inbox vs letterbox

inbox

noun
  • A container in which papers to be dealt with are put. 

  • The aggregate of items that demand one's attention or effort. 

  • An electronic folder serving the same purpose, but for electronic files, especially email. 

verb
  • To put (something) in someone's inbox. 

  • To communicate with (a person) by writing to their electronic inbox. 

letterbox

noun
  • A delivery point for mail, a box, compartment or slot for this purpose. 

  • A hidden container that holds a logbook and rubber stamp, found by following clues as a form of recreation. 

  • A rectangular hole or slot. 

  • A collection point for mail intended for onward delivery, a secure box or receptacle for this purpose. 

verb
  • To transfer a widescreen motion picture to home video formats while preserving the original aspect ratio, with the placing of black bars above and below the picture area. 

  • To hunt for letterboxes (containers with logbook and rubber stamp) by following clues. 

adj
  • Transferred to home video formats while preserving the original aspect ratio, having black bars above and below the picture area. 

How often have the words inbox and letterbox occurred in a corpus of books? (source: Google Ngram Viewer )