Trev's comments are pretty much spot on, but I'll clarify a little*.
The "thou shalt not"s a the bottom of HJ's mail have no contractual standing on their own as they do not consitute an agreement between the parties. However, the admonition not to republish is a clear indication of intent of authorization. Any created work which is substantially original is automatically the copyright of its author, and copyright includes the right to authorize or forbid usage by others.
All other things being equal, the notice not to distribute does not affect the legal standing of a copyright material, it merely makes it harder to argue your way out of the already existing protection that exists under national and international copyright laws. One might argue that posting on a.org consituted an "implicit agreement" for the mail to be reposted as it is a forum in which, generally, such activites are common. Obviously if the posting comes with a notice that clearly states the message is not to be copied, this argument holds no water.
Contrary to the belief of many, the mere fact that a work is published in a public forum does NOT imply license to those reading it to copy or redistribute the work. In copyright terms, such presentation of the material is considered "public display", and is entirely subject to copyright. The fact that the author has posted it in a particular forum clearly indicates that implicit permission is granted to the owners of that forum to reproduce that material in that instance. No such authorization is granted to any visitors to the forum, however. This is pretty obvious if you think about the parallels - clearly if a movie poster is placed in a public location, people passing that poster are not granted the right to reproduce the copyrighted artwork, for instance. A book in a library can be read by anyone, but nobody is granted any right to republish the book. An article published on nytimes.com is just as protected by copyright as if it had been published in the newsprint edition.
As far as I'm aware (copyright protection is so horrendously complex that IANAL does not cover it - lawyers can argue for years about this stuff), if the terms and conditions of a public forum require the waiving of copyright by posters, then posting to the forum would constitute a contractual acceptance on the part of the author that the copyright had been waived (I haven't checked to see whether a.org has such a clause). If there is no such clause in the terms, make no mistake, the original author retains complete ownership and control of copyright.
There are circumstances under which reposting is allowed - for example if the posting might not be considered "substantially original" (for example, a posting that just says "ROFL!" does not count). Fair use laws generally allow for a reasonable proportion of the material to be reproduced under certain circumstances, but not the reproduction of the entire work (except for certain private, archival or academic purposes). The copyright also applies only to the actual composition, not to the informational content of that composition, and therefore the original content may be paraphrased and small parts of it quoted without authorization of the copyright holder.
In an instance where a posting constitutes a "work" under copyright law, and that post is reposted without permission, the individual reposting has breached the author's copyright, and can be held legally liable. What's more, the owner / operator of the forum to which the work is reposted may also be held liable. For example, if HJ posted something here which was "substantially original" and therefore constituted a copyrighted work, and somebody reposted it on anotheramigaboard.com, that person would have performed "direct infringement" of HJ's copyright. If HJ then contacted the owner of anotheramigaboard.com to ask him to remove the post and said owner refused, he or she would have performed at the least "contributory infringement" (see RTC vs Netcom for the classic case on sysop liability).
Now most people aren't going to actually do anything about it, and given that generally there is no financial damage caused by such reposting, there would be little advantage to the original poster in pursuing the copyright infringer, unless they felt like making a point. However this should not be taken as a given. For example, HJ might sneakily chose to register his copyright of a forum posting with the US copyright office, in which case there are statutory fines involved - minimum $15,000 if a US citizen then infringes his rights (such as by reposting), if I remember correctly.
In short, if someone posts something that has a "do not repost" notice, heed that notice. If it's interesting enough to reproduce elsewhere, that suggests it's probably a "substantially original" work, and is subject to copyright. Unless you know for sure that the poster has in other ways granted license to copy (such as by agreeing to waive copyright), you are almost certainly breaking copyright laws. The best guideline when it comes to copyright issues is "If in doubt, don't."
* Apologies, but if this doesn't look like just "a little" to you, you've never delved into copyright legislation!
--- IRONIC DISCLAIMER---
IANAL and might be wrong, but I'm pretty sure I'm not - I've spent a fair bit of time reading up on copyright laws and case studies. I've generally quoted the specifics & terminology of US copyright law (this is a US site), but my generalisations hold for most international copyright laws. For the record, I hereby grant a universal and non-exclusive right to everyone to reproduce this work, under the single condition that it be reproduced unattributed (i.e. you can do what the heck you like with it - I just don't want the blame) ;-). Next time someone comes up with the old "it's in a public forum, it's not copyright" line, feel free to repost.
