The Quarantine Plus feature enables you to view and, optionally, re-queue messages that failed to deliver to your mail server:
To view failed deliveries:
1) Go to the Quarantine+ tab in the SpamHero control panel.
2) Select Failed delivery from the list box next to the search button:
4) Hover over one of the messages and click on the Logs link determine why the delivery failed:
5) After resolving the issue, click on the Deliver link of a single messages to confirm that your mail server is accepting messages again.
6) Optionally, re-queue all failed deliveries by clicking on the Bulk Redeliver button: