Armour (4)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">armour</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I know that mail is a type of armour)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for mail that I've seen before include "Armour; posted items" , "Flexible armour" , "Dispatch - armour" , "Letters and parcels" , "protective cover" .)