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]

WordNet (r) 3.0 (2006):

postmaster
    n 1: the person in charge of a post office

The Free On-line Dictionary of Computing (19 January 2023):

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.


The Jargon File (version 4.4.7, 29 Dec 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.