Email addresses that had an issue getting emails at some point after several attempts are added to the suppression list as a precaution in On Demand to avoid sending alerts to invalid email addresses.
At the moment, to request the removal of an email address in the suppression list, you will need to open a Service Request with Quest Support and provide a CSV file export with the most recent Activity Trail data highlighting the emails you would like to remove.
The product team logged Enhancement Request ODCORE-I-32 to improve and find an easy way to manage email addresses that are added to the suppression list.