The Collaborative International Dictionary of English v.0.48:

Postmaster \Post"mas`ter\, n.
   1. One who has charge of a station for the accommodation of
      travelers; one who supplies post horses.
      [1913 Webster]

   2. One who has charge of a post office, and the distribution
      and forwarding of mails.
      [1913 Webster]

Jargon File (4.4.4, 14 Aug 2003):

postmaster
 n.

   The email contact and maintenance person at a site connected to the
   network. 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.