Administrative users can log into the Administrative Portal and manage the Smart Quarantine for their entire organization. You can access the quarantine from inside the message logs for your domain. There is a dropdown on the top left to either view the messages or the Quarantine folder.
Once you have found the message that you would like to review, click on the message body to view the message details.
Your actions here are to bounce (reject) the message, or to release it to the end user.
* Currently end users do not have direct access to their grouped quarantine messages, this feature is being added.