The  other site  runs a  package  that hasn't  got the  concept of  "send
replies to X, but  deliver errors to Y, and by the way  this comes from Z
on the T list". This package just  knows about "this comes from X, errors
to X,  replies to X". The  gateway uses the "delivery  errors" address as
the value for X. So when the  user replies, his reply goes to the address
for delivery  errors. LISTSERV  never tries to  distribute any  mail that
goes to this  address, it just passes  it to the list  owner. And assumes
it's a delivery error.
 
  Eric