A reverse DNS entry is meant to keep email cloaking spammers from sending emails that 'appear' to come from your domain. Ever get a spam from your own email address? In order to prevent spammers from getting your server on an ISP's or even global spam blacklist, your host can add the reverse DNS for your domains, but that's something that is typically not automatic. If your host doesn't know what a reverse DNS entry is and doesn't recognize that particular error, RUN! AOL loves to bounce emails back for the lack of a reverse DNS entry. I've found it simpler to just not email anyone who uses AOL.
