Verifying connecting hostnames in DNS is a very poor anti-spam measure, precisely because it results in situations like this. Domino supports better mechanisms.
There is a RedBook called the Domino 6 Anti-Spam Survival guide that you can download for free at http://www.ibm.com/redbooks
. There is also a RedBook called Security Considerations in Lotus Notes and Domino 7 with a chapter dedicated to spam that covers the enahncements in 6.5 and 7. Also, there is a huge amount of inforomation on Chris Linfoot's blog at http://chris-linfoot.net