Source : Webster's Revised Unabridged Dictionary (1913)

Postal \Post"al\, a. [Cf. F. postal.]
   Belonging to the post office or mail service; as, postal
   arrangements; postal authorities.

   {Postal card}, or {Post card}, a card sold by the government
      for transmission through the mails, at a lower rate of
      postage than a sealed letter. The message is written on
      one side of the card, and the direction on the other.

   {Postal money order}. See {Money order}, under {Money}.

   {Postal note}, an order payable to bearer, for a sum of money
      (in the United States less than five dollars under
      existing law), issued from one post office and payable at
      another specified office.

   {Postal Union}, a union for postal purposes entered into by
      the most important powers, or governments, which have
      agreed to transport mail matter through their several
      territories at a stipulated rate.

Source : WordNet®

postal
     adj : of or relating to the system for delivering mail; "postal
           delivery"