Rule #12
Do you answer all questions, and pre-empt further ones?
  v7.0 Posted at 10/08/2012 12:31 PM by meganweeks

Often people will hit send on a reply and not realise that they have not answered one of the questions in the email. This creates more traffic that can be avoided. In the same vein, it's a good idea to supply any information the recipient may need, which will avoid another two emails.