Three Sentences is based on the premise to reply to email as it were SMS and keep it short - three sentences is the maximum. I try to use this for all my emails - not just replies.
The Problem E-mail takes too long to respond to, resulting in continuous inbox overflow for those who receive a lot of it. The Solution Treat all email responses like SMS text messages, using a set number of letters per response. Since it’s too hard to count letters, we count sentences instead.
While I am writing my next book on how to handle email and other forms of modern communication I found this interesting article on the growing use of non-vacation auto-responders.
This might be the next step in email communication if we keep going as we did in the past.