I have a program from which I can send emails from ANY existing email, except for gmail and some more. There fore I don't know how this can actually happen. x_x
It's called cloaking I think. It's very easy, you can use PHP scripts and such for it. I'm sure the 'spammer' set it up that the email address the emails were coming from were the same it was going to, if that makes sense?
That goes to spam anyway. But I really though GMail disabled this, really. I know you can do it because I was faking someone's mail with that, but not for any evil purposes.
I could kill people who forward messages without deleting the sender. Just makes it easier for zombies to get your email... -_-"