What happens is that some spammers will steal an email address and use it as a the return address on their emailings.
This doesn't require them to have access to the server on which the email address resides, or to the ISP's network, or to your computer. Just as anybody can send a letter via USPS using a fake or stolen return address, the same can be done with email.
That makes me feel MUCH better! Making sure I understand you correctly. A spammer stole my email address and is sending out emails from their computer using it? There is no virus on my computer? They don't have access to my computer, just my email address?
Still isn't it odd that they are targeting oil and gas stocks?
By the way my old isp is earthlink.