The icon next to the contact's email address communicates the subscribed/unsubscribed state. Green indicates that the email address accepts emails. Red indicates that the email address is unsubscribed. You can click on the icon to change the subscribed/unsubscribed state. If the contact has one email address that is unsubscribed, the system will not allow emails to be sent.
There is no way to toggle an unsubscribed email to make it subscribed. You will have to delete the unsubscribed email address and add the email address back to toggle it from unsubscribed to subscribed.