This happens when the "Owner=" list is misformed, for instance:
 
* Owner= quiet:,[log in to unmask],[log in to unmask]
 
Some people do that because they think  this will make Joe quiet, but not
Jack. They don't realize it makes both  quiet. Others do it on purpose so
that  no one  is  "bothered" with  "unwanted junk  mail".  In both  cases
LISTSERV makes sure that someone is notified. The header parser will also
identify these problems and report them.
 
  Eric