The postifx setup is quite simple at the moment: It rejects all unknown / unregistered mail. Greylisting is not in place yet. I quickly ran pflogsumm to give you an impression:
Per-Day Traffic Summary date received delivered deferred bounced rejected -------------------------------------------------------------------- Sep 27 2009 110 107 153 16 4874 Sep 28 2009 170 855 298 56 6894 Sep 29 2009 229 1235 379 90 6757 Sep 30 2009 176 525 420 38 7477 Oct 1 2009 187 222 377 20 7913 Oct 2 2009 132 132 414 17 8086 Oct 3 2009 112 117 374 18 7790 Oct 4 2009 128 658 294 42 6158 Oct 5 2009 243 1634 509 97 7657
I don't understand these figures. Ah, this is for the complete mail server. That explains why there was say 128 received on Oct 4, yet only a handful in the list archive.
But I tend to agree with you: We might have to face the fact that we have to improve the SPAM filter on our end and open the list. As a first step we could install greylisting and have a bunch of moderators to see how that goes.
Do you at least have spamassassin running? It would be interesting to see how many of the rejected emails spamassassin also decided were spam. Could you configure postfix to keep a copy of the rejected. I can then manually perform some tests with spamassassin and see how good it is.
Andrew