The report can sort by Subject Line or Salesforce Username.
I suspect that the User was deleted in SFDC.
I was able to figure out the answer so I thought that I would come back and share. The issue is of the sender showing as "unknown" was triggered by the sales executive using the Outlook Plugin and then sending to an e-mail address that was not a person in Marketo and tracking with Marketo. This triggered a new person to be added to Marketo but at the time of the e-mail send the person was unknown and that's just how the Sales Insight report picks it up.