issue resolved, but not understood....
So we went two days without email service. We were only able to restore service by re-creating every user account via the ispconfig interface. I do not understand what caused this to happen, or how to prevent it in the future.
All of the mail started being held in /var/spool/mail and there seemed to be no way to identify why or to force the email back into /home/www/web6/user folders.