Others may be able to read your message, even if it's over someone's shoulder. Follow the reporting hierarchy in your office. Sending messages to the company president may be technically possible but tactically unwise. Use humor and sarcasm carefully. Written communications suffers from the absence of gestures and facial expressions.
Nothing is more distracting than mispelled e-mail messages.
It conveys an attitude of carelessness and lack of thought. Do not type in all-caps. That is the equivalent of shouting in e-mail. Keep paragraphs and messages short and to the point. When responding to a message, include that message as a reference.