We did something similar a couple months ago (I provided details about it on the mailing list so it should be in the list archives).
We did as you did by explaining the situation and provided the links to report the scam to the proper authorities, but we took it a step further. We explained that registry domain locking can protect them from these scams *and* we automatically locked all of our customers domains. We provided instructions on how they could easily unlock or lock them again via our management interface. We too received many positive comments on the mailing.. Let me say it again. Registry domain locking will automatically protect your customers from these scams.
