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Fully agree with Dirk - this is the proverbial Very Bad Idea. In addition to the significant issues this can cause with duplicate content, its also going to bork your Analytics. Could also be siphoning link equity if folks are linking to his URLs instead of yours. Especially if he later leaves and  decides to point his domain name to another (possibly competing?!) site. There's no reason (or justification) for the employee to be using URL masking here. If he/she isn't willing to put the work into making a site of their own, they should be made to do a proper 301-redirect of their domain name to the company site. Paul