An email is an imposition on a person’s time. Writing to someone is saying I know you have a finite amount of time and attention today, and in life, and I’m going to take some of it.
Undue formality only wastes more of that time. And it wastes the writer’s time in worrying about exactly how formal to be.
Rarely does an email require more than three sentences. If it does, consider calling or getting together in person. Social interaction is healthy, and more time spent in the inbox isn’t likely to be.
James Hamblin, “How to Email”, The Atlantic (29 September 2016) [http://www.theatlantic.com/health/archive/2016/09/brevity-in-email/501986]