Say please and thank you. Be careful to give the benefit of the doubt. We live in a coarse and informal age, and the simple rules of etiquette set us apart as Christians. Love is in the details. Manners are the oil in the gears of communication.
Promptness counts in email.
Make an effort to answer your email when you open it if you can, unless your answer requires thought and reflection. Your answer doesn't have to be long and drawn out, but you should answer quickly out of respect for the other person's dignity. Clarity is the quality of making your writing understandable to the person reading it, and simplicity helps assure clarity. It's available in every bookstore. It's a short, sweet, beautiful little book.