airmail vs post office

airmail

noun
  • The system of conveying mail using aircraft. 

  • The items of mail so carried. 

verb
  • To send mail by air. 

  • To (unintentionally) throw the ball well over a fielder's head where that fielder is unable to make a play on the ball. 

post office

noun
  • An organisation that delivers letters. 

  • A party game involving the exchange of kisses. 

  • A place (building, office, shop, or counter) concerned with the business of delivering letters, post or mail and selling stamps, etc. 

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