law: Indian Post Office Act, 1898 
  67. Penalty for detaining mails or opening mail bag 
  Whoever, except under
  
  the authority of this Act 21[or of any other Act for the time being
  
  in force] or in obedience to the order in writing of the Central Government or
  
  the direction of a competent Court, detains the mails or any postal article in
  
  course of transmission by post, or on any pretence opens a mail bag in course
  
  of transmission by post, shall be punishable with fine which may extend to two
  
  hundred rupees :
  Provided that nothing in this
  
  section shall prevent the detention of an officer of the Post Office carrying
  
  the mails or any postal article in course of transmission by post, on a charge
  
  of having committed an offence declared to be cognizable by the Code of
  
  Criminal Procedure, 189831, or any other law for the time being in
  
  force.