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

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