Probably nothing bad would happen to you if you did. But it's better Netiquette to ask for permission. Postings from individuals aren't usually carefully thought out; they represent the individual's feelings and opinions at a given moment. Think how you'd feel if a comment you made in a bar after a few beers turned up in the paper the next day.

This would be extremely bad Netiquette. Unfortunately, it's completely possible. In fact, in one unfortunate case, it's happened. Newspapers around the country are plugging into the net. Their reporters go "net trawling" for stories. The reporters hang out in smoky dens, mailing lists that only accept professional journalists, and exchange leads. While conscientious reporters try to contact their sources before running a story, they don't have to, and sometimes they don't. If they use your message as material, they're protected by the First Amendment.

Q. Can I use email a friend sent me to bolster my position in an argument I'm pursuing in a discussion group?

Don't do it without permission. Posting private email to the net without the author's OK is considered very poor Netiquette.