I agree that any message, private email or via a message board, is copyrighted material, and I understand the difference between ownership of copyright and ownership of material object. This is pretty much the same in my legislation and probably in many others.
Still I would like to point out that the term “copy” cannot be applied to electronic messages the same way it works for physical letters. I have clearly copied a letter when there is a second piece of paper with a facsimile on it. The same goes for scanning it into an electronic file. In fact, any manifestation of the letter that is different from the original is to be considered a copy.
That’s where the problem is: What is the “original” in regard to an electronic message? The moment the sender sends the message a copy is made (maybe even before, it may have been moved around in memory or saved to disk). Every relaying server copies (and maybe even “distributes”) the message. Every time I read the message (I assume I am allowed to do that), technically I copy it (but what I copy is already the copy of a copy of a copy …).
So there’s the other thing: distribution. I am allowed to put a letter I have received into a new envelope and send this new letter to someone else. I am allowed to show a letter I have received to guests who visit my house (unless the information in that letter is subject to more stringent confidentiality rules). I may not be allowed to use this letter for advertising in order to attract more guests, whether or not I demand admission fees.
Having said all that, I would argue that forwarding an email to another person will be fair use. Considering that posting messages to a message board usually expresses the sender’s desire that many people read this message, forwarding them to even more people may be fair use (unless confidentiality …).
Others, especially lawyers, may want to contradict or elaborate on these views. I just wanted to point out that forwarding an email is probably not plainly illegal.