When a contact in your address book has used the unsubscribe link in a previous email you sent to them from the Marketing Center, their email address will appear greyed out in the address book, with a  symbol next to their email address:

If you add an unsubscribed address to a list of recipients for a future email or campaign event, we will not attempt to deliver the email to them because they unsubscribed. We recommend deleting unsubscribed recipients from your Marketing Center address book.

You will be emailed when a contact unsubscribes from your Marketing Center emails or campaigns.