They have time to think, go deeper. Treat e-mail as private correspondence. Let them know if it's urgent. Respond rapidly, but when you've had time to consider your response. Don't worry about typos or mispellings.
Meaning is most important.
Don't assume the person knows what you're referring to. Instead of giving a one-word answer, repeat the question. Beware of sarcastic humor, which can easily be misinterpreted. Use short paragraphs, and make messages fit on one screen. The machine that hooks your computer to telephone lines, enabling it to send and receive messages.