Sounds like you have qmail at the root. I ran that with spamassassin, barracuda and some other custom rules for years. Didn't add on the auto response you have described, but really liked it back in the day.
Did I guess kinda close to what you have running?
I'd think the annoying part would be that you've forced the work to prove they are human back on the sender. Might be a good way to go though given how much spam there is.