Actually what is hardest to filter is bounces from gmail servers. I'm not really sure how spammers generate them. They are not in response to anything that I send. It seems like google ignores my SPF records, even though it indicates that it found that the sender forged the From header and sends me the bounce with attached spam that is targeted at me anyway. Quite annoying.

EDIT: I guess I can just reject the gmail bounce if it contains the "Received-SPF: fail (google.com:". Ah!