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

Administration Management Domain
ADMD

    (ADMD) An X.400 Message Handling System
   public service carrier.  The ADMDs in all countries
   worldwide together provide the X.400 backbone.  Examples:
   MCImail and ATTmail in the U.S., British Telecom
   Gold400mail in the U.K.

   See also PRMD.

   [RFC 1208].

   (1997-05-07)