Is this domain no longer active? I keep getting these Undeliverable messages for addresses in this domain when replying to posts.
This is just a discussion list, not a domain host, and since that's someone else's domain, it's outside the realm of Yahoo!
How do you handle dead addresses in your list? Can't you just delete this user from the list if their address is no longer valid? Your list probably receives these bounce messages too, so there should be some way to automate the process of clearing them from the list.