LISTSERV Error Monitoring only catches bounces that are
conformant to the rules.
 
All others it passes on to you.
 
Thus, when you DELETE or NOMAIL the subscriber, you still can be
"stuck" with more bounces between the time that LISTSERV
processed your command and the distributions that are STILL in
the pipeline waiting to be returned to the ERRORS-TO= value in
your list's definition.
 
Pete Weiss at Penn State