From Entrepreneur.com:

“High five! The average worker spends 28 percent of the workweek reading and answering e-mail, according to a study by McKinsey Global Institute. Keeping e-mails brief and to the point can help you reclaim some of this time. Try limiting your messages to five sentences — it not only will increase productivity, but also will improve your chances of getting a reply. For more: entrepreneur.com/5sentences.”