Over the past several weeks, my mail server ended up in the CBL/XBL block lists, with some domains refusing to accept my email. I don't have an open relay or an open proxy, and my anti-virus and patches are up-to-date. I wonder if this would be caused by my autoresponders to the postmaster@domain.com address, which, when replying to spam emails with false addresses, might be causing the server receiving my reply to think that my auto-response is spam.