All,

Many thanks to all who responded to my mystery subscriber (i.e. bouncer)
and provided such thorough explanations of how to solve such a situation.
I found two potential problem subscribers at iname.com (as all suspected I
might) -- one was [log in to unmask] and the other was [log in to unmask]

Given the nature of the list in question (to some it's an tempting target
for attack), and given the suspicious-sounding names of these subscribers
(and of [log in to unmask]), I quiet deleted them rather than investigate
and potentially tip them off that they needed to redouble their efforts to
cause trouble.  If they were legit subcribers, they'll be back.

Thanks.

-- John