Message Handling System

(messaging, standard)

(MHS) The standard defined by ITU-T as X.400 and by ISO as Message-Oriented Text Interchange Standard (MOTIS). MHS is the X.400 family of services and protocols that provides the functions for global electronic mail transfer among local mail systems and MTAs.

It is used by CompuServe, among others.

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messaging system

A system that sends text messages from one user to one or more other users. Email, chat and texting are the major messaging systems. Images are also included.

THIS IS ABOUT EMAIL
The following definition covers the Internet-based email system. For other messaging systems, see text messaging and chat.

Mail User Agent (Client)
The mail user agent (MUA or UA) is the client email program, such as Outlook, Eudora or Mac Mail, which is used to compose, send and receive messages.

Message Transfer Agent (Server)
The message transfer agent (MTA), or "mail transfer agent," is a mail server that forwards messages from user agents (MUAs) or delivers mail to its own message store (MS) for local recipients. Sendmail and Microsoft Exchange are the most widely used MTAs, and in a large enterprise, there may be several MTAs deployed.

Message Store
The message store (MS) holds the mail until it is selectively retrieved and deleted by an access server. A local delivery agent (LDA) writes the messages from the MTA to the message store, and the protocols used to retrieve the mail are POP and IMAP (see POP3 and IMAP4).

The Internet's SMTP
Internet email is based on the SMTP protocol. Prior to the Internet's enormous growth in the late 1990s, numerous proprietary messaging systems were widely used, including cc:Mail, Microsoft Mail, PROFS and DISOSS. See SMTP, messaging middleware, text messaging and instant messaging.

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