>Just out of curiousity, how does listserv, or any given server, determine
>whether or not such a user exists?  What sort of occurence could generate
>these spurious error messages?

LISTSERV operates on the assumption that when a host says the user is
unknown, the host is telling the truth :)  This is why probing and not
using hair-trigger auto-deletes is probably the best way to go.  At least
if you probe someone for several days and the probe fails you have a
fair amount of certainty that the user is actually gone.

I have no idea what could cause a server to tell you that a user is unknown
when he actually exists.  I suppose there are probably all kinds of reasons
for it, mostly due to configuration errors and other system problems on
the user's host.

Nathan