These days, almost all things are copyrighted the moment they are written, and no copyright notice is required.
Copyright is still violated whether you charged money or not, only damages are affected by that.
Postings to the net are not granted to the public domain, and don't grant you any permission to do further copying except perhaps the sort of copying the poster might have expected in the ordinary flow of the net.
Fair use is a complex doctrine meant to allow certain valuable social purposes. Ask yourself why you are republishing what you are posting and why you couldn't have just rewritten it in your own words.
Copyright is not lost because you don't defend it; that's a concept from trademark law. The ownership of names is also from trademark law, so don't say somebody has a name copyrighted.
Fan fiction and other work derived from copyrighted works is a copyright violation.
Copyright law is mostly civil law where the special rights of criminal defendants you hear so much about don't apply. Watch out, however, as new laws are moving copyright violation into the criminal realm.
Don't rationalize that you are helping the copyright holder; often it's not that hard to ask permission.
Posting E-mail is technically a violation, but revealing facts from E-mail you got isn't, and for almost all typical E-mail, nobody could wring any damages from you for posting it. The law doesn't do much to protect works with no commercial value.
