Parapunter,
responding is a poor idea since it confirms that a computer within that pool of addresses is live and responding and therefore ratchets up the likelihood of further spam to that ISP
Rubbish.
Read up on dictionary attacks.
That's the sort of technique used to "get" the majority of email addresses to send to. The rest comes up from email addresses harvested by whatever means.
Sending 1 email or 6,000,000,000 emails costs the disgusting people exactly the same thing.....i.e. pretty much nothing, especially if using botnets..... so what makes you think they're bothering to go through the sort of commercial database cleansing processes that a normal company would ? Remember, for example, that they are using one-off addresses to send, so whether or not they get a deluge of bouncebacks is irrelevant.