A mail transfer agent or MTA (also called a mail transport agent, mail server, or a mail exchanger in the context of the Domain Name System) is a computer program or software agent that transfers electronic mail messages from one computer to another. The domain name system (DNS) stores and associates many types of information with domain names, but most importantly, it translates domain names (computer hostnames) to IP addresses. ... A computer program is a collection of instructions that describe a task, or set of tasks, to be carried out by a computer. ... In computer science, a software agent is an abstraction, a logical model that describes software that acts for a user or other program in a relationship of agency. ... Electronic mail, abbreviated e-mail or email, is a method of composing, sending, and receiving messages over electronic communication systems. ...

It 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. A mail submission agent or MSA is a computer program or software agent which receives electronic mail messages from a mail user agent (MUA) and contacts an mail transfer agent (MTA) for delivery of the mail. ... An email client (or mail user agent [MUA]) is a computer program that is used to read and send email. ...

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. A Mail Delivery Agent (MDA) is software that accepts incoming e-mail messages and distributes them to recipients individual mailboxes (if the destination account is on the local machine), or forwards back to an SMTP server (if the destination is on a remote server). ... Procmail is a mail delivery agent (MDA) or mail filter, a program to process incoming emails on a computer, widely used on Unix systems. ...

According to a survey done in late 2006 by MailChannels.com[1], which covered around 400,000 public mail servers of commercial entities worldwide as listed by a US data agency, the most popular mail server software are Sendmail, Postfix, MS Exchange, qmail and Exim. They also found that many companies use the services of e-mail security services such as Postini, MXLogic or Concentric Hosting to receive e-mail. Sendmail is a mail transfer agent (MTA) that is a well known project of the open source and Unix communities and is distributed both as free software and proprietary software. ... Postfix is an open source mail transfer agent (MTA), a computer program for the routing and delivery of email, that is intended as a fast, easy to administer and secure alternative to the widely-used Sendmail. ... Microsoft Exchange Server is a messaging and collaborative software product developed by Microsoft. ... qmail is a mail transfer agent that runs on Unix. ... Exim is a mail transfer agent (MTA) used in Unix-like operating systems. ...

A survey done in March 2004 by Falko Timme[2], which covered 106,269 public mail servers, mostly in German netblocks, the most popular were Sendmail, Postfix, Exim, with IMail also being numerous but endemic at two ISPs.

See also

• MX record
• List of mail servers
• Comparison of mail servers
• Mail delivery agent

An MX record or Mail exchange record is a type of resource record in the Domain Name System (DNS) specifying how Internet e-mail should be routed. ... This is a list of mail servers: mail transfer agents, mail delivery agents, and other computer software which provides e-mail services. ... This is a list of mail servers: mail transfer agents, mail delivery agents, and other computer software which provides e-mail services. ... A Mail Delivery Agent (MDA) is software that accepts incoming e-mail messages and distributes them to recipients individual mailboxes (if the destination account is on the local machine), or forwards back to an SMTP server (if the destination is on a remote server). ...

External links

• Daniel J. Bernstein, Internet host SMTP server survey, November 2001
• Anonymous, Open source Mail Server Comparison, November 2003
• Dan Shearer, MTA Comparison, January 2007