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Ideally, there are two solutions to this that I can think of right now... (neither solutions has been implemented yet with xDir)
1. Add an Image Verification to the send mail function of xDirectory. You know, like the ones you see everywhere these days.
2. Add a hook into the directory which requires the person sending the contact to enter a valid email address, then sends a conformation to their email with a link that will actually send the original inquiry out. This would get rid of any spammers trying to send span to your clients, without leaving a trail.
I know this doesnt help your site right now, but its just some ideas that could solve that nasty problem.
Cheers,