When I create an unsubscribe form using CM html code, then submit a made up email address that isn't on the list, there doesn't appear to be any sort of validation process before the generic successful unsubscribe page is displayed. Is there a way to check email addresses submitted against the addresses on the list? This would be useful so that people could be sure they're unsubscribing themselves (vs a person who forwarded the email to them).
Currently, your only option is to use the API.
I'm not sure I understand what you mean, though - is your concern that someone will unsubscribe the wrong address? Because if so, then I don't see how this would help? But I might be missing something :)