Why does mail forwarding not work correctly?

Due to the incessant spam I receive on my .mac account, I forwarded my email in mail settings to a service that performs whitelisting. The whitelisted email is then sent back to me through a gmail account. However, I still occasionally get stray spam from my .mac account. I've checked the headers and they are indeed coming directly from .mac. I don't know how to stop this. Any ideas?