X400

X.400 refers to a family of ITU-T recommendations that define an international standard for electronic mail as part of the OSI protocol suite. It was developed to enable interoperable message exchange between heterogeneous networks through a store-and-forward messaging model. The X.400 architecture introduces distinct components and roles, including User Agents (UAs) for end users, Message Stores (mailboxes) for persistent storage, and Message Transfer Systems (MTS) responsible for routing and delivery. The system supports submission, relay, and delivery stages and provides mechanisms such as delivery reports and return receipts to indicate message status.

A defining feature of X.400 is its structured addressing and routing model. X.400 addresses can be hierarchical and domain-based, allowing messages to be routed across administrative boundaries. Address resolution and directory lookups are commonly integrated with directory services such as X.500, enabling naming and routing information to be discovered across heterogeneous networks. The standard emphasizes interoperability with other OSI application services and with directory-based naming to support global messaging.

Security and policy are addressed within X.400 through defined services for privacy, authentication, and message integrity. The framework supports encryption and other security mechanisms appropriate for regulated environments, with provisions for trusted delivery and non-repudiation where required.

History and legacy: work on X.400 began in the 1980s as part of the OSI initiative, and it saw substantial deployment in government, financial, and telecommunications networks during the 1990s. However, the rapid adoption of SMTP-based Internet mail and the complexity of X.400 led to a decline in widespread usage. Today, X.400 deployments are largely maintained in legacy systems or specialized sectors, often connected to SMTP gateways to enable interoperability with the broader Internet messaging ecosystem.