I'm very sorry your contacts have accidentally Unsubscribed. It sounds as though you've included a link to your sign-up form within your email campaign? Or are the contacts clicking the Update Profile link in the footer of your email campaign? Either way, the link may be broken because the email was forwarded. We especially see this happen if you're forwarding the email campaign to a role address or distribution list. If that's the case, you'll want to get specific email addresses for all of your contacts and add them to your account rather than send to a group list.
In the meantime, make sure that anyone who is trying to Resubscribe checks their junk/spam folder (and Preferences folder if it's a Gmail address) for the confirmation email. If it doesn't arrive, it could mean that the email address is Suspended. If you'd like to send us an email to social_support(at)constantcontact(dot)com with your username, the email addressess of the contacts who are trying to resubscribe and a reference to this post, we can look into this for you.
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