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... than all
Message Handling Systems
Electronic mail (e-mail) represents one of the most successful classes of network applications currently enjoyed by many users. Early e-mail systems were network dependent, and their use was limited to the private networks of individual organizations. The CCITT X.400 and the ISO 10021 series of standards for Message Handling Systems (MHS) have paved the way for standardized and widely-available e-mail services.
At the center of MHS is a Message Transfer System (MTS) which handles the delivery of messages. The system consists of the following components:
* A Message Transfer Agent (MTA) is responsible for the routing of complete e-mail messages (called envelopes) through the MTS. MTAs handle envelopes in a store-and-forward fashion.
* A User Agent (UA) manages a user's mailbox. It enables the user to create, submit, and receive messages. The UA may serve an application or provide a user interface for direct interaction. UAs typically run on multi-user systems (e.g., mainframes).
* A Message Store (MS) acts on behalf of a UA running on a system which may not be available on a continuous basis (e.g., personal computers). MSs are typically used within a LAN environment, serving a collection of personal computers.
UAs and MSs make it possible for users to receive messages when they are not personally present, and while even their terminal or personal computer is not switched on. They simply store the messages and notify the user at the earliest opportunity.
Each user has its own UA. Furthermore, each user is identified by a unique address which is structured in a hierarchical fashion (similar to a postal address). The address structure reflects the division of MTAs into domains. Domains exist at various level of abstraction: country, organization, unit, etc.
It consists of contents and addressing information. The contents consists of two parts: heading and body. A heading is comprised of fields such as recipients' addresses, addresses to which the message should be copied, originator's address, subject, etc. Some of the heading information is provided by the user (e.g., recipients' addresses and subject), others are automatically inserted by the UA (e.g., date and originator's address). The body contains the message information itself. It may consist of more than one part, each of which may be of a different type (e.g., text, digitized voice, digitized image).
An envelope is constructed by a UA by deriving envelope addressing information from the heading and adding it to the contents. MTAs only deal with envelopes and their addressing. They have no interest in the contents of an envelope. Each receiving MTA looks at the addressing of an envelope, records on it the last leg of the route so far, time stamps it, and hands it over to the next MTA. The envelope therefore bears a complete trace of its route through the network of MTAs.
MHS uses four protocols (P1, P2, P3, and P7) to provides two types of service:
* The Message Transfer (MT) service supports the handling of envelopes. This service operates between MTAs (P1 protocol), between UAs and MTA/MSs (P3 protocol), and between UAs and MSs (P7 protocol).
* The Inter-Personal Messaging (IPM) service supports the handling of the contents of envelopes. This service operates between UAs (P2 protocol). IPM depends on the MT service for its operation.