Apologies if some recent spam from this list has snuck past your own junk mail filters. The senders have been banned and messages deleted from Groups when they come in.
Since this issue seems to be persisting, the posting policy has been updated to moderate all incoming messages from non-members. Previously non-members could post freely to the list so this will hopefully resolve the issue, albeit at the cost of a slight delay to approve valid posts from non-members.
Nick