The spammers generally obliterate or otherwise fake both your address and the return address. The subject line is often meaningless gibberish to get past spam filters. What shows on the "document" you receive has nothing to do with who they sent it to, or where it came from.
Think of it as a "form letter" that's sent by a machine. It sends it according to its list of suckers potential clients, but it doesn't need to change the document for each addressee if it's all just junk anyway.
You can sometimes find some additional information about where it came from and how it was routed by doing a right click - properties on the header, but doing even this can be enough to trigger a "delivery confirm" that tells them you looked at it - and that puts you on the "live address" list for sale to other spammers.
Don't open, don't read, don't even let it sit "selected" long enough to "open," and don't bother trying to "investigate" (unless it's an isolated item that's particularly obnoxious AND personal) - just delete. And NEVER click on the "click here to be removed from our mailing list" button. That just confirms they've hit on a "live" address, and most of the slimeballs make their main income from selling your address if they can confirm it.