A simple puzzle

A simple puzzle can you solve it 

IT'S A 7 LETTER WORD.. 

IF WE REMOVE 1 LETTER FROM IT, IT REMAINS SAME. 

IF WE REMOVE 2 LETTERS FROM IT, IT REMAINS SAME. 

IF WE REMOVE 3 LETTERS FROM IT, IT REMAINS SAME. 

IF WE REMOVE ALL THE LETTERS FROM IT, STILL IT REMAINS SAME. 

WHAT IS IT ? 

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A POSTBOX