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For more information refer to the Emma article about how to opt-in an unsubscribed contact.
Beware of accidental unsubscribes - How do I forward an Emma email?
Accidental opt outs can happen when a mailing is forwarded to someone and if they opt out, instead of opting themselves out, they opt out the original recipient instead. Luckily, Accidental opts outs can be avoided! To do so, a contact can forward a mailing by using the "Send to friend" option at the bottom of the email, which generates unique links for each new recipient. So if the forwarded recipient opts out, the original recipient will still stay in your Audience.