In my ongoing research on how to advise backscatter-spewing sites how to change their configurations to stop this kind of thing from happening, I recently ran across this very useful blog post from a guy named Alan Hardisty. In his post, he explains how to make sure you Microsoft Exchange 2007 or 2010 server doesn't send backscatter. Turns out, it's as simple as typing "Set-RecipientFilterConfig -RecipientValidationEnabled:$true" in the Exchange Management Shell.
Exchange admins, if you don't already have your server configured this way, please change it over! The rest of us email admins out here on the internet will thank you for it.