Solve the Puzzle..........

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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POSTBOX (he! he! he! he!... )