You can resubscribe a contact who mistakenly opted-out by navigating to Contacts > Other Lists > Bounces & Opt-Outs. Click through and open your Opt-Out list, search the appropriate email address, and click the Remove Opt-out button to the left of the email address.

Please pay attention to pertinent privacy and compliance regulations when removing opt-outs. We encourage you to get written confirmation from your customer before opting them back in.