You may use the export feature within Message Reports under Email > Messages once the Email List has been used to send a Message. This opt out export includes recipients who were opted out prior to receiving the message. It is not recipients who opted out of the specific message. 
  1. Navigate to Email > Messages
  2. Click the graph icon for a sent message to view the message report
  3. Click the Opt-outs link
  4. Click the CSV button in the upper right corner
*Note: The date of when a user opted out of an email is not currently available via opt out report at this time

Additional Resources
How to export email reports
How to view or report on opt outs or unsubscribes