I believe there is a difference between knowing who an "anonymous" e-mailer is and wanting to avoid disclosing such information... and operating a service where finding out is not possible (at least without invasive means).
Its the old story, you cannot be subpoenaed to reveal information that you simply do not have. Whether or not you are permitted to operate a system so structured is another matter.
You *can* be subpoenaed for that information, you just aren't capable of providing it. -- A quote from Petro's Archives: ********************************************** Sometimes it is said that man can not be trusted with the government of himself. Can he, then, be trusted with the government of others? Or have we found angels in the forms of kings to govern him? Let history answer this question. -- Thomas Jefferson, 1st Inaugural