to call tech support for help with resubscribing a contact who mistakenly unsubscribed. I found out the "confirmed opt-in" setting has to be turned on while sending the confirmation email and must remain on until a reply is received from the recipient. Since the "confirmed opt-in" feature is not mandatory, it should be clear when the "resubscribe" button is clicked that the "confirmed opt-in" feature must be turned on in order for it to work.
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