[Mailman-Users] Restricting posting to list members broken

[jwm at plain.co.nz]

AS the title suggests, only it's being _too_ restrictive. I'm getting
(as mailman admin) messages that say message from 'email address I
know is subscribe' cannot post to this list as it's restricted to list 
members only. As this was _my_ address, I though it might be some sort 
of weird local delivery thing, or mailer miscofiguration, but it seems 
to be affecting everyone else, too. 

Anyone else encountered this with mailman 1.0b8? What header info does 
it use to determine origin?

John.