The functional model of X.400/88 differs from X.400/84 in having three new objects: The Access Unit (AU), the Physical Delivery Access Unit (PDAU), and Message Store (MS). The two AU objects represent gateways to other telematics services and to non-compliant X.400 systems. The Message Store is used for storing messages, i.e. archiving
.X.400/88 has consistently followed the OSI recommendations in its standardization and is therefore, in contrast to X.400/84, only located on the application layer. This means that the services of the Message Transfer Service (MTS) and the associated protocol modules are re-described as Application Service Elements (ASE). In addition to the application service elements Application Control Service Element (ACSE), Remote Operation Service Element (ROSE) and Reliable Transfer Service Element (RTSE), the five MHS-specific service elements Message Administration Service Element (MASE), Message Delivery Service Element (MDSE), Message Retrieval Service Element (MRSE), Message Submission Service Element (MSSE) and Message Transfer Service
Element (MTSE) are specified.
's access to the message store. The P protocols P1, P3, and P7 are used for the X.400-specific application service elements (MASE, MDSE, MSSE, MTSE) as well as for the application-independent service elements ACSE, RTSE, and ROSE.Also new to X.400/88 is communication with user groups
, which ispossible on distribution lists (DL). Messages sent to distribution lists are sent directly to all recipients of
the distribution lists.X.400/88 also differs significantly from the previous version in security-related aspects
The 88 version defines a model for the secure transmission of messages that consists of special security services. These can be exploited by using security elements in the implementation. The security in X.400/88 takes into account all unauthorized access and makes use of appropriate cryptological encryption techniques
. With the development of X.500, the naming and addressing concept in the 88 version was also changed. The revised recommendation defines how a Message Handling System (MHS) should use a directory service and what information should be stored for the MHS applications.