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messaging systemA messaging system sends text messages from one user to one or more other users. Image attachments are also typically supported. This definition covers the components of an Internet-based email system. For text and instant messaging, see text messaging and instant messaging.
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.
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.