I get thousands of spams each month. It's extremely rare that any have a valid return address. The exceptions are politicians and Buy.com.
The politicians I understand. Mercifully they have consistent email addresses; a simple entry in my spamcop blacklist takes care of them.
Buy.com I don't get. I was getting one spam a week or so from them, so I clicked on the unsubscribe link. After that I got 3 spams a day - until I finally roused myself to blacklist them.
What the heck were they thinking?
Update 1/18/07: Looks like there were two spam streams, one direct and one through google checkout. My blacklist stopped the direct one, and google checkout supports disabling the mail address they used. So they had me twice. They're toast as far as I'm concerned.