> What do other list owners/ LISTSERV postmasters do in handling rejection
> mail?
 
I generally write a note to the postmaster at the rejecting site and ask
for confirmation (if the message said "no such user").  If traffic on the
list is heavy, I may not wait very long for confirmation, but I generally
get an answer saying, "Oh, yes, that user is gone."  I ignore messages
that complain about obviously transient phenomena.  Once, recently, I
deleted a user immediately upon the very first rejection because it was
a rejection of the message welcoming that user to the list!
                                  John