1. Understand how your email system and network are set up, and just
who can see what.
2. Don't do stupid things.
Unfortunately, etiquette books can't eliminate human carelessness. So,
just as the only 100% effective form of birth control is abstinence, you
could prevent breaches of privacy by refusing to use email. For most of
us, however, neither remedy holds much attraction. A fallback remedy
is to follow your grandmother's advice: Never put anything in writing
that you wouldn't mind seeing on the front page of the
New York Times.
Of course, the rule of etiquette is the same for email as for old-fashioned letters: Never, ever snoop in another person's papers.