Source : Webster's Revised Unabridged Dictionary (1913)

Postmaster \Post"mas`ter\, n.
   1. One who has charge of a station for the accommodation of
      travelers; one who supplies post horses.

   2. One who has charge of a post office, and the distribution
      and forwarding of mails.

Source : WordNet®

postmaster
     n : the person in charge of a post office

Source : Free On-Line Dictionary of Computing

postmaster
     
        The {electronic mail} contact and maintenance person at a site
        connected to the {Internet} or {UUCPNET}.  Often, but not
        always, the same as the {admin}.  The Internet standard for
        electronic mail ({RFC} 822) requires each machine to have a
        "postmaster" address; usually it is aliased to this person.
        See also {webmaster}.