HP 3000 Manuals

Managing HP X.400 Administrator's Guide

What is X.400? 

X.400 is a set of standards defined by the International Telegraph and
Telephone Consultative Committee (CCITT). This set of standards, based on
the Open System Interconnection 
(OSI) model, describe the basic components and service elements needed to
exchange messages between electronic mail systems and users.

MPE/iX 5.0 Documentation