How to query "delivery status = unknown" on CiviMail report
Welcome to SE!  The first thing to note is that if you're on 4.6.36 you really need to upgrade.  There have been numerous security fixes and functionality enhancements since then.  You will also need to upgrade your version of php.  But that's not what you asked...
I needed to investigate mails with a status of 'unknown' recently.  The standard reports did not show what I wanted but the SQL behind them is a useful starting point.
From the Mailings &gt; Scheduled and Sent Mailings screen, find the mailing of interest and click on Report.  On the Successful Deliveries line, click on Report and then go to the Developer tab.  From there you can copy the SQL, modify it a bit and paste it into a SQL session. How you modify it depends on what you want to know, but you will want to remove the 'where' clause that limits it to successful mails.
Things have changed a lot since 4.6 so this may not work for you.