A few months ago, I kept getting spam emails from company(ies)advertising some sort of web financial services. Their emails came from @msn - when I contacted MSN to complain, they discovered that their domain name was being forged elsewhere. MSN complained to the ISP where the forged email came from - and I don't know if it's a coincidence, but it's stopped!
Lesson? A slightly stroppy to the ISP may work wonders. Don't let the spammers get away with it.
Oh - the spammers included some pompous wording to the effect that according to the US Supreme Court, this email isn't spam since it includes an "unsubscribe" mechanism. Yeah, right. Don't be put off, complain anyway!