A great way to provide outstanding service via email is to anticipate the next question and answer that one too. For example, a customer might email to ask if you have a particular item in stock. A good customer service professional would reply quickly with the answer.
A great customer service professional would reply quickly with the answer along with ordering information (such as a phone number or website) if the product is in stock or suggested alternatives if the product is not in stock. This avoids additional emails and makes it easier for your customer.