Every email you send through CorpNote contains an unsubscribe link. When people unsubscribe, you will not be able to send email to that contact even if they are in your CorpNote address book.
If the contact has requested to be unsubscribed from your email list but has not used the CorpNote unsubscribe form, you can unsubscribe for them which will prevent you from accidentally sending emails to that contact. If the contact later re-subscribes through your CorpNote subscribe form, the "unsubscribed" status will be removed and you will be able to send email to the contact again.
You can export unsubscribed contacts as a .csv comma delimited file that can be imported into other contact manager programs that accept this type of file. This is helpful if you need to update unsubscribed contacts in another contact manager.