The way this question is worded is confusing in it's intent. It is asking if it's illegal for someone (anyone) to post online, a copy of an email without asking permission from the person answering. Taken in that context, the answer would be no. Feel free, I'm fine with it, thanks for asking. Have a cookie. Clean up the question a bit so as to avoid an answerer making assumptions please.

Pretty much everything is copyrighted, from e-mails to unpublished works to published novels. This http://www.whatiscopyright.org/ contains a good general explanation of copyright protections. Only the author of the work, or someone acting with the authority or permission of the author, may reproduce any work, including e-mails. Copyright protection exists from the moment the work is put "in fixed form", so as soon as you write it, type it, or record it, it's yours and no one can legally copy it without your permission. The key word there, however, is "legally". If someone copies a work without the permission of the author, the only real remedy is through the legal system. Aside from proving the authorship of the piece, and therefore determining who the work belongs to, there may be little benefit to litigation. Recovering damages, especially for something like an e-mail, is highly unlikely. Since e-mails are generally not income-producing works, a court is unlikely to grant any monetary compensation for such an infringement, and violation of copyright laws rarely lands anyone in jail. So yes, it is illegal, but there isn't a lot you can do about it short of a simple "cease and desist" letter. Further links: US Government Copyright Office http://www.copyright.gov 10 Big Myths about copyright explained http://www.templetons.com/brad/copymyths.html