Thanks, Jacob, your comments clarified things.  The current setting is:

Auto-delete= Yes,Semi-Auto,Delay(8),Max(2)

I currently receive copies of the errors that presumably are not formatted in
such a way that LISTSERV knows what to do with them.

- Kathy M.

Jacob Haller wrote:

> You may wish to temporarily set your mailing list to
>
>         Auto-Delete= Yes,Semi-Auto,...
>
> (I assume your current setting is Yes,Full-Auto); LISTSERV will then
> forward bounces from addresses not subscribed to the mailing list to
> the listowner for inspection.  That should clarify what the bounces
> related to the "ghost" addresses are.