RCVD_BAD_ID - Spam Scoring

[MGSteve]

Hi, a client just sent an email from his work address to his home address (home address is using Zimbra) and for some reason it got a whacking great spam score.

X-Spam-Status: Yes, score=7.643 tagged_above=-10 required=6.6
tests=[AWL=-1.222, BAYES_99=3.5, DNS_FROM_OPENWHOIS=1.13, HTML_MESSAGE=0.001, MIME_QP_LONG_LINE=1.396, RCVD_BAD_ID=2.837]

Now, BAYES_99 I understand from reading around is scored a bit high by default, so I can reduce that down a bit. DNS_FROM_OPENWHOIS I understand is no longer used in later builds, so I'll zero that out, which leaves RCVD_BAD_ID which a google shows two results for, both emails!

What is this RCVD_BAD_ID and what triggered it?