After deleting a contact, emails from that email address do not come into the system. Is there a way to 'un-delete' or re-enable new tickets from that email address?

Yes, you can restore the contact so that all emails from the contact come in as valid tickets and are not marked as spam. Go to the Customers tab > Contacts > click on the hamburger menu on the top left on this page to choose 'Deleted' contacts and restore the contact from this page.