"misc" covers mainly the situations where qpsmtp finds that the smtp protocol is not followed properly, arithmetically it is all rejected situations which are not covered by the RBL or Executible columns.
As far as I can make out many "built in" Virus and Spam engines often do not "wait" for the correct responses etc, but just try t send email as fast as possible to the receivers. My own experience on my servers is that this seems to eliminate most Viruses, I only rarely get an "executible" rejection and NEVER get a Virus detected by Clam.
Hope that helps. Others may have other thoughts on this.