Here's an excerpt from an article in the the Sept Communications of the ACM. Sorry, it's a subscription site so no linky, but just google for "Intel" and "Hamidi", and I'm sure there's tons of stuff about it. I seem to recall that Wired had a big article about it in the summer.

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Legally speaking: Unsolicited communications as trespass?
Intel v. Hamidi

[snip]

Ken Hamidi is a former Intel engineer who in 1995, with others, formed an organization known as Former and Current Employees of Intel (FACE-Intel) to share critical information about Intel's employment and personnel practices.1 On behalf of FACE-Intel, Hamidi sent six email messages to thousands of Intel employees over approximately a two-year period. These messages criticized Intel's practices and policies as abusive, encouraged recipients to join FACE-Intel, and suggested that Intel employees should find other employment. The email also informed recipients they could be removed from the FACE-Intel mailing list if they wished to do so, and Hamidi honored such requests.

Property rights are, of course, vitally important, but they are not the only issue in a trespass to chattel case. Trespass to chattel law doesn't just require unwanted use of another's property, but also some showing of harm. Authoritative sources on trespass to chattel law indicate the harm must be to the chattel itself.

The California Supreme Court majority opinion in Hamidi pointed out "[t]he consequential economic damage Intel claims to have suffered, i.e., the loss of productivity caused by employees reading and reacting to Hamidi's messages and company efforts to block the messages, is not an injury to the company's interest in its computers�which worked as intended and were unharmed by the communications�any more than the personal distress caused by reading an unpleasant letter would be an injury to the recipient's mailbox, or the loss of privacy caused by an intrusive telephone call would be an injury to the recipient's telephone equipment." Intel had, in the majority's view, not suffered a kind of injury trespass to chattel law was meant to redress.

But this was not the end of the California Supreme Court's inquiry. The court went on to consider whether trespass to chattel law ought to be extended to encompass the kinds of harms Intel claimed against Hamidi and whether the law should dispense with the harm requirement altogether in cyberspace trespass cases.

The Court was reluctant to do the former in part because it recognized that Intel employees were not distracted from their work because of the quantity of email sent by Hamidi, but rather because of the content of the email, that is, by the statements and opinions Hamidi expressed in these messages. The contents upset some employees and caused discussion among employees and between employees and their supervisors. The California Supreme Court majority observed that "Intel connected its email system to the Internet and permitted its employees to make use of this connection both for business and, to a reasonable extent, for their own purposes. In doing so, the company necessarily contemplated the employees' receipt of unsolicited as well as solicited communications from other employees and individuals. That some communications would, because of their contents, be unwelcome to Intel management was virtually inevitable."

The California Supreme Court recognized that extending trespass to chattel law as far as Intel wanted would create a precedent under which virtually any unsolicited communication could be challenged as a trespass insofar its contents were unwelcome by the recipient (an Intel employee) or the intermediate transmitter (Intel) by "fictionally recharacterizing the allegedly injurious effect of a communication's contents on recipients as an impairment to the device which transmitted the message." Fictions of this sort, the Hamidi majority concluded, "promise more confusion than clarity in the law."

The California Supreme Court was even more reluctant to rule that unsolicited communications could be challenged as trespass without any showing of harm. This would, as a law professor brief in support of Hamidi's appeal pointed out, mean that "each of the hundreds of millions of [Internet] users must get permission in advance from anyone with whom they might want to communicate and anyone who owns a server through which their message might travel." This would substantially reduce freedom of communications over the Internet�even unwanted linking could be a trespass according to this theory.

It'd be difficult to trace it back to a specific person (although in some cases it's a matter of how motivated someone is to find out). It's generally easier to figure it out from the syntax, diction, and content of the e-mail rather than any technical details about the e-mail itself.

is an email that is critical of a company illegal