One of the clearest ways to determine if an email is the best way to communicate with the recipient is to ask yourself if it focuses on who, what, when, or where. If, however, your email delves into why or opinion, you may want to think twice about sending it. The recipient only has your words on the page to decipher your meaning, intent, and tone. Without visual clues or the sound of your voice your message can easily be misunderstood.
- Peter Post, Emily Post's The Etiquette Advantage in Business