Just FYI:

Spamhalter for Mercury trying to detect this kind of faked sender's address. It is mail from local e-mail adress, but it was received from foreign server by unauthorized SMTP. So, messages with faked sender is marked as SPAM automaticly, regardless on message content.

I realized that it makes sense that a spammer would assume that your own address would probably be on a whitelist, but they wouldn't have an inkling of an idea about anyone else. Therefore, they can't spoof just anyone's address... they have to spoof your own. The auto-whitelisting concept only breaks down (loses it's robustness) for your own address.