In X.400 messaging, a message-handling system (MHS) that is managed by a registered private agency. Administrative Management Domains (ADMDs) are usually large public telecommunications carriers such as MCI WorldCom and AT&T.
This is in contrast to a Private Management Domain (PRMD), which represents a message-handling system managed by a private corporation. In the X.400 world, PRMDs cannot communicate directly with each other; instead, they must communicate through ADMDs.
All the ADMDs in the world therefore form a kind of messaging backbone for global X.400 communication.