Copyright Legitimacy vs. Defending Clients?

trmj asks: "I run a hosting company, and today I received an e-mail, claiming to be from an animation firm, about one of my clients hosting pirated information. As usual, before entertaining a claim of this sort, I researched the claim (there was a bit of legitimacy in it) and what the laws are governing this issue. After that, I replied in the appropriate manner, however, found that the original e-mail address was not valid, leading to the discovery that neither was the e-mail. As a hosting provider, how far should I go to protect the rights of my clients while upholding the law, and still verify every claim that comes in?"