Professional Reading: Does Forwarding A Personal Email Violate the Sender's Privacy?

Yes, according to Arkansas law prof Ned Snow. See his A Copyright Conundrum: Protecting Email Privacy. Here's the abstract:

The practice of email forwarding deprives email senders of privacy. Expression meant for only a specific recipient often finds its way into myriad inboxes or onto a public website, exposed for all to see. Simply by clicking the “forward” button, email recipients routinely strip email senders of expressive privacy. The common law condemns such conduct. Beginning over two-hundred-fifty years ago, courts recognized that authors of personal correspondence hold property rights in their expression. Under common-law copyright, authors held a right to control whether their correspondence was published to third parties. This common-law protection of private expression was nearly absolute, immune from any defense of “fair use.” Accordingly, the routine practice of email forwarding would violate principles of common-law copyright.

The issue of whether common-law copyright today protects email expression turns on whether the Federal Copyright Act preempts common-law copyright. The Copyright Act includes a fair-use defense to infringing uses of unpublished works, and that defense likely applies to email forwarding. A strong argument exists, however, that the Act does not preempt common-law rights of expression which protect privacy. Federal preemption extends only as far as the Constitution permits. According to the Copyright Clause in the Constitution, federal property rights in expression are limited to rights that forward a utilitarian end. Rights of privacy do not forward a utilitarian end. The Act should therefore be construed as not preempting common-law copyright's protection of privacy. Email forwarding must yield to privacy protection.