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Recipients of email from Emma can opt-out from a sender entirely, or "update their preferences" and unsubscribe from certain topics. Either can happen accidentally, and sometimes people want to opt back in or resubscribe. The easiest way to accomplish this is to send them a link to the sub-account's sign-up form. Submitting the sign-up form will opt the person back into receiving mail from that sub-account. Additionally, they will be resubscribed to any topics they select on the sign-up form. If your sub-account doesn't have a sign-up form, please see Emma's documentation for how to create a sign-up form.
For more information refer to the Emma article about how to opt-in an unsubscribed contact.