[Mailman-Users] allow non-subscribed users to send email

I know this is so not what mailing list admin normally should allow but it has been asked for and the one asked worries more about complains such as "why this person do not have permission to send to this list" etc. Anyway, how do i allow anyone valid mail address to a list?

-- Regards,

Umarzuki Mochlis http://debmal.my