I use ISPConfig on Debian. In the last 24 hours, I've been receiving a flood of returned mail as a spammer is faking one of our legitimate e-mail addresses as his FROM address. I've looked at /var/log/mail.log and it looks like Yahoo and other servers now think the originating address (and therefore my server!) is a spammer. I've looked at the tutorials on securing your mail server, but they mostly deal with spam and so far that's not been (too) much of a problem. What can I do to stop the flood and exclude the spammers from going through the server?