OOO should be configured to respond only to messages that have made it past the spam filter.
Instead, what's happening is this:
1. Spammer sends spam;
2. OOO responds to the spam
3. Spam is caught by the spam filter.
Surely 2 and 3 should be reversed, otherwise Dreamhost is broadcasting active/responsive email addresses to spammers and botnets. I hope I don't have to explain why this is a bad thing...