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messaging systemAlthough the term may refer to text or instant messaging, it generally refers to an electronic mail delivery system made up of the following functional components, which may be packaged together or independently.
Mail User Agent
The mail user agent (MUA or UA) is the client email program, such as Outlook, Eudora, Mac Mail or Mozilla Thunderbird, which is used to compose, send and receive messages.
Message Transfer Agent
The message transfer agent (MTA), also called the "mail transfer agent," either forwards the message from user agents (MUAs) and other mail servers (MTAs) or delivers it to its own message store (MS) if the recipient is local. Sendmail and Microsoft Exchange are the most widely used MTAs on the Internet. In a large enterprise, there may be several MTA servers (mail servers) dedicated to Internet email while others support internal email.
The message store (MS) holds the mail until it is selectively retrieved and deleted by an access server. In the Internet world, a local delivery agent (LDA) writes the messages from the MTA to the message store, and the primary mailbox access protocols used to retrieve the mail are POP and IMAP (see POP3 and IMAP4).
The Internet's SMTP
Internet email, the most ubiquitous messaging system in the world, 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.