This article is from the Copyright Law FAQ, by Terry Carroll with numerous contributions by others.
Almost certainly. They meet the requirement of being original works of
authorship fixed in a tangible medium of expression (see section 2.3).
They haven't been put in the public domain; generally, only an expiration
of copyright or an unambiguous declaration by an author is sufficient to
place a work into public domain.
However, at least with Usenet postings, there are two doctrines which
probably allow at least some copying: fair use (see sections 2.8 and 2.9)
and implied license.
Whether a particular use of a Usenet posting is a fair use is, as always,
a very fact-specific determination. However, it's probably safe to say
that it's a fair use if the use was not commercial in nature, the posting
was not an artistic or dramatic work (e.g.,, it was the writer's opinion,
or a declaration of facts, and not something like a poem or short story),
only as much of the posting was copied as was necessary (e.g., a short
quotation for purposes of criticism and comment), and there was little or
no impact on any market for the posting.
A similar argument can be made for quoting of private email messages. Of
course, revealing the contents of a private email message could run afoul
of any of a number of non-copyright laws: defamation, invasion of
privacy, and trade secrecy, to name a few. So even if you won't be
violating any copyright laws, you should consider other factors that may
expose you to legal liability before revealing a private message's
Proponents of the implied license idea point out that Usenet postings are
routinely copied and quoted, and anyone posting to Usenet is granting an
implied license for others to similarly copy or quote that posting, too.
It's not clear whether such implied license extends beyond Usenet, or
indeed, what "Usenet" really means (does it include, for example,
Internet mailing lists? Does it include netnews on CD-ROM?). If a
posting includes an express limitation on the right to copy or quote,
it's not at all certain whether the express limitation or the implied
license will control. No doubt it depends on the specific facts. For
example, was the limitation clearly visible to the person who did the
copying? Was the limitation placed such that it would be visible only
after the person who did the copying invested time and money to get the
posting, believing it to be without any limitation?
With private email messages, a copier who relies solely on the implied
license argument will probably lose, since it's hard to argue that by
sending the private message to a limited audience, the sender intended
for it to be copied and quoted. For email messages to a public mailing
list, the implied license argument may still be sound.
These theories are largely speculative, because there has been little
litigation to test them in the courts. As a practical matter, most
postings, with a small number of notable exceptions, are not registered
with the Copyright Office. As such, to prevail in court, the copyright
holder would need to show actual damages (see section 2.5). Since most
of these cases will result in little or no actual damage, no cases have
been be brought; it's simply too expensive to sue for negligible damages.