you would be surprised at the number of email
addresses that are floating around on your own machine.
You ever receive emails that have been forwarded on, and where the forwarder has neglected to remove all the other previous addressees?
Ever send emails to more than one person that "may" be forwarded on to another group?
Ever see how many Facebooks "friends" people have that only have the most tenuous and vague connection with each other.
Ever play the "6 degrees of Kevin Bacon"? Google it.
Email addresses do not just live in someone's address book. They are in cached copies of websites, documents, etc.