law: Indian Evidence Act, 1872 
  16. Existence of course of business when relevant 
  When there is a
  
  question whether a particular act was done, the existence of any course of business,
  
  according to which it naturally would have been done, is a relevant fact.
  Illustrations
  (a) The question is,
  
  whether a particular letter was dispatched.
  The facts that it was the
  
  ordinary course of business for all letters put in a certain place to be
  
  carried to the post, and that that particular letter was put in that place are
  
  relevant.
  (b) The question is,
  
  whether a particular letter reached A. The facts that it was posted in due
  
  course, and was not returned through the Dead Letter Office, are relevant.