Mailing list moderation

I've had a recent surge in spam e-mail being sent to the mailing
list. I have been filtering this manually for non-subscribers as a
service, but it has gotten a bit overwhelming recently. I will not be
passing through e-mail from non-subscribers for a while in an effort
to let the worst storm of spam pass. Let me know if you have
suggestions on how to enable mail from non-subscribers to best
simplify life for those trying to cross-post with lists like linux-