The sort of process I envisage would work something like this:
(i) Mail arriving at the list goes through the (presumably already
installed) automatic filter (Spamassassin?).
(ii) Of what's left, anything from a known serious poster passes
through.
(iii) Anything which is a response to an existing article passes
through.
(iv) The rest stays in limbo for up to 24 hours; during that time, a
moderator may accept it or reject it;
(v) After 24 hours (moderator on holiday?) articles will get passed
through to the list.