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 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.


The Free On-line Dictionary of Computing (30 December 2018):

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.


Bouvier's Law Dictionary, Revised 6th Ed (1856):

POSTMASTER, or DEPUTY POSTMASTER. An officer of the United States appointed
by the postmaster general to hold his office. during the, pleasure of the
former. Before entering on the duties of his office, he is required to give
bond with surety to be approved by the postmaster general. Act of 3d March,
1825, s. 3. 12. Every postmaster is required to keep an office in the place
for which he may be appointed; and it is his duty to receive and forward by
mail, without delay, all letters, papers, and packets as directed; to
receive the mails and deliver, at all reasonable hours, all letters, papers
and packets to the persons entitled thereto.
     3. In lieu of commissions allowed deputy postmasters by the 14th
section of the act of 3d March, 1845,.the postmaster general is authorized
by the act of March 1, 1847, s. 1, to allow, on the proceeds of their
respective offices, a commission not exceeding the following rates on the
amount received in any one year, or a due proportion thereof for less than a
year: On a sum not exceeding one hundred dollars, forty per cent; on a sum
over the first hundred and not exceeding four hundred dollars, thirty-three
and one-third per cent; on a sum over and above the first four hundred
dollars and not exceeding twenty-four hundred dollars, thirty per cent.; on
a sum over twenty-four hundred dollars, twelve and one-half per cent.; on
all sums arising from the postage on newspapers, magazines, and pamphlets,
fifty per cent.; on the amount of postages on letters or packets received
for distribution, seven per cent.: Provided, That all allowances,
commissions, or other emoluments, shall be subject to the provisions of the
forty-first section of the act which this is intended to amend; and that the
annual compensation therein limited shall be computed for the fiscal year
commencing on the first of July and ending the thirtieth of June each year,
and that for any period less than a year the restrictions contained in said
section shall be held to apply in a due proportion for such fractional
period: And, provided further, That the compensation to any,, deputy
postmaster under the foregoing provisions to be computed upon the receipt at
his office of a larger sum shall in no case fall short of the amount to
which he would be entitled under a smaller sum received at his office.
     4. By act of congress approved March 3, 1851, Sec. 6, it is enacted,
That to any postmaster whose commissions may be reduced below the amount
allowed at his office for the year ending the thirtieth day of June,
eighteen hundred and fifty-one, and whose labors may be increased, the
postmaster general shall be authorized, in his discretion, to allow such
additional commissions as be may deem just and proper Provided, That the
whole amount of commissions allowed such postmaster during any fiscal year,
shall not exceed by more than twenty per centum the amount of commissions at
such office for the year ending the thirtieth day of June, eighteen hundred
and fifty-one.
     5. Although not subject to all the, responsibilities of a common
carrier, yet a postmaster is liable for all losses and injuries occasioned
by his own default in office. 3 Wils. Rep. 443; Cowp. 754; 5 Burr. 2709; 1
Bell's Com. 468; 2 Kent. Com. 474; Story on Bailm. Sec. 463.
     6. Whether a postmaster is liable for the acts of his clerks or
servants seems not to be settled. 1 Bell's Com. 468, 9. In Pennsylvania it
has been decided that he is not responsible for their secret delinquencies,
though perhaps he is answerable for want of attention to the official
conduct of his subordinates. 8 Watts. R. 453. Vide Frank; Post Office.