- Mail transfer agent
A mail transfer agent (MTA) (also called a "mail transport agent", "message transfer agent", or "smtpd" (short for
SMTPdaemon)), is a computer programor software agentthat transfers electronic mailmessages from one computer to another.
The term mail server is also used to mean a
computeracting as an MTA that is running the appropriate software. The term mail exchanger (MX), in the context of the Domain Name Systemformally refers to an IP addressassigned to a device hosting a "mail server", and by extension also indicates the server itself.
An MTA receives mail from another MTA (relaying) or from a
mail user agent(MUA). The MTA works behind the scenes, while the user usually interacts with the MUA. Every time an MTA receives an e-mail, it will add a "Received:" trace header field to the top of the message. In this way, there is a record of which MTAs handled the e-mail and in which order. Upon final delivery, the "Return-Path:" header will also be added to record the return path.
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 procmailcan provide more sophisticated functionality.
According to one survey,
sendmail, Microsoft Exchange Server, Postfix, and Eximtogether control over 85% of market share for SMTP service.Fact|date=September 2008
Another survey suggests a more balanced playing field, though it included hosted e-mail services such as
Postini.MailChannels [http://www.oreillynet.com/pub/a/sysadmin/2007/01/05/fingerprinting-mail-servers.html Mail Server Survey] , 2006]
List of mail servers
Comparison of mail servers
Mail user agent
Mail delivery agent
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