Contents of a letter box (4)

I believe the answer is:

mail

(Other definitions for mail that I've seen before include "letter?" , "Form of armour; post" , "Dispatch - armour" , "protective cover" , "some self-defence material" .)

I've seen this clue in the USA Today.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.