Mail transfer agent

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A mail transfer agent (MTA) (also called a mail transport agent, message transfer agent, smtpd (short for SMTP daemon), is a computer program or software agent that transfers electronic mail messages from one computer to another.

The term mail server is also used to mean a computer acting as an MTA, that is running the appropriate software. The term mail exchanger (MX), in the context of the Domain Name System formally refers to an IP address assigned to a device hosted by a mail server, and by extension also indicates the server itself.

[edit] Overview

An MTA receives messages from another MTA (relaying), a mail submission agent (MSA) that itself got the mail from a mail user agent (MUA), or directly from an MUA, thus acting as an MSA itself. The MTA works behind the scenes, while the user usually interacts with the MUA.

The delivery of e-mail to a user's mailbox typically takes place via a mail delivery agent (MDA); many MTAs have basic MDA functionality built in, but a dedicated MDA like procmail can provide more sophistication.

According to various surveys ^[1] ^[2] ^[3] ^[4] the most popular mail server software are sendmail, Postfix, Microsoft Exchange Server, Exim, IMail (by Ipswitch, Inc.), MDaemon by Alt-N Technologies, MailEnable, Merak Mail Server and qmail. ^[5] The MailChannels survey^[2] also found that many organizations use the services of e-mail security services such as Postini, MXLogic or Concentric Hosting to receive e-mail.