As The Web Turns: The Ten Commandments of E-Mail

1. Thou shalt include a clear and specific subject line.
2. Thou shalt edit any quoted text down to the minimum thou needest.
3. Thou shalt read thine own message thrice before thou sendest it.
4. Thou shalt ponder how thy recipient might react to thy message.
5. Thou shalt check thy spelling and thy grammar.
6. Thou shalt not curse, flame, spam or USE ALL CAPS.
7. Thou shalt not forward any chain letter.
8. Thou shalt not use e-mail for any illegal or unethical purpose.
9. Thou shalt not rely on the privacy of e-mail, especially from work.
10. When in doubt, save thy message overnight and reread it in the light of the dawn.

And, here's the "Golden Rule" of E-Mail:
That which thou findest hateful to receive, sendest thou not unto others.