However, E-mail is not, unless previously agreed, secret. You can even quote parts of it to demonstrate. Frankly, somebody who sues over an ordinary message might well get no damages, because the message has no commercial value, but if you want to stay strictly in the law, you should ask first. On the other hand, don't go nuts if somebody posts your E-mail. Copyright is still violated whether you charged money or not, only damages are affected by that.
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. The ownership of names is also from trademark law, so don't say somebody has a name copyrighted. 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.