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