My wife was telling me about how a lot of these things work.
Bots go out to sites and look for things like "Contact Us", "Feedback" etc. and then take text from that and stuff it into an email. Where do they get the email addresses to mail to? They mine the same site for as many email addresses they can find. Then, they email those addresses with the text they pulled from the site so that the email "looks" like it should "go" with the site. They attach their worm or virus and hope you open it!
There are many, many, many other schemes that do this too but I thought I'd put a bit of info out there for how this happens. The email doesn't come from GT40s.com or anything like that, but it'll have a line identifying GT40s.com somewhere in hopes to fool you.
Ron