Filter on To: and Cc: headers?

Dear all,

I operate a number of mailinglists that are open for posting to anyone. Unfortunately, this openness is being abused by spammers that send out a lot of spam to the lists. As is common with most spam, they send to fake or non existing addresses with the real recipient address only being given in BCC headers.

How do I put up such a filter that only mail for a given domain is accepted for the list?

Right now I have a rc.local.s10 like this:

:0 * !(To|Cc):.*starchat.net ! spameater@webconquest.com

SUBJ=`formail -zx Subject:` :0fw * ! ^Subject:.*[routing]] * ! ^Subject:.Re: * | formail -I "Subject: [routing] $SUBJ"

But that doesn't seem to do the trick.

Any insights?