[Mailman-Users] Can receive but not send?

[Andrei]

Thanks to Sean I've made some progress. It seems that local users on
lists.host.org server can receive and send emails. Mailman on
lists.host.org can
also receive emails. However, there is an an error message when Mailman
tries to send email out to list subscribers. The error message I see in the
logs is "Unrouteable address" for each email address in the list (e.g.
email at yahoo.com).