You can reactivate Hard Bounces from your portal.

Just go to Reports > Email Log, search for the unsubscribed (or bounced) email, select it and then click on Reactivate:

The reactivation will take a few seconds per email. 

Please select only 1 email at a time and wait for the reactivation process to complete before reactivating another email.  Once the reactivation is complete the status should changed to Reactivated:

Once that is done, wait for another minute or two to ensure the reactivation tags are also cleared from our email provider. 

After that, try sending emails again. 

If they bounce back again, please check if the email is still correct, or try sending a direct (non-Prism) email to that recipient to see if that bounces too.  If normal emails send while Prism's emails still bounce, please send them to us at support@prism.horse so we can check further.